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The pattern of the earliest cell divisions in a vertebrate embryo lays the groundwork for later developmental events such as gastrulation, organogenesis, and overall body plan establishment. Understanding these early cleavage patterns and the mechanisms that create them is thus crucial for the study of vertebrate development. This chapter describes the early cleavage stages for species representing ray-finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians and summarizes current understanding of the mechanisms that govern these patterns. The nearly universal influence of cell shape on orientation and positioning of spindles and cleavage furrows and the mechanisms that mediate this influence are discussed. We discuss in particular models of aster and spindle centering and orientation in large embryonic blastomeres that rely on asymmetric internal pulling forces generated by the cleavage furrow for the previous cell cycle. Also explored are mechanisms that integrate cell division given the limited supply of cellular building blocks in the egg and several-fold changes of cell size during early development, as well as cytoskeletal specializations specific to early blastomeres including processes leading to blastomere cohesion. Finally, we discuss evolutionary conclusions beginning to emerge from the contemporary analysis of the phylogenetic distributions of cleavage patterns. In sum, this chapter seeks to summarize our current understanding of vertebrate early embryonic cleavage patterns and their control and evolution.
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© Springer International Publishing Switzerland 2017
F. Pelegri et al. (eds.), Vertebrate Development, Advances in Experimental
Medicine and Biology 953, DOI 10.1007/978-3-319-46095-6_4
Chapter 4
Vertebrate Embryonic Cleavage Pattern
Andrew Hasley, Shawn Chavez, Michael Danilchik, Martin Wühr,
and Francisco Pelegri
Abstract The pattern of the earliest cell divisions in a vertebrate embryo lays the
groundwork for later developmental events such as gastrulation, organogenesis, and
overall body plan establishment. Understanding these early cleavage patterns and
the mechanisms that create them is thus crucial for the study of vertebrate develop-
ment. This chapter describes the early cleavage stages for species representing ray-
finned fish, amphibians, birds, reptiles, mammals, and proto-vertebrate ascidians
and summarizes current understanding of the mechanisms that govern these pat-
terns. The nearly universal influence of cell shape on orientation and positioning of
spindles and cleavage furrows and the mechanisms that mediate this influence are
discussed. We discuss in particular models of aster and spindle centering and orien-
tation in large embryonic blastomeres that rely on asymmetric internal pulling
forces generated by the cleavage furrow for the previous cell cycle. Also explored
are mechanisms that integrate cell division given the limited supply of cellular
building blocks in the egg and several-fold changes of cell size during early devel-
opment, as well as cytoskeletal specializations specific to early blastomeres
A. Hasley • F. Pelegri (*)
Laboratory of Genetics, University of Wisconsin—Madison,
Genetics/Biotech Addition, Room 2424, 425-G Henry Mall, Madison, WI 53706, USA
S. Chavez
Division of Reproductive & Developmental Sciences, Oregon National Primate Research
Center, Department of Physiology & Pharmacology, Oregon Heath & Science University,
505 NW 185th Avenue, Beaverton, OR 97006, USA
Division of Reproductive & Developmental Sciences, Oregon National Primate Research
Center, Department of Obstetrics & Gynecology, Oregon Heath & Science University,
505 NW 185th Avenue, Beaverton, OR 97006, USA
M. Danilchik
Department of Integrative Biosciences, L499, Oregon Health & Science University,
3181 SW Sam Jackson Park Road, Portland, OR 97239, USA
M. Wühr
Department of Molecular Biology & The Lewis-Sigler Institute for Integrative Genomics,
Princeton University, Icahn Laboratory, Washington Road, Princeton, NJ 08544, USA
including processes leading to blastomere cohesion. Finally, we discuss evolution-
ary conclusions beginning to emerge from the contemporary analysis of the phylo-
genetic distributions of cleavage patterns. In sum, this chapter seeks to summarize
our current understanding of vertebrate early embryonic cleavage patterns and their
control and evolution.
Keywords Blastomere • Spindle orientation • Cleavage plane determination
• Aster centering • Scaling • Cytoskeleton • Compaction • Cell cleavage type
• Evolution
4.1 Introduction
The pattern of early cell divisions in vertebrate embryos varies widely. It is important
to understand this patterning and its origin, since, in most organisms, the arrangement
of cells resulting from the early cleavages is responsible for generating the earliest
features of the embryo’s body plan. The cleavage pattern of the early embryo, and the
arrangement of cells that results, generates the early embryonic anatomy. During
activation of zygotic gene expression at the midblastula transition (MBT), new gene
expression initiates new patterns of cell behavior, generating morphogenetic move-
ments that further modify the embryo. However, early cleavage pattern constrains
subsequent developmental processes. Moreover, many cellular decisions, including
axis induction, germ layer specification, and germ cell formation, occur prior to
MBT. The early embryonic anatomy, which originates from the interaction between
the initial egg structure and dynamic processes driven by maternally inherited com-
ponents, must facilitate, or at least be compatible with, such inductive processes.
Various factors, such as embryo size, patterns of yolk deposition (in nonmam-
malian vertebrates), the symmetry of yolk deposition with respect to oocyte polar-
ity, localization of molecular cues, and cell shape, influence patterns of cell division.
The resulting cellular assembly in combination with inductive processes lays the
foundation for embryonic morphogenesis. This chapter addresses these cellular and
molecular processes, which generate this late blastula architecture, and their rela-
tion to the early pattern of the embryo. We explore some of the variety in vertebrate
embryonic cleavage patterns and discuss processes involved in their creation. We
first examine the two main classes of embryonic cleavage in vertebrates (meroblas-
tic and holoblastic cleavage). Later, we describe cellular mechanisms required for
furrow placement during early embryonic development, an essential factor in gen-
erating the early embryonic cleavage pattern. We also address additional cellular
and developmental mechanisms underlying morphological landmarks of the
embryo, such as the formation of specialized cytoskeletal structures in large embry-
onic cells and cellular compaction, as well as the regulated use of maternal building
blocks in cleaving embryonic cells. We also summarize current knowledge on
patterns and underlying molecular cues in other vertebrates, including mammals,
and in a well-studied proto-vertebrate system.
A. Hasley et al.
4.2 Main Patterns of Embryonic Cleavage in Vertebrates:
Holoblastic Versus Meroblastic
Early embryonic cell division patterns in vertebrates can be broken into two broad
categories, holoblastic cleavage (e.g., most amphibians and mammals) and mero-
blastic cleavage (e.g., birds, reptiles, and teleost fishes) (Fig. 4.1). In holoblastic
cleavage, the entire egg undergoes cellularization, and yolk platelets are either
absent (e.g., in mammals) or present as cytoplasmic inclusions that partition among
cells (e.g., in amphibians). Cleavage furrows encompass the embryo completely,
from the animal pole (corresponding to the region containing the meiotic spindle in
the oocyte) to the vegetal pole. In meroblastic cleavage, in contrast, cell division
does not divide the embryo in its entirety. Instead, embryonic cells divide in the
animal pole independently of the vegetally located yolk, with cells typically remain-
ing syncytial to the yolk cell for a period of time that varies among species. We
would like to point out that pure holoblastic or meroblastic cleavage patterns are
only idealized extremes. We will discuss that many embryos cleave with an inter-
mediate geometry, in which the entire embryo cleaves but the strong asymmetry of
yolk leads to a bias of cleavage planes. In any of these cases, dividing cells are
called blastomeres regardless of whether they contain yolk.
In all vertebrates, determination of the animal and vegetal poles of the egg devel-
ops during oogenesis, with the oocyte typically containing a pre-existing animal
region with distinct properties (see Chap. 5). This animal region will contain the
nuclear DNA after fertilization and, in embryos with meroblastic cleavage, accumu-
lates yolk-free ooplasm after fertilization. The vegetal region of the egg, in addition
to acting as the site for yolk storage in meroblastic species, is also an essential
player in early embryogenesis, containing signals involved in early axial patterning
Fig. 4.1 Types of embryonic cleavage. Holoblastic cleavage encompasses the entirety of the
embryo, involving meridional planes that cleave through the animal and vegetal poles of the
embryo. Meroblastic cleavage involves an early separation between cells at the animal pole and
yolk at the vegetal pole of the egg, and cell furrows encompass only the animal-most region of the
embryo, leaving the yolk intact
4 Vertebrate Embryonic Cleavage Pattern Determination
and cell specification that are transmitted to blastomeres (see Chaps. 68). In the
following sections, we explore the factors that govern holoblastic and meroblastic
cleavage patterns, especially the substantial variation observed within these two
broad categories.
4.3 Cellular Mechanisms Underlying Cell Cleavage
Pattern Determination
The pattern of early cell division in an embryo influences parameters of the forming
blastula, such as the number of cells, depth of stacking within a cell layer, and rela-
tionship to extraembryonic spaces and structures. The cleavage pattern depends on
cell division planes in individual blastomeres. Here, we address cellular mecha-
nisms that influence the furrow plane and which lead to the cellular arrangement of
the early embryo.
4.3.1 Induction of Cell Cleavage Plane by Chromatin
and Amphiasters
Typically, the cell divides perpendicular to its longest axis into two equal daughter
cells. At the core of this process is the spindle apparatus (Fig. 4.2) with the two
microtubule asters (amphiasters) emanating from its centrosomes. The spindle
apparatus is an arrangement of microtubules that organize around the microtubule-
organizing centers (MTOCs) and the DNA during M phase of the cell cycle and is
responsible for chromosome segregation during cell division. Kinetochore microtu-
bules connect the spindle pole to chromosomes. The microtubules between the
poles that are not connected to the kinetochore are called interpolar microtubules.
While we currently know little about microtubule length distribution in the mitotic
spindles of early development, in the closely related meiosis II spindles, interpolar
microtubules are very short compared to spindle length (Needleman et al. 2010).
Another set of spindle-associated microtubules are astral microtubules, which ema-
nate outward from each of the MTOCs in a radial fashion. Typically, microtubule
themselves are arranged with their minus ends pointing toward the closest MTOC
in the spindle pole. As described below, signals both from chromatin at the spindle
midzone and from astral microtubules are involved in furrow induction and there-
fore cleavage plane determination.
Early observations indicated a correlation between the appearance of the furrow
plane and the location of the spindle apparatus, with the furrow forming perpen-
dicular to the spindle at a location equidistant from the spindle poles (Hertwig
1893). This plane normally corresponds to that of aligned chromosomes during the
preceding metaphase, as well as the region where astral microtubules overlap.
A. Hasley et al.
Fig. 4.2 Dynamic changes in spindle morphology during the cell cycle. The spindle apparatus
begins to form before metaphase. During metaphase, interpolar microtubules (ipMTs) link the
spindle poles, and short metaphase asters (ma) form that are too short to reach the cortex (Wühr
et al. 2010). Asters during anaphase appear to be much more developed, reaching a wider radius.
A zone of microtubule exclusion, the microtubule interaction zone, forms in the region in which
asters meet and which presages the furrow (arrows). AurB, a CPC protein, shows recruitment of
this complex to the spindle midzone (arrowhead at late anaphase) and at the tip of FMA tubules in
more distal regions of the furrow (arrowhead in telophase; see also Yabe et al. 2009). A zone of
microtubule exclusion, the microtubule interaction zone, forms in the region in which asters meet
and which presages the furrow (arrows). Microtubules in red, AurB in green, DNA in blue. Figure
adapted from Yabe et al. (2009)
4 Vertebrate Embryonic Cleavage Pattern Determination
Chromosomes therefore can typically be separated equally between the two daugh-
ter cells. Analyses of various cell types have shown furrow-inducing activity in
midzone microtubules of the spindle (Martineau et al. 1995; Cao and Wang 1996).
Studies in amphibians show that neighboring asters induce a cleavage furrow, but
only if chromatin is located between them (Brachet 1910), showing a key role for a
chromatin-derived factor in furrow induction. However, experimental manipula-
tions of sand dollar embryos by Rappaport, which created a barrier between the
poles of a forming spindle to force overlap of normally nonadjacent asters in other
regions of the blastomere, showed the induction of an ectopic furrow in the region
of overlap in the absence of nearby chromatin, confirming a role for astral microtu-
bules in furrow induction (Rappaport 1961, 1996). These experiments indicate that
signals present in both the midzone and astral microtubules contribute to furrow
induction (Rappaport 1996; Mishima 2016). The degree to which one of these two
mechanisms alone is able to induce a cleavage furrow varies between different spe-
cies (Brachet 1910; Rappaport 1961; Rappaport and Rappaport 1974; Su et al.
2014; Field et al. 2015). Because the location of the midzone and that of astral
microtubule overlap typically occur in the same position, both of these structures act
together to establish a robust positioning mechanism to place the furrow at a plane
halfway between spindle poles.
The large cells of some embryos like fish or amphibians require some special
adaptations related to spindle structure and function. A major adaptation involves
aster morphology. In smaller cells, nearly all microtubules’ minus ends are believed
to be close to the centrosome. However, such an arrangement, due to its radial nature
within the volume of the blastomere, would lead to severely reduced microtubule
densities near the cortex in very large cells. Instead, large cells such as the zebrafish
and Xenopus embryonic blastomeres implement an alternative strategy, in which
sites of microtubule nucleation are evenly distributed throughout these large asters
(Figs. 4.2 and 4.4). The implementation of these internal microtubule nucleation
sites results in microtubule density remaining near constant, independent of dis-
tance from the aster center (Wühr et al. 2009). The induction of microtubule nucle-
ation sites within the aster can be explained with a chemical trigger wave that relies
on microtubule-dependent nucleation (Ishihara et al. 2014).
Another major adaptation of asters which is particularly apparent in very large
embryos is the formation of an aster–aster interaction zone, a region depleted of
microtubules at the site of overlap between adjacent asters (Wühr et al. 2010;
Nguyen et al. 2014; Figs. 4.2 and 4.4). As described below, this interaction zone
seems to enable to communicate the proper plane for cell division from the mitotic
spindle apparatus to the cell cortex, which can be separated by several hundred
micrometers. Furthermore, the interaction zone preempts the barrier of the future
cleavage plane allowing the aster to center and orient along the longest axis of the
future daughter cell before it actually exists.
Analysis of the function of the Chromosomal Passenger Complex (CPC) compo-
nent Aurora kinase B (Aur B) in zebrafish embryos provides additional insight into
this redundancy as it applies to the large embryonic cells (Yabe et al. 2009). Embryos
from females homozygous for a maternal-effect mutant allele in the gene cellular
A. Hasley et al.
island, which encodes Aur B, exhibit defects in furrow formation. However, in these
embryos, cytoskeletal structures associated with the furrow (see below) appear to be
induced relatively normally in the center of the blastomere, whereas they are entirely
absent in more distal regions of the furrow. As expected, Aur B protein in these
embryos is found both at the spindle midzone and the tips of astral microtubules that
contact the forming furrow (Fig. 4.2). Genetic analysis of the Aur B maternal-effect
cellular island mutant allele indicates that it retains some functional activity, as
homozygotes for complete loss of function alleles are zygotic lethal and do not
survive to adulthood, in contrast to homozygotes for the maternal-effect mutant
allele. A comparison of the maternal-effect cellular atoll phenotype, which allows
formation in the medial furrow region, with the effects of a specific Aur B inhibitor,
which cause furrow inhibition throughout the furrow, suggests that the partial activ-
ity in the maternal-effect cellular island allele is provided by Aur B function present
in the spindle. Such spindle-provided Aur B may be at a higher concentration or
have a higher functional activity than that present in astral microtubule ends.
Consistent with this interpretation, embryos maternally mutant for both futile cycle,
which fails to form spindle structures, and cellular island lack furrow-associated
structures throughout the length of the furrow, in both medial and distal regions.
Altogether, these observations suggest that, in large embryonic blastomeres, CPC
activity and potentially other signals from astral microtubules are essential for fur-
row induction in distal regions of the cell, which are presumably too distant to be
influenced by inducing signals from chromatin present at the spindle midzone (Figs.
4.2 and 4.4). Indeed, in several amphibians, early cleavage furrow advance may
depend on signals propagated solely through the cortex via the furrow’s distal ends
(Sawai 1974, 1980; Mabuchi et al. 1988; Sawai and Yomota 1990). These spatial
functional specializations of furrow-inducing activity may be a necessary adapta-
tion to the small coverage of the spindle relative to the much larger embryonic
Furrow-inducing activity from the spindle midzone and astral microtubules is
concentrated at the point halfway between the spindle poles, and in a plane perpen-
dicular to that of the spindle, thus functionally linking cleavage patterning to the
mechanistic determinants of spindle orientation within dividing blastomeres. This
linkage will be discussed in the next section.
4.3.2 Centering and Orienting Asters and Spindles
For over 125 years, scientists have known that mitotic spindles tend to align along
the longest axis of a dividing cell and that cleavage furrows tend to form perpen-
dicular to the mitotic spindle (Hertwig 1893). The tendency holds even when cell
shape is deliberately manipulated to change the orientation of a cell’s longest axis.
In the original manipulations, artificially elongating frog embryos by compression
generated a reorientation of the cleavage plane consistent with realignment of the
spindle (Pflüger 1884; Hertwig 1893; Black and Vincent 1988). This phenomenon,
4 Vertebrate Embryonic Cleavage Pattern Determination
named Hertwig’s rule, has been consistently observed in numerous cell types,
including both embryonic and somatic cells, in a variety of organisms including
vertebrates, invertebrates, and unicellular organisms. The sensitivity of spindle ori-
entation to cell shape underlying Hertwig’s rule has been explained by interactions
between astral microtubules and the cell cortex, thought to generate forces that
become balanced and energetically favorable when the spindles become aligned to
the long axis of the cell (Bjerkness 1986; Grill and Hyman 2005; see below).
An important question to understand cleavage plane determination is how astral
structures sense cellular geometry. Spindles may sense cell shape through their
asters, which extend outward in a radially symmetric manner and are therefore ide-
ally suited to sense intracellular surroundings and the cortex. However, in some
large embryos, astral microtubules of the spindle are too short to reach the cortex.
In these cells, the task of sensing cellular geometry is performed by the cell- spanning
anaphase asters of the preceding cell cycle. For the first cell cycle, the task is per-
formed by the sperm-aster, a mono-aster that forms in the zygote immediately after
fertilization (Chambers 1939). The Role of the Sperm-Aster
In most vertebrate lineages after fertilization, centrioles are inherited through the
sperm, having been lost during oogenesis. This arrangement is thought to be essen-
tial to maintain a constant number of centrioles from one generation to the next
(reviewed in Delattre and Gönczy 2004). Surprisingly, however, there are numerous
variations on this general theme. Sperm may bring a pair of centrioles, a pair with
an incomplete centriole, or a single centriole. In the latter two cases, biogenesis of
the second centriole is completed in the zygote after fertilization. Sperm-derived
centriolar pairs are thought to act as a template to mediate the reconstitution of a
centrosome by nucleating maternally derived centrosome components (Lessman
2012). During interphase of the first cell cycle, this centrosome acts as an MTOC to
generate the structure with a single aster, termed the sperm-aster.
The primary function of the sperm-aster is to mediate pronuclear fusion. During
fertilization, the maternal pronucleus is formed after reinitiation of meiosis II trig-
gered by egg activation, whereas the paternal pronucleus is introduced by the sperm.
After fertilization, the paternal pronucleus remains in close proximity to the centro-
some, and the sperm-aster is required for the movement of the maternal pronucleus
toward the MTOC and closely associated paternal pronucleus. This movement
occurs through the movement of the maternal pronucleus toward the microtubule
minus end at the MTOC at the center of the sperm-aster. This movement has been
shown in a number of vertebrate species (Chambers 1939; Navara et al. 1994), to be
mediated by transport via the minus-directed microtubule-based motor dynein
(Reinsch and Karsenti 1997).
In most vertebrates, multiple layers of regulation are in place to inhibit fertiliza-
tion by more than one sperm (Just 1919). If polyspermy is induced artificially, each
sperm produces its own sperm-aster (Brachet 1910). The asters space each other out
A. Hasley et al.
in the embryo and multiple mitotic spindles each induce their own cleavage furrow.
This condition is lethal for the embryo. However, in most urodeles (newts and sala-
manders), polyspermy is the natural mode of fertilization (Fankhauser 1932).
Although each sperm gives rise to its own sperm-aster, only the one sperm-aster that
reaches the female pronucleus becomes dominant to span large parts of the cell. The
other sperm-asters disintegrate. It seems necessary that the DNA of the supernumer-
ary asters must also somehow be destroyed later on to prevent polyploidy of subsec-
tions of the embryo. The molecular mechanisms underlying this fascinating
phenomenon are not understood.
In the zebrafish, the sperm-aster has also been shown to facilitate the multimer-
ization of maternally inherited ribonucleoparticles (RNPs) that confer the germ cell
fate, termed the germplasm. This appears to be achieved by the action of sperm-
aster ends on an actin-based network associated with these RNPs, where the radial
growth of sperm-aster microtubules generates a wave of RNP aggregation (Theusch
et al. 2006; Nair et al. 2013), in anticipation of their accumulation at the furrows
produced during the early cell divisions (Eno and Pelegri 2013; reviewed in Eno and
Pelegri 2016). A similar function for the sperm-aster in germplasm RNP multimer-
ization in other model systems has not yet been shown, although in some systems,
such as in frogs and chicken, germplasm RNPs exhibit patterns of accumulation at
the furrows that are similar to those occurring in the zebrafish (see Chap. 8). Nuclear–Cytoskeletal Attachment During Nuclear Fusion
In the zebrafish, transport of the paternal pronucleus along the sperm-aster has also
been shown to require the function of another maternal-effect gene, futile cycle/
lrmp (Lindeman and Pelegri 2012). This gene encodes a KASH-domain protein
with gene products localized to the nuclear envelope of maternal and paternal pro-
nuclei. This protein is thought to mediate a link between the outer nuclear mem-
brane and the microtubule cytoskeleton. Interestingly, the futile cycle/lrmp-dependent
nuclear–microtubule connection is also required to maintain the close attachment of
the paternal pronucleus to the centrosome at the sperm-aster MTOC (Lindeman and
Pelegri 2012). Thus, attachment of the outer nuclear membrane to the cytoskeleton
is essential for both long-range transport of the maternal pronucleus toward the
MTOC and local attachment of the paternal pronucleus to the MTOC. This coordi-
nated set of processes drives the movement of the maternal pronucleus toward the
MTOC while keeping the paternal pronucleus close to this structure, thus mediating
the encounter of both pronuclei leading toward their fusion.
Nuclear envelope–cytoskeletal interaction likely continues throughout early
embryonic blastomere divisions, as suggested by the highly localized pattern for
futile cycle/lrmp proteins at the nuclear membrane–centrosome interphase
(Lindeman and Pelegri 2012). This nuclear–centrosomal linkage, coupled to the
centering of the (aster-containing) spindle apparatus, guarantees the even distribu-
tion of chromatin content among the newly formed blastomeres.
4 Vertebrate Embryonic Cleavage Pattern Determination
The process of pronuclear fusion is additionally integrated with aster centering.
In amphibians, sperm entry occurs at random locations in the animal hemisphere,
generating an immediate need for aster centering. Even in zebrafish, where the
mature egg contains a sperm entry site at the approximate center of the animal pole
that will constitute the future blastodisc area, the aster still likely centers itself along
the depth of the forming blastodisc (see below). Below, we describe how aster cen-
tering is achieved in a large embryonic cell. Tug-of-War Forces and Aster Centering
Early studies showed growing microtubules can generate pushing forces against an
object (Hill and Kirschner 1982), which suggested a mechanism for aster (and bipo-
lar spindle) centering in small cells such as yeast (Tran et al. 2001). However, the
larger size of most metazoan cell types necessitates longer astral microtubules in
order to contact the cortex, which, due to their greater length and greater tendency
to undergo buckling, limit the force that might be transmitted through a pushing
mechanism (Dogterom et al. 2005). Instead, in these cells, aster centering has been
hypothesized to depend on pulling forces. Indeed, experiments in fertilized sand
dollar eggs showed that generating a zone of microtubule polymerization (by local-
ized inactivation of the microtubule inhibitor colcemid) generates movement toward
the zone of polymerization (Hamaguchi and Hiramoto 1986), not away, as would
have been predicted by a pushing model for aster centering (Fig. 4.3a). This led to
a model in which astral microtubules are centered by pulling, rather than pushing
forces, a mechanism that was subsequently supported by experiments in
Caenorhabditis elegans and yeast (Bukarov et al. 2003; Grill et al. 2003; Grill and
Hyman 2005). Consistent with the colcemid-inhibition experiments in sand dollar
embryos (Hamaguchi and Hiramoto 1986) and a microtubule pulling model, the
converse experiment, involving partial aster depolymerization via UV-mediated
uncaging of the microtubule inhibitor combretastatin 4A in early zebrafish embryos,
results in spindle movement away from the site of depolymerization (Wühr et al.
2010; Fig. 4.3b).
These pulling interactions were assumed to occur between microtubules and the
cell cortex (Dogterom et al. 2005; Grill and Hyman 2005; Kunda and Baum 2009).
Due to astral microtubule orientation, with minus ends at the aster center and plus
ends facing away, such pulling forces could be mediated by the minus end-directed
microtubule-based motor dynein (reviewed in Kotak and Gönczy 2013). However,
pulling from the cortex under most circumstances will not lead to aster centering but
rather MTOC pulling close to the cortex. Grill and Hyman suggested a scenario in
which a limited concentration of cortical dynein compared to the number of plus-
end microtubules reaching the cortex could lead to stable aster centering (Grill and
Hyman 2005).
Nevertheless, studies have suggested that, at least in large cells such as those in
early zebrafish and Xenopus embryos, dynein is not exerting a force by pulling from
the cortex but rather by anchoring of astral microtubules to internal elements of the
A. Hasley et al.
cytoplasm. This was initially suggested by the observation that both the sperm-aster
after fertilization and bipolar spindles during cleavage become centered within the
cell before astral microtubules reach the cortex. Similarly, in the abovementioned
microtubule inhibitor uncaging experiment (Wühr et al. 2010), spindle movement
away from the site of local microtubule depolymerization occurred before the aster
reached the cortex. As in cultured cells, interference with dynein function in zebraf-
ish blastomeres (in these experiments through injection of p150-CC1, a dominant-
negative form of the dynein partner dynactin) results in spindles that are uncentered
as well as misoriented (Wühr et al. 2010). A similar requirement for dynein function
was demonstrated for the centering of the sperm-aster immediately after fertiliza-
Fig. 4.3 Demonstration that asters are centered by pulling, not pushing, forces. (a) Treatment of
embryos with colcemid leads to inhibition of microtubule polymerization, and localized inactiva-
tion of colcemid with UV light results in the movement of the aster toward the site of microtubule
growth (Hamaguchi and Hiramoto 1986). (b) Local uncaging of the microtubule inhibiting drug
combretastatin 4A results in the movement of the aster away from the site of microtubule inhibi-
tion (Wühr et al. 2010). A model in which asters become centered by microtubule pushing forces
predicts the opposite effects on aster movement and is ruled out by observations
4 Vertebrate Embryonic Cleavage Pattern Determination
tion in Xenopus eggs (Wühr et al. 2010). These studies suggest that, in zebrafish and
Xenopus, dynein-dependent pulling forces are required for the centering of astral
microtubules during pronuclear fusion and blastomere divisions. Because in these
species the centering and experimentally induced movements occur prior to astral
microtubules contact with the cortex, the pulling force is unlikely to be generated at
the cortex, but is instead generated along presently unknown internal elements of
the cytoplasm. Knockdown experiments in C. elegans suggest that, in this system,
vesicles are likely the anchor for cytoplasmic dynein to generate pulling forces on
microtubules (Kimura and Kimura 2011). Distribution of such internal anchors
through the cytoplasm could result in a length-dependent force, such that longer
microtubules contribute larger pulling forces than shorter ones, resulting in net aster
movement. A similar model for microtubule length-dependent pulling depending on
cell volume (i.e., internal stores) as opposed to cell surface has been derived through
cell shape manipulations and mathematical modeling (Minc et al. 2011). Thus,
although pulling from cortical anchors has been documented to orient the spindle in
smaller differentiated cells (McNally 2013), large embryonic blastomeres appear to
use a tug-of-war pulling mechanism from internal sites, whose consequences for
embryonic cleavage patterning are described below.
4.3.3 Mechanisms Underlying Spindle Orientation in Large
Embryonic Cells in Fish and Amphibians
Two model systems, zebrafish and Xenopus, provide insight on the mechanisms that
drive cleavage patterning in early vertebrate embryos with large blastomeres.
Embryos from these species exhibit similar behaviors with regard to the spatial
arrangement of the spindle and associated DNA in relation to the blastomere center.
Due to the small size of the spindle relative to the large size of the blastomere, each
of the resulting nuclear masses is at the end of anaphase in a location relatively close
to the previous furrow, off-center with respect to the newly formed daughter blasto-
meres. Thus, in preparation for the next cell division, an important initial require-
ment is the centering of the forming spindle within the daughter blastomeres.
Additionally, the cell cleavage plane in these embryos is known to alternate with
each cell division, with each cleavage plane at a 90o angle relative to that of the
previous one. Spindle Orientation Based on Cell Geometry Can Be Overridden
by Molecular Cues
As described above, aster-mediated forces influence the position of both sperm-
asters and amphiasters emanating from bipolar spindles. As will be discussed
shortly, in the case of bipolar spindles, stress forces and/or coordinate centering of
A. Hasley et al.
astral microtubules at each of the spindle poles is thought to additionally contribute
to the alignment of the spindle within the cell. In smaller cells, such alignment is
clearly influenced by interactions of the cell boundaries and the metaphase spindle,
according to Hertwig’s rule of alignment with the longer cell axis. The idea that
astral centering occurs by internal pulling forces can be reinterpreted in this context,
since forces on MTOCs and the spindle structure will be dependent on the length of
astral microtubules emanating from these structures. Since the cortex acts as a bar-
rier to microtubule growth, the long axis of the cell allows for longer microtubule
growth and consequently stronger internal pulling forces, which contribute to spin-
dle alignment along this axis. In this manner, cell shape can influence the orienta-
tion of the spindle.
The general principle of cells forming a cleavage furrow perpendicular to their
longest axis can be overridden by molecular cues, where mechanisms of spindle
orientation are influenced by asymmetrically distributed factors (reviewed in Sousa-
Nunes and Somers 2013; Williams and Fuchs 2013; Rose and Gönczy 2014;
Schweisguth 2015). This mechanism tends to be most common during the division
of polarized cells, such as those found in tissue epithelia and cells that are beginning
to differentiate into specific lineages. This phenomenon has been observed in some
cell fate determination systems that depend on asymmetric cell division mediated
by the orientation and/or position of the spindle with regard to intracellular polarity
factors, such as in C. elegans embryonic development, Drosophila neuroblast
formation, micromere formation in echinoid embryos, and neural precursor
divisions in the vertebrate nervous system. As exemplified in C. elegans, off-center
positioning of the spindle in addition to its orientation also mediates the formation
of different cell sizes of daughter cells. Controlled orientation of spindle and cell
cleavage plane has also been observed during extension of the vertebrate axis, with
cleavage plane orientation mediating axis elongation.
However, in many vertebrate embryos such as zebrafish and Xenopus, early blas-
tomeres are generated through a relatively uniform process in the absence of appar-
ent signals that generate cell asymmetry. Under these conditions, how is the spindle
orientation of a sequence of daughter cells determined, and how can this explain the
overall cleavage pattern of an early embryo? Transmission of Spindle Orientation Cues During Rapid Cycling
While a cortex-sensing mechanism can explain spindle alignment in smaller cells,
studies in zebrafish and Xenopus have shown that it cannot explain spindle orienta-
tion in the large embryonic blastomeres in those species. In such embryos, spindles
become aligned immediately prior to metaphase even before microtubule asters are
long enough to contact the cortex (Fig. 4.4). It is only during anaphase, which in
these cells coincides with interphase for the following cell cycle, when astral growth
becomes extensive enough to reach the cortex. The early commitment of the spindle
orientation by the metaphase spindle and refinement of orientation by the telophase
astral microtubules has been demonstrated by following the effects of induced cell
4 Vertebrate Embryonic Cleavage Pattern Determination
shape changes at different time points (Wühr et al. 2010). By applying pressure to
Xenopus blastomeres to artificially impose a cleavage plane bisecting the elongated
cell axis, the authors found that in prophase, prior to nuclear breakdown, the form-
ing spindle (as determined by the inter-centrosomal axis) was already aligned with
the long axis of the cell, within about 5o (compared to 45o if randomly oriented).
Between anaphase and cytokinesis, when the telophase aster would be expected to
have an effect, this alignment improved slightly yet significantly, to 1–2o. These
experiments clearly show that (1) the metaphase spindle largely acquires its final
orientation at a time when astral microtubules do not reach the cortex and (2) ana-
phase asters contribute to the fine-tuning of spindle alignment.
The second of these structures—anaphase astral microtubules—appears to pro-
vide two functions to the ongoing cell cycle. As in the case for the sperm-aster (see
above), anaphase astral microtubules experience dramatic growth due to continuous
nucleation at internal sites, resulting in microtubules becoming nucleated as they
expand outward toward the cortex (Ishihara et al. 2014). This internal priming
mechanism may allow astral tips to reach the cortex in spite of the large size of the
Fig. 4.4 Spindle alignment occurs early in the cell cycle and conforms to an alternating perpen-
dicular pattern. Shown are microtubules (green) and DNA (blue, arrowhead in insert) in a frog
embryo shortly before the first cleavage. Alignment of the axis of the incipient spindle is concur-
rent with telophase for the previous (in this embryo, first) mitotic spindle. Arrow in insert points to
the nascent second mitotic spindle indicating the spindle axis. The forming spindle is parallel to
the microtubule interaction zone, which indicates the location of the first cleavage furrow (thick
arrow in the main panel; Wühr et al. 2010). The second spindle is therefore oriented perpendicular
to the spindle of the previous cell cycle. Scale bar corresponds to 500 μm. Figure adapted from
Wühr et al. (2010)
A. Hasley et al.
blastomeres (see above). For the ongoing cell cycle, a primary function of the ana-
phase aster appears to be furrow induction at the site of overlap of microtubules
from each spindle pole, presumably by providing furrow induction signals such as
CPC factors as discussed above. A second function, indicated by the slight improve-
ment in spindle alignment observed in anaphase, seems to be to add precision late
in the cell cycle to the orientation of the alignment of the spindle acquired earlier in
the cell cycle. The precise mechanistic nature of this late alignment has not been
studied, but presumably involves length-dependent internal pulling forces and/or
(because anaphase microtubules do reach the cortex) cortex-sensing mechanisms.
The second function, fine-tuning of orientation, can be explained by a cortex-
sensing mechanism. But, how does the spindle largely acquire its future orientation
early in the cell cycle, even prior to its ability to sense the cortex and possibly cell
shape? A key concept for understanding this mechanism of early spindle alignment
is that, in the rapidly cycling blastomeres of the early embryo, cytoskeletal struc-
tures involved in cell division exhibit a degree of overlap between cell cycles. Thus,
the spindle for a given cell cycle is starting to form at a time in which telophase
astral microtubules of the previous cell cycle are still carrying out essential cell divi-
sion functions. In particular, telophase astral microtubules, in addition to executing
the above-described functions on furrow induction and spindle alignment fine-
tuning for an ongoing cell cycle, provide cues for the early orientation of the meta-
phase spindle for the following cell cycle (Wühr et al. 2010).
This influence of furrow orientation from one cycle to the next appears to depend
on a zone of microtubule exclusion that forms at the site of anaphase astral micro-
tubule overlap, which develops in the plane in which the future cytokinetic furrow
will cleave (Figs. 4.2, 4.4, and 4.5). As mentioned above, metaphase asters are rela-
tively small and do not reach the cortex. On the other hand, anaphase asters grow
dramatically to fill in the entire space of blastomeres and contact the cortex, where
they provide signals to induce furrowing during cytokinesis. As telophase asters
from opposite sides of the spindle reach the midzone, a microtubule-free (microtu-
bule interaction) zone appears which clearly delineates the site of the forming fur-
row. This zone of microtubule exclusion generates a “dome”-shaped aster, where
the sides of the dome correspond to the new spindle’s long axis, aligned parallel to
the plane of the forming furrow (Wühr et al. 2010). This dome shape, with microtu-
bules being longer on the side of the spindle opposite the microtubule interaction
zone at the furrow (Figs. 4.4 and 4.5), generates an asymmetric force that moves the
MTOCs toward the center of the future daughter cells and aligns the nascent spindle
along the axis parallel to the forming furrow. The mechanism underlying the
formation of the microtubule interaction zone is poorly understood. Embryos
mutant for the zebrafish maternal-effect gene motley, which encodes an isoform of
the CPC component survivin, do not exhibit a microtubule interaction zone at the
furrow. In these mutants, anaphase astral microtubules instead cross the furrow
boundary from opposite directions to generate a diffuse region of overlap (Nair
et al. 2013).
The series of cellular events that result in early spindle alignment in zebrafish
and Xenopus embryos (and possibly other vertebrates with large blastomeres) can
4 Vertebrate Embryonic Cleavage Pattern Determination
be described as follows (Fig. 4.5). During anaphase, at a time when nuclear enve-
lope membranes are being reformed, a microtubule interaction zone forms at the
plane along which the furrow will cleave. Because of the rapid cycling of early
blastomeres, this time period also coincides with the separation of centrosomes and
start of formation of the spindle for the following cell cycle. The nascent spindle is
located in relatively close proximity to the microtubule interaction zone at the fur-
row corresponding to the previous cell cycle. The close proximity of astral
microtubules to this aster interaction zone limits the length of astral microtubules
directed toward the furrow. Under these circumstances, pulling forces on these
shorter astral microtubules (extending between the interaction zone and the aster
center) are weaker than forces on the opposite side of the aster center (extending
toward the central and larger portion of the blastomere). This pulling force differen-
tial results in a net force on the spindle that moves it away from the furrow plane
toward the center of the blastomere, resulting in spindle centering. Asymmetric
pulling forces on the radially structured metaphase astral microtubules can also
Fig. 4.5 Mechanism of spindle centering and alignment in large embryonic cells. Rapid cell
cycling results in temporal overlap between processes corresponding to different cell cycles (see
also Fig. 4.4). Anaphase astral microtubules for a given cycle (depicted in red for the first spindle
in the top panel) expand all the way to the cortex. The asters form a microtubule exclusion region,
the microtubule interaction zone, at the site of aster overlap (left insert). Formation of an incipient
spindle for the following cell cycle (depicted in green for the second spindle in the bottom panel
and right insert) occurs simultaneous with events associated with the previous cell cycle, including
microtubule furrow array (FMA) formation and reorganization (see also Fig. 4.10). The influence
of the microtubule interaction zone on the forming spindle for the following cycle results in asym-
metric microtubule growth. Coupled to pulling from internal sources, the resulting astral microtu-
bule length asymmetry generates a centering force (blue arrow) for the forming spindle apparatus
within the forming blastomere. This same asymmetric influence, coupled to spindle elongation and
a hypothesized stress force within the spindle, results in spindle alignment in an orientation paral-
lel to the furrow (and perpendicular to the spindle) for the previous cell cycle (green arrows)
A. Hasley et al.
explain the alignment of the spindle in the same direction as the furrow from the
previous cycle, if the sister centrosomes are connected and exert a stress force on
each other. Because aster microtubules oriented in the same orientation as the aster
interaction zone (for the previous cell cycle) do not experience a microtubule length
limit, internal pulling along the same in the direction of the previous furrow would
not be restricted. Together, these forces result in alignment of the spindle for the
new cell cycle along the same axis as the furrow plane for the previous cell cycle.
This in turn results in the orientation of the new furrow, for any given cell cycle, in
a plane perpendicular to that of the previous cell cycle. In zebrafish, where this
sequence of events has been studied in more detail, the process is repeated to gener-
ate the embryonic cleavage pattern at least until about the sixth cell cycle, when the
blastomeres become small enough to be contacted by the metaphase asters. As
described in the following sections, this mechanism has been proposed to mediate
the stereotypical cleavage patterns in zebrafish and Xenopus early embryos. Cleavage Pattern Determination in Zebrafish
In the zebrafish, embryonic cleavage is known to be not only highly synchronous, a
characteristic of pre-MBT cleavage patterns (see Chap. 9), but also stereotypical in
terms of the orientation of the furrow plane during each cell cycle (Kimmel et al.
1995). As mentioned above, zebrafish embryos have meroblastic cleavage. This
cleavage pattern is presaged by the structure of the oocyte, which has two primary
regions, a wedge-shaped region at the animal pole of the egg where the oocyte
nucleus is arrested in metaphase II of meiosis and the remaining region that contains
a mixture of ooplasm and yolk granules (Selman et al. 1993). In zebrafish, the sperm
site of entry is found at a specific location within the oocyte animal pole region
which corresponds to the approximate center of the blastodisc that forms after egg
activation (Hart and Donovan 1983; Hart et al. 1992). This likely facilitates sperm-
aster centering with respect to a plane parallel to that of the blastodisc itself (which
we refer to as the xy plane). Although not yet shown, centering in a perpendicular
dimension (in the z-direction, along the height of the forming blastodisc and the
animal–vegetal axis of the embryo) may still occur. Similar to sperm-aster centering
in Xenopus (Wühr et al. 2010), sperm-aster centering along the z-axis in zebrafish
could occur through asymmetric forces on the sperm-aster due to microtubule
length restriction on the side of the cortex (see below).
The earliest embryonic cleavage cycles exhibit a largely stereotypic pattern
(Kimmel et al. 1995; Fig. 4.6a). During the first five cycles, blastomeres divide
along the xy plane (with x being the dimension along the first cell division and y
along the second cell division) and in an orientation that alternates 90o every cell
cycle. This generates, in subsequent cell cycles, a pattern of one-tiered blastomere
arrays of 2 × 1 (two-cell embryo), 2 × 2 (four-cell), 4 × 2 (eight-cell), 4 × 4 (16-cell),
and 8 × 4 (32-cell). Furrow positioning for the sixth cell cycle bisects the blasto-
meres along a z-plane, generating a two-tiered 8 × 4 blastomere array (64-cell
embryo). This cleavage pattern is remarkably constant, although detailed live imag-
4 Vertebrate Embryonic Cleavage Pattern Determination
ing has found variations in the cell cleavage pattern during the earliest cell cycles
(Olivier et al. 2010). Later cell cycles continue to be temporally synchronized but
exhibit a pattern of furrow placement of increased variability. Because furrow
induction and positioning are determined by the spindle midzone and its orienta-
tion, the observed pattern is generated by changes in spindle alignment in each suc-
cessive cleavage.
The mechanistic model described above is consistent with spindle orientation
changes in the early zebrafish and therefore its largely invariant cleavage pattern.
According to observations and the model, metaphase asters in the early cell cycles
are too small to sense the shape of the cell via microtubule–cortex interactions.
However, aster alignment depends on internal pulling forces that are asymmetric
due to the microtubule interaction zone at the cleavage plane for the previous cell
cycle. As described above, these asymmetric forces result in alignment of the spin-
dle in an orientation parallel to the furrow (and perpendicular to the spindle from the
previous cell cycle). During each cell cycle, this asymmetric force results in both
spindle orientation and spindle centering and, importantly for the overall cleavage
pattern, cell furrows forming at alternating perpendicular angles.
This regular cleavage pattern explains the alternating furrow orientation pattern
for the first cell divisions, but why do blastomeres stay in a single plane, forming a
one-tiered structure, and why does the pattern change during the sixth cell cycle to
generate a two-tiered structure? In terms of the spindles, why do spindles lie in the
xy plane during the first five cell cycles, whereas during the sixth cell cycle,
spindles reorient vertically along the z-axis? The first question, of why spindles
remain along a single x–y plane, may be related to a cell shape-sensing mechanism.
The cell cortex gradually becomes close enough to the spindle to allow astral micro-
tubules to increasingly contact the cortex and respond according to shape-sensing
forces (Wühr et al. 2010; Xiong et al. 2014). Blastomeres are initially relatively
elongated along the x–y plane compared to the z-dimension, i.e., they are longer and
wider rather than taller, which would tend to align spindles along the x–y plane. The
answer to the second question, of why spindles tend to reorient along the z-axis dur-
ing the sixth cell cycle, may be related to the same mechanism if, as blastomeres
divide and acquire a smaller size, their dimensions along the x and y axes become
smaller, relative to the z-axis, which has remained relatively unchanged (e.g., blas-
tomeres become taller, in the z-axis, than wider, along the x–y dimensions). Thus, a
shape-sensing mechanism may cause spindles to realign from the x–y plane to the
z-axis when new blastomere dimensions promote this realignment. An effect of
changing cell dimensions on cleavage plane orientation has also been implicated in
the regulation of the thickness of epithelial layers at later stages of development (Da
Silva-Buttkus et al. 2008; Luxenburg et al. 2011; Lázaro-Diéguez et al. 2015). The
stereotypic pattern of division orientation in zebrafish embryos ceases to be appar-
ent at the seventh cycle and beyond (Kimmel et al. 1995; Hoh et al. 2013).
The emerging picture is that blastomeres of the early embryo may utilize a mech-
anism in which cell shape is sensed by the combined action of the mitotic spindle,
oriented by an asymmetry defined by the zone of microtubule exclusion, and inter-
phase asters allowing cell shape sensing. Together with changes in blastomere
A. Hasley et al.
dimensions, this combined mechanism results in changes in spindle orientation that
begin to transform a two-dimensional single-cell layer into a three-dimensional
blastula. Cleavage Pattern Determination in Xenopus
The mechanistic model described above, which explains the early zebrafish cleav-
age pattern, is also consistent with the generation of cell cleavage pattern in Xenopus
laevis, another embryo with large blastomeres (Wühr et al. 2010; Fig. 4.6b). As in
zebrafish, blastomere cleavage planes exhibit an alternating 90o pattern (Nieuwkoop
Fig. 4.6 Cleavage pattern orientation in early zebrafish and Xenopus embryos. (a) Stereotypic
cleavage pattern in zebrafish. During the first five cell cycles, spindles (double arrows) remain
aligned to a single, horizontal (x–y) plane parallel to the blastodisc plane, alternating 90° every cell
cycle. During the sixth cell cycle, spindles tend to align in the vertical (z-axis) orientation, generat-
ing two tiers of blastula cells. Spindle orientation becomes random during the seventh cell cycle.
(b) Canonical cleavage pattern in Xenopus. The first two cell cycles have spindles aligned to the
x–y plane, also exhibiting an alternating 90° pattern, resulting in meridional cleavages. Spindle
tends to reorient along the z-axis during the third cell cycle to generate a cleavage plane parallel to
the equator. During the following cell cycles, the spindle aligns with the longest axis of the cell
(Strauss et al. 2006; Wühr et al. 2010). Graded shading represents asymmetric yolk distribution,
which is enriched in vegetal regions. Asymmetrically distributed yolk is hypothesized to interfere
with microtubule length and/or pulling forces to generate a force that places the spindle in an
eccentric position, biased toward the animal pole. In both (a) and (b), spindle orientation is indi-
cated with green double arrows (which appear as a dot when entering the plane of the page). The
order of furrow formation (corresponding to the cell cycle in which those furrows form) is indi-
cated by Roman numerals
4 Vertebrate Embryonic Cleavage Pattern Determination
and Faber 1967; Sawai and Yomota 1990). Also similar to zebrafish, spindles
acquire their final orientation early in the mitotic cycle, before astral microtubules
are long enough to reach the cortex (Wühr et al. 2010).
In contrast to zebrafish, fertilization of anuran amphibian eggs, such as Xenopus,
does not occur through a spatially defined sperm entry point, but instead occurs at
random locations in the animal hemisphere (Elinson 1975; Schatten 2012). This
generates in the Xenopus embryo an immediate need for sperm-aster centering
along the x–y plane. This centering likely occurs by internal pulling forces acting on
the sperm-aster, formed immediately after fertilization and generated by reconstitu-
tion of a centrosome around the sperm-derived centrioles, which act as an MTOC
for the sperm-aster. The location of the sperm centrosome immediately below the
surface will automatically result in centering: the membrane generates a restriction
on the astral microtubule lengths, and pulling forces from the opposite (internal)
side generate an overall asymmetric force that centers the sperm-aster (Wühr et al.
2010). The aster’s closeness to the cortex induces an asymmetry generating a long
sperm-aster axis that is roughly parallel to the tangent of the cortex at the sperm
entry point. The aster seems to sense this long axis and transfer it to the nascent first
mitotic spindle (see below). This might explain the old observation that the first
cleavage plane typically cuts through the sperm entry point (Roux 1903).
During the first division cycles, Xenopus MTOCs tend to move toward the
animal- most third of the embryo (Wühr et al. 2008). This movement in the animal
direction may also be explained if, as in zebrafish, yolk granules present in more
vegetal regions limit astral microtubule attachments. More yolk could inhibit micro-
tubule growth or result in fewer cytoplasmic structures for dynein to pull on, thus
again providing a force differential that pulls the spindles animally until reaching an
equilibrium point at the observed location. However, the proposed mechanism by
which asters and yolk interact is not understood. Induction of furrows in Xenopus
initiates at the animal region and only gradually moves vegetally (Danilchik et al.
1998), consistent with asymmetric location of the spindles, which can act to induce
a furrow first on the more closely located animal cortex. The first two divisions
occur with spindles aligned along an x–y plane parallel to the equator, alternating 90
°C to generate four cells whose furrows span meridians along the animal–vegetal
axis. This arrangement is similar to the cleavage pattern in zebrafish and, even with
the obvious contrast that cleavage in Xenopus is holoblastic, is easily explained by
the microtubule exclusion model described above. As in zebrafish, maintenance of
the spindles in the x–y plane may rely on a differential force that precludes z-axis
tilting during these cycles. The frog egg is not perfectly spherical but slightly oblate,
and vegetal yolk likely weakens pulling forces on asters in the z-direction, two con-
ditions that would promote spindle orientation along the x–y plane.
Xenopus embryos, also like zebrafish, exhibit a transition of spindle alignment
from the x–y plane to a z-axis orientation (as in zebrafish, the latter also corresponds
to the animal–vegetal axis of the embryo). However, this transition normally occurs
in the third cycle in Xenopus, compared to the sixth cycle in zebrafish. This earlier
transition in Xenopus may reflect a differing balance between microtubule pulling
lengths along the x–y axis (limited by membranes laid out between blastomeres)
A. Hasley et al.
and vegetally located yolk. The result of this z-axis shift in spindle orientation is cell
furrowing along a plane parallel to the equator (above the equator due to the spin-
dles being located closer to the animal pole), generating four smaller animal blasto-
meres and four larger vegetal blastomeres. Subsequent cleavage planes generally
follow a rule for alternating shifts in spindle orientation albeit showing increasing
variation: cleavage in cycle 4 again tends to generate longitudinal furrows, reflect-
ing a realignment of the spindle with the x–y plane, whereas cleavage in cycle 5
tends to generate furrows parallel to the equator, reflecting a second z-axis reorien-
tation. The alternating alignment of the spindle along the x–y plane and the z-axis
may reflect, as in zebrafish, a cell shape-sensing mechanism influenced by changing
dimensions of the blastomeres as they undergo cell division (Strauss et al. 2006;
Wühr et al. 2010).
Thus, and in spite of exhibiting an entirely different type of cleavage pattern
(meroblastic compared to holoblastic), the global pattern of cleavage orientation in
zebrafish and Xenopus embryos can be explained spatially and temporally through
the cleavage stages using the same simple mechanistic model (Wühr et al. 2010).
This model initially relies on microtubule length-dependent forces and the influence
of the furrow from the previous cell cycle, together with additional intracellular
modulation, such as the distribution of yolk. Over time cells acquire a smaller size
and the patterning system may transition to the spindle being able to directly sense
the cortex (Strauss et al. 2006; Wühr et al. 2010; Xiong et al. 2014). Together, these
influences generate a three-dimensional blastula.
The fact that species as phylogenetically distant as teleosts and amphibians
appear to obey a conserved set of cell-biological mechanisms, which generate
manifestly different cleavage patterns from different initial starting conditions,
suggests that a common set of rules may provide the basis for the various cell
arrangements observed in many early vertebrate embryos. During evolution, such
unifying rules may be all that is necessary to accommodate limited changes in start-
ing blastodisc dimensions and/or the influence of modifying factors (i.e., embryo
size, affinity of internal anchors, amount or nature of yolk particles) to generate
cleavage pattern variation. It will be interesting to test this hypothesis through fur-
ther studies in additional species.
4.3.4 Cell Cleavage Orientation in Other Vertebrate and Proto-
vertebrate Systems
Studies in amphibians and teleosts have, until now, contributed the most to our
mechanistic understanding of cleavage plane determination in vertebrate embryos.
However, a full comparative picture will involve patterns in other vertebrate systems
such as Aves, reptiles, and mammals. The chordates include invertebrates such as
tunicates in addition to vertebrates, and mechanisms involved in cell cleavage pat-
tern have been well described in ascidian (sea squirt) tunicate embryos. We sum-
marize our current knowledge in these systems below.
4 Vertebrate Embryonic Cleavage Pattern Determination
138 Meroblastic Cleavage in Aves and Reptiles
Most knowledge concerning the cleavage stages of avian embryos comes from stud-
ies in the domestic chicken (Gallus gallus). However, understanding of cellular and
molecular processes during cleavage stages is limited even in this well-known
developmental model organism (Lee et al. 2013; Sheng 2014; Nagai et al. 2015).
This is because, by the time the egg is laid, the embryo is long past the blastula
stage. Thus, studying cleavage-stage embryos in the chick requires the use of meth-
ods for obtaining eggs still developing in utero (Lee et al. 2013).
The chick embryo undergoes meroblastic cleavage, with blastomeres dividing
atop a large yolky mass (reviewed in Sheng 2014; Fig. 4.7). Fertilization is notable
because, while only a single female and single male pronucleus will give rise to the
zygotic genome, polyspermy is not uncommon (Lee et al. 2013). These supernu-
merary sperm, which can be few or many, are found in both yolk and blastoderm.
Their function, if any, is unclear, but they are capable of producing pseudo-furrows
that do not fully ingress.
Another notable feature of these embryos is that the point at which the first two
cleavage furrows meet is not centered at the middle of the embryo (Sheng 2014;
Nagai et al. 2015). There is a known correlation between asymmetric inheritance of
maternally deposited factors and establishment of PGCs, but whether this is associ-
ated with off-center early cleavages is unclear. It has also been proposed that the
off-center cleavages cause blastomeres to inherit heterogeneously deposited maternal
factors asymmetrically as they cellularize, leading to symmetry breaking and axis
induction. However, the ability to experimentally change the dorsal–ventral axis
after egg laying challenges this early-establishment hypothesis (reviewed in Sheng
The first two cleavages in the chick are stereotypical, with the second forming
perpendicular to the first in the plane of the blastoderm. The third furrow follows
this pattern, though evidence suggests that divisions at this point become asymmet-
ric, resulting in smaller cells in the interior and larger cells at the periphery (Lee
et al. 2013). Cell division becomes asynchronous at the fourth cleavage, and pre-
sumably some asynchronous division continues (Sheng 2014).
It is also roughly around this time that the embryo begins to form two layers of
blastomeres. As cleavage progresses, the embryo will reach 5–6 layers of cells in
thickness before thinning again during and after gastrulation (Sheng 2014). A study
found that approximately 75 % of surface cells (i.e., those in the uppermost layer)
divided in a direction parallel to the blastoderm plane, yielding two daughters in
the same layer, while deeper cells show more variation, with approximately 56 %
dividing in a direction 30–90° from that of the blastoderm plane (Nagai et al. 2015;
see below). These data suggest that cleavage orientation only partly explains
increasing layer number, but other mechanisms, such as rearrangement of already
cellularized blastomeres, may also be involved. The increase in layers also roughly
corresponds to the onset of formation of the subgerminal cavity, a space that sepa-
rates part of the blastoderm from the yolk. This structure may also influence blasto-
derm thickness.
A. Hasley et al.
Study of chick cleavage stages reveals some striking similarities to groups out-
side amniotes. For example, evidence suggests that zygotic gene activation in the
chick occurs around the seventh or eighth cell division (precision is difficult due to
cell cycle asynchrony). This places the midblastula transition somewhere between
cycles 7 and 9, quite similar to the timing observed in zebrafish and Xenopus (Nagai
et al. 2015). These researchers also find evidence of a yolk syncytial layer in the
chick, though it is unclear if it plays a regulatory role similar to the YSL in zebrafish
(see Chap. 7). The division of a blastomere with the cleavage plane parallel to the
blastoderm, resulting in an inner cell and an outer cell that will go on to different
fates, is reminiscent of a similar process in Xenopus (Sheng 2014). This evidence,
combined with overall morphological similarity of cleavage-stage embryos in birds,
reptiles, and teleost fish, suggests deep ancestry or convergent evolution of many
characteristics of cleavage-stage embryonic development in vertebrates (Nagai
et al. 2015).
Reptiles warrant a mention here. They are an important group to study to under-
stand evolution of embryonic cleavage patterning in vertebrates and are known to
undergo meroblastic cleavage. Unfortunately, data on their earliest cleavage stages
Fig. 4.7 Cleavage pattern in the early chick embryo. Early cleavage in chick embryos during the
early to mid-cleavage stages (EGKI–EGKIV) tends to exhibit a perpendicularly alternating pat-
tern, diagrammed in (a) and visualized through scanning electron micrographs in (b). (c) Diagram
of a side view of the animal pole of the embryo, depicting meroblastic cleavage, with cellularized
cells in blue and the number of layers indicated by numbers. Reproduced from Lee et al. (2013) (a)
and Nagai et al. (2015) (b, c), with permission
4 Vertebrate Embryonic Cleavage Pattern Determination
are scarce (Wise et al. 2009; Matsubara et al. 2014). Reasons for this can include
difficulty of establishing breeding colonies in the lab, small clutch sizes, fertiliza-
tion time uncertainty for wild-caught pregnant females, difficulty in culturing
embryos due to extreme temperature and humidity sensitivity, and difficulty isolat-
ing embryos because of embryo adhesion to the egg’s inner surface. However,
recent work has begun to overcome these obstacles (reviewed in Wise et al. 2009;
Matsubara et al. 2014). A method for culturing embryos of the Japanese striped
snake (Elaphe quadrivirgata) (Matsubara et al. 2014) is a major step forward. While
the earliest stage depicted in the study was a gastrula-stage embryo, it appeared
quite similar to a chick embryo at the same stage. The data focused on somitogen-
esis, but this method has promise for examining early cleavage stages in snakes.
Other studies have suggested various lizards, such as the leopard gecko
(Eublepharis macularius), as a model for development in that group (reviewed in
Wise et al. 2009) and presented staging series. So far though, the majority of these
focus on embryos in eggs that have already been laid, which is too late for charac-
terization of cleavage stages. However, easy husbandry of these animals, combined
perhaps with methods similar to those mentioned above for snakes and chicks, has
potential to further the study of cleavage patterning in this phylogenetically impor-
tant group of reptiles. Early Cleavage Divisions in Mammals
Analogous to fish, amphibians, birds, and reptiles, mammalian zygotes initially
undergo a series of cleavage divisions following fertilization to produce an increas-
ing number of progressively smaller cells without changing the overall size of the
embryo. However, the introduction and optimization of in vitro fertilization (IVF)
and embryo culture techniques have revealed several notable differences in how
these early divisions occur between mammals and other vertebrate animals. First,
mammalian species exhibit rotational cleavage, whereby meridional division is
observed along the animal–vegetal axis in the first cleavage, but during the second
cleavage, the daughter cells can divide either meridionally or equatorially by divid-
ing perpendicular to the animal–vegetal axis (Gulyas 1975; Fig. 4.8). As a conse-
quence, each blastomere inherits equivalent cytoplasmic material from both the
animal and vegetal region at the two-cell stage and potentially differentially allo-
cated animal and vegetal portions when the embryo divides from two cells to four
cells (Gardner 2002). The type of second division each daughter cell undergoes
determines which of the four distinct classes (meridional–meridional, meridional–
equatorial, equatorial–meridional, or equatorial–equatorial) a four-cell embryo will
become, and this may impact both cell fate and developmental potential as previ-
ously suggested (Piotrowska-Nitsche and Zernicka-Goetz 2005). More specifically,
it has been shown that four-cell mouse embryos containing at least two blastomeres
with both animal and vegetal material are much more likely to develop to term than
embryos where all blastomeres have either only animal or only vegetal cytoplasmic
inheritance (i.e., the equatorial–equatorial class; Piotrowska-Nitsche et al. 2005).
A. Hasley et al.
Based on their differential distribution between the animal and vegetal poles in the
zygote, several candidates have been proposed to mediate this cleavage-related
asymmetry (reviewed in Ajduk and Zernicka-Goetz 2015). Apart from findings of
asymmetric localization, however, none of these candidate proteins have been
shown to function in the determination of whether a meridional or equatorial divi-
sion occurs at the two-cell stage. Thus, the exact molecular mechanism(s)
Fig. 4.8 Cleavage pattern in the early mammalian embryo. The first division is meridional, along
the animal and vegetal poles, and the second division can be either meridional or equatorial. This
generates four possible cellular arrangements at the four-cell stage with meridional–equatorial and
equatorial–meridional divisions distinguishable by virtue of one of the blastomeres in the two-cell
embryo dividing prior to the other. In embryos that exhibit the equatorial–equatorial pattern, puta-
tive factors localized to the animal and vegetal poles segregate to different daughter cells. See text
for details
4 Vertebrate Embryonic Cleavage Pattern Determination
mediating the developmental fate of each blastomere and distinction between the
four classes of four-cell mammalian embryos remains to be determined (Ajduk and
Zernicka-Goetz 2015).
In addition to unique cell orientation, the time between cleavage divisions in
mammalian embryos is more prolonged, typically between 8 and 24 h apart depend-
ing on the species, compared to that in many nonmammalian vertebrates. For
instance, the time between the first and second mitosis in mice is approximately 20
h and for the majority of other mammals, including humans, 8–12 h (O'Farrell et al.
2004; Wong et al. 2010; Weinerman et al. 2016). Besides longer time intervals, blas-
tomeres in early cleavage-stage mammalian embryos also undergo asynchronous
cell division rather than simultaneously dividing like in other vertebrates. Therefore,
mammalian preimplantation embryos do not increase exponentially in cell number
from the two- to four- or four- to eight-cell stage but can contain an odd number of
blastomeres at certain times in development (Gilbert 2000). Lastly, in contrast to
other vertebrate animals, the mammalian genome becomes activated much earlier,
and many of the mRNA transcripts and protein products produced from embryonic
genome activation (EGA) are required for subsequent cell divisions. Because of this
requirement, the stage of embryo development that coincides with EGA is most
susceptible to cleavage arrest or “block” in several mammalian species (Ko et al.
2000). The mouse embryo exhibits EGA earliest, at the two-cell stage; however,
minor transcription of certain mRNAs also occurs in mouse embryos at the one-cell
stage and is often referred to as zygotic gene activation (ZGA) (Flach et al. 1982;
Ko et al. 2000; Hamatani et al. 2004; Wang et al. 2004; Zeng et al. 2004). Similar to
the mouse, human embryos have also been shown to undergo minor transcriptional
activity of preferential mRNAs prior to the major wave of EGA, and some of the
transcripts include cell cycle regulators (Dobson et al. 2004; Zhang et al. 2009;
Galán et al. 2010; Vassena et al. 2011). Thus, it is likely that preimplantation
embryos from other mammalian species also exhibit “waves” of gene expression
during the transition from maternal to embryonic transcriptional control, which may
impact embryo behavior and cell division timing if these cell cycle-related genes are
aberrantly expressed. Regardless of which wave it occurs under, the production of
embryo-derived transcripts is clearly established by day 3 of human preimplanta-
tion development even in embryos that arrested prior to the eight-cell stage (Dobson
et al. 2004; Zhang et al. 2009; Galán et al. 2010; Vassena et al. 2011), suggesting
that EGA is a function of time rather than cell number per se.
Several studies have shown that male mammalian preimplantation embryos may
actually cleave faster than female embryos cultured in vitro (Xu et al. 1992;
Pergament et al. 1994; Peippo and Bredbacka 1995), although other studies detected
no difference in the sex ratios between early- or late-cleaving human embryos (Ng
et al. 1995; Lundin et al. 2001). This suggests that potential sex-related differences
in the cleavage rate of male versus female human embryos do not occur until later
during post-implantation development or that, alternatively, this phenomenon is
restricted to only certain mammalian species. Nevertheless, early cleavage in gen-
eral is a strong indicator of embryo viability since human embryos that undergo the
first mitotic division sooner appear to have greater potential for successful implanta-
A. Hasley et al.
tion and positive pregnancy outcome (Lundin et al. 2001). More recent findings,
however, suggest that it is the duration of the first cleavage division rather than its
onset, together with the time intervals between the first three mitotic divisions, that
is highly predictive of which human embryos will reach the blastocyst stage (Wong
et al. 2010). Since this initial report, other studies have confirmed the importance of
early cleavage divisions and identified additional cell cycle or morphological
parameters predictive of developmental success (Cruz et al. 2011, 2012; Meseguer
et al. 2011, 2012; Azzarello et al. 2012; Dal Canto et al. 2012; Hashimoto et al.
2012; Hlinka et al. 2012; Rubio et al. 2012; Liu et al. 2014, 2015; Stensen et al.
2015) as well as underlying chromosomal composition (Chavez et al. 2012, 2014;
Campbell et al. 2013; Basile et al. 2014; Yang et al. 2014). Whether the first three
mitotic divisions are similarly predictive of embryo viability and/or chromosomal
status for other mammalian species is still under investigation, but an examination
of early mitotic timing in murine, bovine, and rhesus monkeys has suggested that
this is likely the case (Pribenszky et al. 2010; Sugimura et al. 2012; Burruel et al.
2014). As mentioned above, however, the precise timing between the first cell divi-
sions can vary between different mammalian species (O'Farrell et al. 2004; Wong
et al. 2010; Weinerman et al. 2016), and the underlying cause(s) remains largely
unknown. Besides a later EGA onset in comparison to the mouse, human embryos
have also been shown to express diminished levels of cell cycle checkpoints and
robust expression of cell cycle drivers at the cleavage stage (Harrison et al. 2000;
Los et al. 2004). This can impact not only embryo chromosomal stability, as shown
by the high incidence of whole chromosomal abnormalities (aneuploidy) in
cleavage- stage human embryos (Vanneste et al. 2009; Johnson et al. 2010; Chavez
et al. 2012; Chow et al. 2014), but may also produce preimplantation embryos that
cleave at a faster rate over other mammals. Moreover, time-lapse monitoring of
early embryonic development has demonstrated that human embryos also fre-
quently undergo multipolar divisions, whereby zygotes or blastomeres divide into
three or more daughter cells rather than the typical two. Indeed, it has been esti-
mated that approximately 12 % of human zygotes cultured in vitro are characterized
by multipolar divisions (Chamayou et al. 2013), and this phenomenon could further
explain differences in mitotic timing between mammalian species. While the poten-
tial impact of higher-order divisions at the two-cell stage and beyond may not be as
detrimental and is still being investigated, embryos that exhibit multipolar divisions
at the zygote stage are much less likely to form blastocysts and implant than zygotes
that undergo a bipolar division (Hlinka et al. 2012). Proto-vertebrates
Studies in ascidians have provided important insights into patterns of cell cleavage
in a lineage basal to vertebrates, which may reflect ancestral developmental mecha-
nisms. Cleavage pattern in ascidian species is holoblastic, invariant, and character-
ized by bilateral symmetry (Conklin 1905; Nishida and Satoh 1983; Nishida 1987).
The pattern of cell cleavage orientation in ascidians provides insights into our
4 Vertebrate Embryonic Cleavage Pattern Determination
general understanding of cell cleavage patterning in early embryos. The mechanism
appears to involve symmetric cleavages in alternating orientations, consistent with
shape-sensing spindle orientation mechanisms acting in large embryonic cells. This
underlying cell division pattern is further modified in a subset of cells by a special-
ized structure capable of influencing the orientation and position of the spindle
through multiple cell cycles (at least cycles 3–6), adding asymmetric details to the
blastula, which begin to sculpt the embryo. This modified structure, capable of
influencing spindle placement and cell pattern arrangement, also contains factors
that function in cell shape specification. Thus, basal mechanisms of cell division
interact with a specialized structure to coordinate cell division cues and cell fate
signals to generate the basic body plan (Fig. 4.9).
The first two cleavage planes in ascidian embryos are meridional, passing
through the animal and vegetal poles, with the second cleavage oriented perpen-
dicular to the first to generate four equal-sized blastomeres. The third cleavage
plane is also perpendicular to the first and second cleavage but equatorial, divid-
Fig. 4.9 Early embryonic cleavage pattern in ascidians. Early cleavages in general follow an alter-
nating perpendicular pattern, consistent with a dependence on cell shape. Spindle orientation in the
posterior-vegetal cell is influenced by the attraction of one of the spindle poles to the CAB (blue),
which becomes condensed and attached to the posterior-vegetal cortex in the eight-cell and 16-cell
embryo. This causes the eccentric placement of the spindle at the posterior end of the posterior-
vegetal blastomere during these stages, resulting in consecutive asymmetric cell divisions that
generate a larger blastomere anteriorly and a smaller blastomere posteriorly. Spindle orientation is
also influenced by the condensing CAB during formation of the two-cell embryo, resulting in a
shift of the cleavage plane axis with respect to the polar bodies at the embryo animal pole and
animal–vegetal axis (orange arrow) of the embryo, as well as during formation of the four-cell
embryo, resulting in the observed protrusion of posterior-vegetal blastomeres (B4.1) at this stage.
Spindle centering in anterior- and posterior-animal blastomeres, which do not contain the CAB,
occurs normally resulting in symmetric cell division. Attraction of the spindle pole by the CAB is
dependent on the function of PEM, a maternal product localized to the CAB. Reproduced, with
permission, from Negishi et al. (2007)
A. Hasley et al.
ing each blastomere to generate an eight-cell blastula with four cells in each of
the animal and vegetal halves of the embryo. These first three cleavages are
nearly symmetric, with the exception of an outward tilt of the division axis for
the two posterior blastomeres during the third cell division, resulting in a slight
outward protrusion of the resulting posterior-vegetal blastomeres. The next
three cell cycles continue a cell division pattern that is symmetric in animal and
anterior-vegetal blastomeres but is asymmetric in posterior-vegetal blastomeres.
These posterior-vegetal blastomeres instead divide asymmetrically, each cell
cycle generating a small cell posteriorly and a larger cell anteriorly. The result-
ing cells in the blastula exhibit unique lineages and cell fates (Conklin 1905;
Nishida 1987).
The ascidian cleavage pattern thus exhibits some similarities to those observed in
vertebrate systems, especially vertebrates such as teleosts and amphibians. In par-
ticular, the holoblastic, mutually orthogonal divisions of ascidians during the first
three cycles are aligned in a pattern that is at least superficially identical to that in
the canonical cleavage pattern in Xenopus: two divisions with spindles oriented
along the x–y plane (to generate blastomeres along a single plane) and then a third
along the z-axis (to generate two tiers of blastomeres). Although not yet directly
tested, this symmetric, alternating pattern may result from the same mechanisms
described above that govern spindle orientation in other vertebrates, namely, a com-
bination of asymmetric forces generated by the furrow for the previous cell cycle
and cortex sensing mediated by astral microtubules (see Sect. 4.3.3 and below).
With regard to cell cleavage pattern, two differences stand out in ascidians com-
pared to amphibians. The first difference is the bilateral symmetry of the embryo.
Subcellular mechanisms maintaining bilateral symmetry in cell arrangement have
not yet been studied. It is possible that this feature involves no more than the absence
of cell mixing between the two embryonic halves prior to differentiation, itself pos-
sibly reflecting strong intercellular adhesive forces along the earliest cleavage
planes, with cell division tightly coordinated with cell-autonomous fate
A second unique characteristic of ascidian cleavage pattern is the asymmetric
cell division of posterior-vegetal cells. A posteriorly localized cytoplasmic struc-
ture, termed the centrosome-attracting body (CAB), has been shown to be involved
in both the posterior tilting of the cleavage axis during the third cell cycle, gener-
ating the posterior-vegetal protrusion of the blastula, and the asymmetric cell divi-
sion of the posterior-vegetal cells during the next three cell cycles (Hibino et al.
1998; Nishikata et al. 1999). The CAB is derived from cytoplasm associated with
the posterior- vegetal cortical region that is enriched in cortical ER (cER) and
associated factors. The cER becomes enriched at the vegetal pole through cyto-
plasmic reorganization during the first cell cycle (Roegiers et al. 1995, 1999;
Sardet et al. 2003; Prodon et al. 2005) and subsequently undergoes a posterior
displacement to coalesce by the third cell cycle into a tight mass at the posterior
cortex, constituting the CAB (Hibino et al. 1998; Iseto and Nishida 1999;
Nishikata et al. 1999). Interestingly, observed patterns of cER reorganization are
similar in three evolutionarily distant ascidian species, Halocynthia roretzi, Ciona
4 Vertebrate Embryonic Cleavage Pattern Determination
intestinalis, and Phallusia mammillata, indicating a high degree of conservation
for cytoplasmic mechanisms involved in cell cleavage in ascidian lineages (Sardet
et al. 2003; Prodon et al. 2005). Ablation of the posterior-vegetal cytoplasm from
which the CAB forms results in the absence of spindle tilting and asymmetric cell
division at subsequent cell cycles (Nishida 1994, 1996). The CAB has an elec-
tron-dense appearance at the ultrastructural level (Iseto and Nishida 1999) and is
known to contain a number of localized mRNAs, the so-called type I postplasmic/
PEM RNAs, some of which are known to have cell- determining functions (Nishida
2002; Sardet et al. 2006).
The CAB is also able to associate with the cytoskeleton. Indeed, the posteriorly
located CAB appears to direct the rearrangement of microtubules between the
CAB and the interphase nucleus, forming a bundled array of microtubules which
undergo shortening. This results in the posterior movement of the nucleus, which
is followed by the attachment of one of the centrosomes of the metaphase spindle
to the CAB. The combined process results in a posteriorly displaced spindle appa-
ratus, which is attached through one centrosome to the posterior cortex. This
eccentric placement of the spindle results in the asymmetric cell division that
occurs in these cells. Segregation of the CAB to the posterior-most membrane
insures that the posterior- most blastomere inherits this structure, which continues
to promote the eccentric spindle location in the following cell cycle. The electron-
dense nature of the CAB as well as its ability to localize mRNAs is similar to that
of nuage or germplasm, the specialized cytoplasm that specifies primordial germ
cells (Wylie 2000; see Chap. 8), suggesting that these may be related structures.
Consistent with this interpretation, the smallest, most posterior blastomeres in the
64-cell blastula are fated to become primordial germ cells (Shirae-Kurabayashi
et al. 2006). Although a short microtubule bundle is not observed at the third cell
cycle stage, the posterior tilting of the spindle that occurs during this earlier stage
in posterior-vegetal blastomeres is thought to have the same underlying cause as
spindle eccentric movement, namely, the attraction of one centrosome toward the
CAB (Negishi et al. 2007). The influence of CAB precursor components at the
posterior-vegetal cortex has been proposed to influence spindle orientation as
early as the second cell cycle, resulting in an observed shift of the second division
cleavage plane with respect to polar bodies at the animal pole of the embryo
(Negishi et al. 2007).
One of these CAB-localized mRNAs that code for the novel protein posterior
end mark (PEM) has been shown to be directly involved in CAB-induced microtu-
bule reorganization. In embryos with PEM functional knockdown, the CAB appears
to form normally, but the microtubule bundle linking the centrosome to the posterior
cortex does not form (Negishi et al. 2007). Embryos with PEM functional knock-
down also lack the tilting of the spindle characteristic of the third cell cycle as well
as the cleavage plane shift at the second cell cycle (Negishi et al. 2007). Thus, PEM
mRNA is localized to the CAP, and its protein product has a function essential for
the association of the CAP at the posterior cortex with the spindle centrosome,
involved in both the cell division tilting and the eccentric placement of the spindle
leading to asymmetric cell division.
A. Hasley et al.
4.4 Cell Division Machinery During the Early Cleavage
Cell division in the early embryo is influenced by features characteristic of the egg-
to- embryo transition, such as the shift from meiotic to mitotic cycles, the inheri-
tance of a limited supply of cellular building blocks, and specializations for large
cellular size and unique embryo architecture. We address these topics in this
4.4.1 Maternal Loads and Scaling of Spindle Size
During Early Cell Divisions
The early embryo develops with unique restrictions since, prior to zygotic gene
activation at the midblastula transition (see Chap. 9), all embryonic processes by
necessity are driven solely by maternal products. Thus, while cells at later stages of
the embryo and the adult produce on their own new products essential for cell
growth and division, cells in the early embryo are limited to the supply of cellular
building blocks originally stored in the mature egg. Species have evolved special-
ized systems for the storage in the egg and controlled use of maternal products dur-
ing early embryonic development. In addition to energy and essential building
blocks, the embryo must generate subcellular structures as it becomes multicellular.
It has been shown that the overall protein composition from the fertilized egg to the
midblastula transition only changes minimally (Lee et al. 1984; Peshkin et al. 2015).
Hence, the embryo must generate vital subcellular structures that can fulfill their
tasks under drastically different dimensional scales, while the cell size changes by
orders of magnitude from the fertilized egg to the midblastula transition. In particu-
lar, the cellular machinery must adapt to use a finite initial supply of building blocks
for vital subcellular structures as they are being used by the embryo and in the con-
text of a several-fold change in blastomere size. Interestingly, the limited supply of
histones and replication factors and their utilization by the exponentially increasing
DNA amount have been shown to trigger the onset of the midblastula transition
(Newport and Kirschner 1982a, b; Collart et al. 2013; Amodeo et al. 2015). In this
section, we use the scaling of spindle-associated structures as an example of mater-
nal product inheritance and adaptation to different length scales in the early cleav-
ing vertebrate embryo.
An important advance in the analysis of intracellular processes involved in early
embryonic cell division was the ability to reconstitute asters and bipolar spindles in
Xenopus extracts, first from oocytes to generate structures analogous to meiotic
spindles (Lohka and Maller 1985; Sawin and Mitchison 1991; Mitchison et al.
2013) and later from early embryos to generate sperm-asters and mitotic spindles
(Wühr et al. 2008). In both situations, remarkably normal spindles formed, but, in
spite of the absence of any cell boundaries in this in vitro system, the spindles exhib-
4 Vertebrate Embryonic Cleavage Pattern Determination
ited similar sizes to the in vivo equivalent. Furthermore, an upper limit to spindle
size was also shown to occur in Xenopus embryos: during the first four cell cycles,
the spindle size is relatively constant at an upper limit similar to that observed in
spindles formed in vitro using an early mitotic or meiotic extract (Wühr et al. 2008;
Good et al. 2013; Hazel et al. 2013). The small size of the mitotic spindle compared
to the cell size requires some special adaptation for proper DNA segregation. While
the mitotic spindle still is responsible for the initial separation of sister chromatids,
the majority of the DNA movement into the center of the future daughter cell is
executed by cell-spanning anaphase/telophase asters (see Sect.
Starting at the fifth cell cycle, spindle length begins to scale with blastomere
length, exhibiting an approximately linear relationship. These observations showed
a transition between spindle length control mechanism, with very early and late
blastomeres exhibiting different control mechanisms. The observations that the
upper spindle length limit is observed in in vitro-reconstituted spindles (Wühr et al.
2008) as well as in embryos where cells are too large to be contacted by metaphase
spindle asters (Wühr et al. 2010; see above) suggest the presence of a length-
determining mechanism intrinsic to the spindle. The precise nature of this mecha-
nism remains unknown, but it is thought to depend on a balance between microtubule
nucleation dynamics and the function of microtubule-associated motors, as pro-
posed for the meiotic spindle (Burbank et al. 2007; Cai et al. 2009; Dumont and
Mitchinson 2009; Reber et al. 2013). Subsequently in smaller cells, spindle length
does become reduced coordinately with blastomere cell size.
As stated above, spindle size does scale with blastomere size during the later
blastomere cycles (Wühr et al. 2008). A simple model by which subcellular struc-
tures may scale to the decreasing size of embryonic blastomeres invokes a limiting-
component mechanism, developed through studies in the nematode C. elegans
embryo (Decker et al. 2011). Under this mechanistic model, the size of subcellular
structures, in this case centrosomes, scales according to cell volume due to the
inheritance during cell division of a limiting amount of structure precursor material
that necessarily decrements with each cell division.
The limiting-component mechanism appears to also apply to spindle formation
in Xenopus laevis, as shown by the analysis of spindles forming in cytoplasmic
compartments produced by microfluidic technology (Good et al. 2013; Hazel et al.
2013). In these compartments, spindle size correlates with cell volume. By deform-
ing the compartments to maintain cell volume while changing droplet diameter, the
authors showed that spindle length depends on a volume-sensing mechanism as
opposed to a boundary-sensing mechanism. Direct measurements shows a decrease
in free cytoplasmic tubulin in smaller blastomeres, consistent with spindle scaling
being dependent on the concentration of a limiting spindle factor precursor (Good
et al. 2013). Interestingly, encapsulated cytoplasm from early stages shows an upper
limit in spindle length in vivo as well as in within large droplets (Good et al. 2013;
Hazel et al. 2013). Together, these findings indicate that spindle size is regulated by
two separate yet interacting systems, one dependent on cytoplasmic composition
which imparts an upper limit to spindle length in large cells and a second which
A. Hasley et al.
depends on cytoplasmic volume and a limiting subunit such as free tubulin and
results in spindle scaling in smaller cells.
A limiting-component mechanism may be a simple and widely used property of
animal embryos, including those of vertebrates. Spindle size in early mammalian early
embryos is also consistent with a limiting cytoplasmic factor (Courtois et al. 2012), and
centrosome size in the early zebrafish embryo is significantly larger in early blastomeres
than in later ones (Lindeman and Pelegri 2012). Various components may also interact.
For example, centrosome size is known to influence spindle length (Greenan et al.
2010), and it will be interesting in the future to assess the role of centrosome apportion-
ing to spindle scaling. Mechanisms similar to those proposed to regulate spindle and
centrosome size, dependent on limiting components inherited in the egg, likely act in the
regulated generation of other subcellular organelles in the early embryo.
4.4.2 Specialization of the Cytoskeleton in the Early Embryo
The morphogenetic forces that produce each species’ unique embryonic cleavage
pattern must integrate with several other cellular activities during cell division to
construct the pregastrular embryo. These activities include the establishment of a
specialized basolateral membrane domain in each cleavage plane, the zippering
together of apical–basolateral margins along advancing furrows to produce a tight-
junctional osmotic barrier, and, ultimately, the osmotically driven inflation of inter-
stitial spaces such as the blastocoel. Where the egg begins with only a single outer
(apical) surface, the blastula must develop a functional, polarized epithelium to
physiologically isolate the interstitial space and/or blastocoel from the outside
world. During cleavage, blastomeres become adherent and distinct apical and baso-
lateral membrane domains develop, separated by apical tight junctions (Muller and
Hausen 1995; Merzdorf et al. 1998; Fesenko et al. 2000). This process, referred to
as compaction, occurs at different developmental times in different organisms. For
example, as discussed below (Sect. 4.4.4), in mammalian embryos, compaction is a
distinct phase beginning at about the eight-cell stage or later (Ducibella and
Anderson 1975; Fleming et al. 2000), while in holoblastically cleaving eggs of
amphibians and sturgeon (Zotin 1964; Bluemink 1970; Kalt 1971a, b; Bluemink
and deLaat 1973), it occurs contemporaneously with the earliest cleavages. Basolateral Membrane Formation in Xenopus Cleavage
Amphibian early embryos are distinctive among dividing cells for the compara-
tively large amount of membrane added continuously during the cleavage process
(Bluemink and deLaat 1973). Cleavage furrowing is said to be unipolar because it
begins at one pole of the egg, and the contractile band then assembles and travels as
an arc extending progressively around the egg surface, eventually forming a com-
plete ring which then constricts inward while large amounts of new surface area are
4 Vertebrate Embryonic Cleavage Pattern Determination
added near the base of the advancing furrow. The new plasma membrane therefore
has a composition different from that of the original egg surface (Kalt 1971a, b;
Sanders and Singal 1975; Byers and Armstrong 1986; Aimar 1997). The main
source of the new basolateral membrane appears to be a pool of post-Golgi vesicles
produced during oogenesis which contributes membrane lipids, glycoproteins, and
extracellular matrix components to the growing surface (Kalt 1971a, b; Servetnick
et al. 1990). Roberts et al. (1992) provided direct evidence that Golgi-derived vesi-
cles generated during late oogenesis can fuse specifically with the new membrane. Microtubule-Dependent Exocytosis of Basolateral Membrane
in the Cleavage Plane
The massive localized delivery of new basolateral membrane to the advancing
cleavage furrow is known to be microtubule dependent in both Xenopus and zebraf-
ish. A distinctive array of antiparallel microtubule bundles, referred to as a furrow
microtubule array, develops along the base of advancing furrows in both Xenopus
(Danilchik et al. 1998) and zebrafish (Jesuthasan 1998; Fig. 4.10). Their appearance
and general geometry distinguish furrow microtubule arrays from other
Fig. 4.10 FMA reorganization in Xenopus and zebrafish. (ac) FMA in Xenopus: overview (a)
and magnified views at the base of the furrow (b) and in more vegetal regions of the advancing
furrow (c). (df) FMA in zebrafish furrows: early furrow shows a parallel arrangement of FMA
tubules, oriented perpendicular to the plane of the furrow (d), maturing furrow shows distally
accumulating FMA tubules in a tilted, V-shaped arrangement (e), and complete furrows show
FMA disassembly (f). Scale bar in (d) corresponds to 10 μm for panels (df). Panels (d)–(f) cour-
tesy of Celeste Eno
A. Hasley et al.
microtubule- containing structures in the cleavage plane, such as interzonal spindle
microtubules. The rapid expansion of new membrane sustained by the furrow
microtubule array is sometimes regarded as an embryonic amplification of the mid-
body-dependent localized exocytosis required for abscission of dividing cells of
organisms ranging from yeasts to plants and animals (Straight and Field 2000;
Glotzer 2001). Protrusive Activity During Cleavage Furrow Closure
Various kinds of membrane protrusions develop along cleavage furrow margins
(Danilchik and Brown 2008; Danilchik et al. 2013). Long microvilli are seen associ-
ated with stress folds at the cortex, and lamellipodia and filopodia extend near the
furrow base itself. These protrusions are actively motile and can make relatively
robust contacts across the extracellular space separating the furrow margins.
Abrogating normal actin assembly by microinjection of constitutively active rho
and cdc42 disrupts normal furrow margin protrusive activity and cleavage furrow-
ing, suggesting that the protrusions play a role in normal furrow closure and blasto-
mere adhesion (Danilchik and Brown 2008). Blastocoel Formation
In amphibian embryos, the initiation of blastocoel formation is evident well before
the first contractile ring has finished closing (Kalt 1971a, b). During advance of the
cleavage furrow, a new domain of plasma membrane becomes inserted in the plane
of the plasma membrane on either side of the contractile ring, resulting in two
expanding basolateral surfaces facing each other between separating blastomeres
(Bluemink 1970; Kalt 1971a, b). A similar phenomenon likely occurs in the zebraf-
ish (Feng et al. 2002; Urven et al. 2006; Eno et al. 2016). As discussed above, this
rapid expansion of new membrane depends on localized exocytosis of maternally
derived vesicles immediately behind the advancing contractile ring. The new cleav-
age planes express maternally encoded C- and EP-cadherins which facilitate adhe-
sion between sister blastomeres (Heasman et al. 1994a, b; Kühl and Wedlich 1996)
and integrins. Each successive cleavage event inserts more basolateral surface
between dividing daughter cells; the blastocoel, resting at the intersection of all the
early cleavage planes, thus is entirely lined by basolateral surface and, with the
development of apical–basolateral tight junctions, becomes osmotically isolated
from the outside world. Although definitive extracellular matrix fibers, including
fibronectin, do not develop until mid- to late-blastula stage (Boucaut et al. 1984;
Davidson et al. 2004, 2008), the volume of the blastocoel is nevertheless entirely
filled with extracellular matrix at all stages (Keller 1986, Danilchik, unpublished).
The blastocoel undergoes continuous expansion and change in shape throughout the
cleavage stage, via osmotic uptake of water (Slack and Warner 1973; Han et al.
1991; Uochi et al. 1997), as well as the progressive epibolic thinning of the
4 Vertebrate Embryonic Cleavage Pattern Determination
blastocoel wall (Longo et al. 2004). Other forces involved with blastular morpho-
genesis remain obscure but certainly include localized regulation of cell–cell inter-
actions, as illustrated by the effective obliteration of the blastocoel following
interference with cadherin-dependent adhesion or Eph–ephrin-mediated cell–cell
repulsion (Heasman et al. 1994a, b; Winning et al. 1996).
4.4.3 Transition of Cell Division Factors from Oocytes
to Embryos
Egg activation, typically coincident with fertilization, is regarded as a natural
boundary between egg and zygote. Indeed, egg activation involves many processes
that turn a quiescent cell into an actively dividing one. A dramatic example of this
sharp transition is the cortical calcium wave associated with egg activation and/or
sperm entry, which triggers the exocytosis of cortical granules and remodeling of
surrounding egg membranes to act as a block against polyspermy (see Chap. 1).
Another dramatic example is the initiation of maternal transcript polyadenylation
upon egg activation, which results in the translation of their cognate protein prod-
ucts for use during embryonic development (see Chap. 2).
Besides the abrupt physiological transition brought about by egg activation and
the associated process of fertilization, several other profound alterations in cellular
processes are known to take place between the end of oogenesis and early embry-
onic development. This is exemplified by the transition between two different
mechanisms for spindle formation in mouse embryos (Courtois et al. 2012).
Vertebrates exhibit degeneration of centrioles during oogenesis; centrioles are pro-
vided solely by the sperm, an arrangement that ensures a constant centriole number
through generations and is an obstacle to parthenogenetic development (Symerly
et al. 1995; Delattre and Gönczy 2004). Thus, in vertebrates, spindle formation dur-
ing meiosis utilizes a centriole-independent pathway in which microtubules self-
organize into a pair of microtubule foci which, although wider than those organized
by centriole pairs, can nevertheless direct the formation of a barrel-shaped bipolar
spindle (Heald et al. 1996, 1997; Gaglio et al. 1997; Walczak et al. 1998; Schuh and
Ellenberg 2007). During cleavage stages, embryonic cells typically use bipolar
spindles whose formation relies on centrioles inherited through the sperm. However,
in rodent lineages, not only oocyte centrioles but also sperm centrioles degenerate
(Woolley and Fawcett 1973; Schatten 1994; Manandhar et al. 1998), and the early
rodent embryo has to rely on a centriole-independent mechanism to generate
MTOCs and spindles. Courtois et al. (2012) found that, unlike most organisms, the
mechanism for MTOC formation in the early mouse embryo fails to undergo a sharp
transition at fertilization. Instead, early mouse embryos form MTOCs and spindles
that have morphological properties similar to those in oocytes during meiosis.
During the first eight embryonic cleavages, MTOC and spindle morphology change
gradually from the meiotic pattern to one that is typical of later embryonic stages,
A. Hasley et al.
e.g., a centriole-dependent MTOC nucleation and the presence of conventional cen-
triole markers. These studies further revealed a similar requirement for the centriole-
independent formation of meiotic and early mitotic spindles on microtubule-dependent
motors, such as dynein and kinesin-5 (Schuh and Ellenberg 2007; Courtois et al.
2012), showing not only a continuing reliance on meiotic factors during the early
mitotic divisions but also the deployment of these factors in similar cellular
Another example of gradual transitioning from oocytes to embryos is the finding
in zebrafish of copies of housekeeping genes, which are specialized for maternal
expression and which function during both meiosis and early mitosis. This phenom-
enon has been observed in a maternally expressed form of the protein survivin,
which, as mentioned above, is a component of the CPC complex involved in furrow
induction and maturation. A mutation in the gene motley was found to affect one of
two survivin (a.k.a. birc) genes in the zebrafish genome, birc5b (Nair et al. 2013).
This gene exhibits predominantly maternal expression, whereas the related gene
birc5a is expressed both maternally and throughout zygotic development. Mutations
in motley/birc5b as well as Birc5b protein localization indicate a specialized role for
this gene copy in cytokinesis during both meiotic divisions and early zygotic mitotic
divisions. A similar dual function in meiotic and early embryonic mitotic divisions
is observed in the case of another maternal zebrafish gene, tmi (Nair and Pelegri,
unpublished). These examples highlight the continuation of cellular programs
across the generational boundary occurring at fertilization, possibly because, in the
absence of ongoing transcription, reutilization of programs involved in oocyte for-
mation is an effective way to implement processes subsequently required for early
embryonic development. Further studies will be required to understand the preva-
lence of such cellular program reutilization across the fertilization boundary as well
as the role of gene duplication in the generation of genes acting in such processes.
4.4.4 Mammalian Embryo Compaction
In mammals, the cleavage-stage embryo undergoes several mitotic divisions until
compaction, or intracellular adhesion, occurs to form a morula. Initially described
by Mulnard and Huygens (1978) in mouse embryos and further characterized by
others (Ziomek and Johnson 1980; Batten et al. 1987; Natale and Watson 2002), the
formation of a morula represents the first morphological disruption in embryo radial
symmetry (Fig. 4.11). It is thought that compaction is required for subsequent mor-
phogenetic events such as lineage specification, but how this process is regulated is
generally not well understood (Kidder and McLachlin 1985; Levy et al. 1986). In
the mouse, compaction is mediated by the formation of adherens junctions, or pro-
tein complexes between cells, of which epithelial cadherin (E-cadherin) is a major
component (Vestweber et al. 1987). As a type-1 transmembrane protein, E-cadherin
relies on calcium ions (Ca2+) and its intracellular binding partners, alpha-catenin
(α-catenin) and beta-catenin (β-catenin), to function. However, since E-cadherin
4 Vertebrate Embryonic Cleavage Pattern Determination
and α-/β-catenin are expressed in mouse embryos as early as the zygote stage, it is
unclear how the process of compaction is initiated (Vestweber et al. 1987; Ohsugi
et al. 1996). Of note, compaction still occurs even if transcription is inhibited begin-
ning at the four-cell stage (Kidder and McLachlin 1985) and in fact can be prema-
turely induced by incubating four-cell embryos with protein synthesis inhibitors
(Levy et al. 1986). This suggests that all the components required for embryo com-
paction have been synthesized by the four-cell stage, and, given that premature
compaction is observed in the presence of protein kinase activation as well, it also
indicates that compaction is under the control of posttranslational regulation via
phosphorylation (reviewed in Cockburn and Rossant 2010). Indeed, both E-cadherin
and β-catenin become phosphorylated at the time of compaction (Pauken and Capco
1999), and a recent report demonstrated that E-cadherin-dependent filopodia are
responsible for the cell shape changes necessary for compaction in mouse embryos
(Fierro-González et al. 2013). Using live-cell imaging and laser ablation, this study
determined that filopodia extension is tightly coordinated with blastomere elonga-
tion and that the inhibition of filopodia components, E-cadherin, α-/β-catenin,
F-actin, and myosin-X, prevented cellular elongation and mouse embryo compac-
tion. Whether other mammalian embryos establish and/or maintain cell elongation
by similarly extending filopodia is not known, but an earlier time for the initiation
of compaction correlates with implantation success in human IVF embryos (Landry
et al. 2006; Skiadas et al. 2006).
Along with cell elongation, intracellular polarization also occurs during compac-
tion, whereby the outward-facing, or apical, region of each blastomere becomes
distinct from the inward-facing (basolateral) regions at least in mouse embryos. In
particular, the blastomere nuclei move basolaterally, whereas both actin and micro-
tubules concentrate apically concomitant with differential localization of membrane
and polarity protein complexes (Reeve and Kelly 1983; Johnson and Maro 1984;
Houliston and Maro 1989). Cell–cell contact appears to be essential for establishing
the orientation of polarity, since blastomeres polarize along the axis perpendicular
to cell contact and apical poles form farthest from the contact point. However,
additional mechanisms are also likely involved (Ziomek and Johnson 1980; Johnson
Fig. 4.11 Mammalian embryo compaction and blastulation. The processes of compaction, intra-
cellular adhesion, and polarization result in the formation of a morula at the eight-cell and 16- to
32-cell stage in mouse and human embryos, respectively. In mammals, compaction is mediated by
the development of adherens junctions and results in the development of the first developmental
asymmetry in the embryo. Once compaction is complete, the assembly of tight junctions in the
embryo initiates cavitation and the formation of a fluid-filled cavity called the blastocoel. Most
mammalian species, including humans, form blastocysts between days 5 and 6, whereas mouse
embryos begin blastulation earlier between days 3 and 4 and bovine embryos later between days 7
and 8
A. Hasley et al.
and Ziomek 1981a, b). Once the embryo has compacted and polarized, subsequent
cell divisions are influenced by the orientation of the cleavage plane so that the
established polarity is inherited in the daughter cells (Fig. 4.12). If a blastomere
divides at an angle parallel to its axis of polarity, both daughter cells will be polar
and remain on the outside of the embryo. However, if a blastomere undergoes mito-
sis perpendicular to its axis of polarity, one daughter cell will be polarized and
contribute to the outside of the embryo, whereas the other daughter cell will be api-
cal and become a part of the inside of the embryo (Johnson and Ziomek 1981a, b;
Sutherland et al. 1990). These symmetric versus asymmetric cleavage divisions
eventually result in the generation of two distinct cell populations; the cells on the
inside will become a part of the inner cell mass (ICM), and the cells on the outside
will contribute to the trophectoderm (TE) layer of the blastocyst.
As expected, much of what we know about embryo compaction and blastocyst
formation has been derived from studies in mice, and therefore relatively little is
Fig. 4.12 Cell division in a polarized epithelium results in different patterns depending on spindle
orientation. Spindle orientation perpendicular to the direction of cell apical–basal polarization
(green) results in a cleavage plane that maintains polarization in both daughter cells. Spindle ori-
entation parallel to the direction of cell polarization (red) results in one daughter cell that is polar-
ized and another that is not and which exits the epithelium. Spindle orientation is indicated by a
double arrow
4 Vertebrate Embryonic Cleavage Pattern Determination
known about these processes in other mammalian species, including humans.
Notably, compaction occurs much earlier in mouse embryos, at the eight-cell stage,
than in human embryos, where it begins at the 16-cell stage and, even later, at the
32-cell stage, in bovine embryos (Steptoe et al. 1971; Edwards et al. 1981; Nikas
et al. 1996; Van Soom et al. 1997). Almost immediately following compaction, the
formation of a fluid-filled cavity called the blastocoel is initiated by the assembly of
tight junctions, which includes occludin, cingulin, as well as other components, and
the establishment of high epithelial resistance in TE cells until the 32-cell stage
(Fleming et al. 1993; Sheth et al. 1997, 2000). Once the blastocoel is formed, human
embryos are likely to undergo at least one additional round of cell division to form
a ~256-cell blastocyst, whereas mouse blastocysts typically comprise ~164 cells
(Niakan and Eggan 2013). Until recently, it was unknown how these differences in
the timing of compaction or number of cells may affect polarization and asymmetric
cell divisions in the human embryo. In contrast to the mouse, wherein TE and ICM
fates are established in a positional and cell polarization-dependent manner at the
morula stage as described above, human embryos appear to establish the TE as well
as the epiblast and primitive endoderm lineages concurrently at the blastocyst stage
(Petropoulos et al. 2016). This study also noted that human embryo compaction is
not as prominent as in the mouse, with only partial compaction occurring in a cer-
tain number of blastomeres on embryonic day 4. Consequently, it is not until embry-
onic day 5 and upon blastocyst formation that distinct inner and outer compartments
are observed in human embryos. Whether other mammals undergo compaction via
a cell polarization-dependent mechanism at the morula stage for lineage specifica-
tion or establish the first lineages concomitant with blastocyst formation is unclear,
but partial compaction has been observed in bovine, porcine, and rabbit embryos
(Reima et al. 1993; Koyama et al. 1994). Taken together, this suggests that although
mammalian embryos morphologically resemble each other during preimplantation
development, there are several notable species-specific differences that may limit
extrapolation between mammals.
4.5 Evolutionary Relationship Between Holoblastic
and Meroblastic Cleavage Types
The phylogenetic distribution of holoblastic and meroblastic cleavage indicates that
the latter has evolved independently five times in craniates (a phylogenetic group
containing the vertebrates and hagfish (Myxini)) (Collazo et al. 1994; Collazo 1996;
Fig. 4.13). Several closely related groups outside of craniates, such as ascidians,
tunicates, and echinoderms, exhibit holoblastic cleavage, suggesting that this type
of cleavage is the ancestral mode. Within craniates, meroblastic cleavage appears to
have evolved independently in Myxini (hagfish), Chondrichthyes (sharks, skates,
and rays), Teleostei (largest infraclass of ray-finned fishes), Actinistia (coelacanths),
and Amniota (certain non-eutherian mammals (e.g., egg-laying monotremes), birds,
A. Hasley et al.
and reptiles). Convergent evolution of meroblastic cleavage is further supported by
differences in early development between the various lineages that undergo mero-
blastic cleavage (Collazo 1996). Independent evolution of meroblastic cleavage
appears to reflect a selective advantage. Once arisen within a lineage, meroblastic
cleavage is typically not lost, again consistent with an evolutionary advantage. A
notable exception to this pattern is the inferred reversion of cleavage pattern within
Amniota, from meroblastic to holoblastic as found in eutherian mammals and
In amphibian holoblastic cleavage, like that observed in the model vertebrate
Xenopus, all blastomeres eventually contribute to one of the three germ layers. The
ancestral nature of holoblastic cleavage is largely responsible for the widely held
assumption that amphibian-like cleavage represents the ancestral form of holoblas-
tic cleavage in vertebrates. However, evidence from bichir (Polypterus), a basal acti-
nopterygian (ray-finned fish), and lamprey (Lampetra japonica), a basal vertebrate,
challenges this view (Takeuchi et al. 2009). Vegetal cells in embryos of these organ-
isms do not express mesodermal or endodermal markers as in the case in amphibi-
Fig. 4.13 Independent appearance of meroblastic cleavage in various vertebrate phylogenetic lin-
eages. Phylogenetic tree of lineages from sea urchin to vertebrates, showing that meroblastic cleav-
age (black rectangles) arose multiple times and independently within these lineages. Diagram
reproduced from Collazo et al. (1994), with permission
4 Vertebrate Embryonic Cleavage Pattern Determination
ans. Analyses of such embryos suggest that some vegetal cells do not contribute to
any of the three germ layers and are instead nutritive yolk cells only. This fact,
combined with the evolutionary position of basal fish like bichirs and lampreys,
points to the conclusion that, while holoblastic cleavage is ancestral in vertebrates,
the particular form observed in amphibians, with all blastomeres contributing to
embryonic tissues, is derived. This conclusion is further bolstered by the observa-
tion that a maternally expressed homologue of vegT, which is crucial for early
amphibian endoderm development, appears to only be found in amphibians and not
other vertebrate species, such as mice, bichirs, lampreys, and teleosts (Takeuchi
et al. 2009).
The ancestral trait of having explicitly nutritive yolk cells in vertebrates may
have provided early embryos an evolutionarily advantageous ability to implement
cell division in the absence of yolk granules while nevertheless maintaining embry-
onic nutritive stores, an advantage that may have been maintained in meroblastic
cleaving embryos. Developing embryos are known to rely on exquisitely precise
cellular processes, such as cytoskeletal reorganization and the recycling of mem-
brane particles during cell division, and it is easy to imagine that the presence of
yolk particles may interfere with, or add variability to, this process. Selection
against such interference could be one cause of a transition to a meroblastic cleav-
age system in an animal’s lineage.
The inference of explicitly nutritive yolk cells in ancestral vertebrates may also
make it easier to understand precisely how meroblastic cleavage might evolve from
holoblastic cleavage. For example, fusion of yolky blastomeres into a single nondi-
viding mass is proposed to be the second of two changes that occurred leading to
evolution of the teleost embryo, the first being loss of bottle cells that are still pres-
ent near the beginning of gastrulation in more basal taxa (Collazo et al. 1994). Given
this, groups such as bowfins (Amia) and gars (Lepisosteus) (Ballard 1986a, b; Long
and Ballard 2001), which exhibit cleavage patterns that appear to be partially mero-
blastic (Fig. 4.14a), may be representative of ancestral transitional states along a
continuum from holoblastic to meroblastic cleavage.
An important correlation that has long been observed in the study of cleavage
pattern evolution is that meroblastic cleavage often correlates with large egg size
(Collazo 1996). Egg size correlation can be most clearly seen in amniotes where
large eggs and meroblastic cleavage are predominant, with the small embryos of
eutherian mammals and marsupials having returned to essentially holoblastic cleav-
age. Furthermore, as discussed above, the generally small eggs of amphibians
exhibit a likely derived form of holoblastic cleavage. In such cases, selected traits
such as differing degrees of reliance on egg nutritive stores may underlie the corre-
lation between cleavage type and egg size, although other explanations have been
proposed (Collazo 1996).
Teleosts constitute a major exception to this correlation, as there appears to be a
sharp decrease in egg size in their stem phylogenetic lineage (Collazo 1996).
Conversely, the Puerto Rican tree frog, Eleutherodactylus coqui, has an egg approx-
imately 20 times the size of Xenopus laevis but in spite of this enormous size main-
tains a holoblastic cleavage pattern. How these particular species may escape the
A. Hasley et al.
general rule of egg size and type of cleavage is unknown. However, blastula embryos
for E. coqui exhibit a distinct population of nutritional vegetal cells that are not
destined to become endodermal tissue (or any other embryonic cell type) (Buchholz
et al. 2007; Fig. 4.14b), and, as mentioned above, the presence of such nutritional
cells may represent an important step in the evolutionary transition from holoblastic
to meroblastic cleavage (Buchholz et al. 2007; Elinson 2009).
Thus, it appears that holoblastic cleavage is the ancestral pattern of cleavage in
craniates. Meroblastic cleavage, in which only a portion of the embryo is made up
of dividing blastomeres, has evolved independently at least five times within this
group. This evolution is sometimes, though not always, correlated with increased
egg size. Meroblastic cleavage evolution in teleosts is a significant exception and
has been shown to involve two evolutionary changes, loss of bottle cells and fusion
of yolk into a single mass. The presence of a population of nutritive cells, whether
ancestral or derived in a lineage, may be a key innovation on the way toward mero-
blastic cleavage. These innovations likely conferred a selective advantage, possibly
an increased ability to implement subcellular programs required for early
4.6 Conclusions
In this chapter, we have described mechanisms underlying patterns of cell cleavage
arrangement in early vertebrate embryos. Much of our knowledge on this topic stems
from studies in tractable developmental systems such as amphibians and teleosts.
Fig. 4.14 Intermediates between holoblastic and meroblastic cleavage patterns. (a) Partially
meroblastic cleavage in bowfin fish (A. calva). The yolk-rich vegetal region ceases to cleave after
the formation of 16 cleavage furrows. (b) Formation of nutritional endoderm in the Costa Rican
tree frog (E. coqui). Yolk-rich vegetal cells divide although do not eventually become part of the
embryo proper (Buchholz et al. 2007). Diagrams reproduced from Ballard (1986a, b), in panel (a)
and Buchholz et al. (2007) in panel (b), with permission
4 Vertebrate Embryonic Cleavage Pattern Determination
These studies highlight major challenges that the cleavage-stage embryo, in these
and likely other vertebrate species, has to overcome. These include structures such as
spindle asters that are too small relative to the large early blastomeres, a limited
supply of cellular building blocks within a changing landscape of cell size and orga-
nization, and the requirement for cytoskeletal specializations adapted to very large
cells. We discuss how the vertebrate embryo appears to use simple rules to drive
development even under these limitations, such as the use of interphase astral micro-
tubules from a given cycle to orient the spindle for the following cell cycle or the use
of limiting inherited reagents, such as tubulin, to scale spindles according to cell size
in later-stage embryos. Such simple rules provide elegant solutions to overcome con-
straints associated with the transition from an egg into a three-dimensional embry-
onic blastula. Notably, we examine how a combination of cell shape-sensing cues,
including those from a microtubule exclusion zone at the furrow for the previous cell
cycle to orient the spindle, explain the sequence of blastomeric divisions leading to
the basic cell arrangement in both zebrafish and Xenopus embryos, and possibly
other vertebrates as well, though this remains to be determined. Interestingly,
dynamic changes in the embryonic developmental landscape contribute to develop-
mental decisions as they occur. For example, changes in cell dimensions in teleost
blastomeres likely result in the eventual shift of the spindle plane from a horizontal
(x–y) axis to a vertical (z) axis, generating a two-tiered blastula.
Although not as well studied, similar rules may exist across the range of verte-
brate species, for example, mechanisms for spindle scaling in mammals and mecha-
nisms of cleavage plane positioning in proto-vertebrates such as ascidians,
suggesting that cellular mechanisms involved are highly conserved. This hypothesis
is bolstered by examination of the phylogenetic distribution of major cleavage pat-
terns across vertebrates and their close relatives.
It is possible that a wide variety of cleavage patterns can be explained with the
same simple rules but with different initial parameters or conditions. For example,
the presence of yolk enriched in the vegetal pole in Xenopus likely creates a pulling
force bias on the spindle, resulting in the animal movement of the spindle and even-
tually an asymmetric cell division leading to a smaller animal pole cell and a larger
vegetal pole cell. As another example, a change in shape in the initial blastodisc
may lead to changes in the relative proportions of blastomere allocated to different
dimensions in the resulting blastula. Basic cell shape-sensing mechanisms also
interact with specialized cytoplasmic structures, such as the CAB in ascidians,
resulting in the generation of added cell cleavage pattern variation. The observed
embryonic patterns thus appear to be the outcome of a temporal sequence based on
initial embryonic conditions (starting shape, amount, and type of relevant factors) as
they are modified by ongoing cycles of cell division by the factors inherited by the
egg itself. Future studies will continue to address detailed mechanistic aspects that
drive these early embryonic processes and will also allow us to understand how
changes in various conditions and parameters lead to diversity in blastomere
arrangements encountered in different species. Cell cleavage pattern is a backdrop
on which cell fate decisions are overlain, and it will also be important to better
understand the interconnection between these two types of processes. Future studies
A. Hasley et al.
will surely continue to provide us with a view of the elegant mechanisms embryos
use to solve the unique problem of transforming an egg into a multicellular
Acknowledgments D.H. was supported by NIH grant TG 2 T32 GM007133-40 and NSF grant
1144752-IGERT, as well as the Graduate School and the College of Life Science and Agriculture
at U. Wisconsin, Madison, and thanks Danielle Grotjahn for the help and discussions with related
work. S.C. gratefully acknowledges the National Centers for Translational Research in
Reproduction and Infertility (NCTRI)/NICHD (P50 HD071836), Howard and Georgeanna Jones
Foundation for Reproductive Medicine, Medical Research Foundation of Oregon, and the Collins
Medical Trust for funding. Research in the laboratory of M.D. is supported by the National Science
Foundation (IOS-1557527). M.W. was supported by the Charles A. King Trust Postdoctoral
Fellowship Program, Bank of America, N.A., Co-Trustee. Research in the laboratory of F.P. is
supported by NIH grant RO1 GM065303.
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4 Vertebrate Embryonic Cleavage Pattern Determination
... During early embryonic mitotic divisions of non-mammalian vertebrate organisms, nuclear positioning typically determines spindle positioning, which in turn determines cleavage plane location (Hasley et al, 2017). Positioning the nucleus at the centre of nascent daughter cells prior to nuclear envelope breakdown (NEBD) is therefore critical for ensuring symmetrical division of early embryonic cells (or blastomeres). ...
... Contrary to the above findings, it is well documented in other models that nuclear movement during early embryonic divisions involves microtubule-based pulling mediated by the minus-end directed microtubule motor, dynein (Hasley et al, 2017). To test whether pulling forces might be involved, we treated embryos with the recently characterised small-molecule dynein inhibitor, dynarrestin (Hoing et al, 2018), after first mitotic anaphase was complete but prior to the start of centre-to-cortex movement. ...
... Microtubules have predominantly been found to play procentring roles in embryos (Hasley et al, 2017). In frogs and fish, forces mediated by the minus-end directed microtubule motor, dynein, position nuclei at the cell centre so that equal-sized blastomeres are generated during the ensuing division (Wuhr et al, 2010). ...
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Microtubules typically promote nuclear centring during early embryonic divisions in centrosome-containing vertebrates. In acentrosomal mouse zygotes, microtubules also centre male and female pronuclei prior to the first mitosis, this time in concert with actin. How nuclear centring is brought about in subsequent acentrosomal embryonic divisions has not been studied. Here, using time-lapse imaging in mouse embryos, we find that although nuclei are delivered to the cell centre upon completion of the first mitotic anaphase, the majority do not remain stationary and instead travel all the way to the cortex in a microtubule-dependent manner. High cytoplasmic viscosity in 2-cell embryos is associated with non-diffusive mechanisms involving actin for subsequent nuclear centring when microtubules again exert a negative influence. Thus, following the first mitotic division, pro-centring actin-dependent mechanisms work against microtubule-dependent de-centring forces. Disrupting the equilibrium of this tug-of-war compromises nuclear centring and symmetry of the subsequent division potentially risking embryonic development. This circuitous centring process exposes an embryonic vulnerability imposed by microtubule-dependent de-centring forces.
... In all vertebrates, the early embryonic cell division patterns can be divided into two broad categories: 1) holoblastic, complete cleavage which mostly occurs in amphibians, mammals and chondrosteans, and considered as ancestral for vertebrates; and 2) meroblastic, incomplete cleavage which has evolved five times in each vertebrate lineage, including hagfish, sharks and other cartilaginous fishes, coelacanths, amniotes and teleosts, (Collazo et al., 1994;Elinson, 2009). This transition (holoblastic to meroblastic) is generally associated with an increase in egg size: yolk volume relative to the ancestral condition of the lineage (Collazo et al., 1994;Elinson, 2009;Hasley et al., 2017). ...
... Generally, all amphibian eggs cleave holoblastically: the egg is divided completely by the first few cleavage furrows. Nonetheless, in some species, particularly those with a large yolky mass, the first few cleavage planes fail to pass entirely through the A-V region (Buchholz et al., 2007;Collazo and Keller, 2010;Hasley et al., 2017). For example, many species of different anuran taxonomic groups with a "pseudo-meroblastic" pattern, including E. eschscholtzii, E. coqui and Hyperolius puncticulatus, are particularly appropriate for such comparative studies because they have eggs of varying sizes and yolk volume which are larger than those of X. laevis (Buchholz et al., 2007;Chipman et al., 1999;Collazo and Keller, 2010). ...
Generally, holoblastic cleavage in embryos (as in amphibians) in which all blastomeres contribute to one of the germ layers, are preserved as a stem lineage of vertebrates, and meroblastic cleavage has evolved independently in each vertebrate lineage. The increasing egg size: yolk volume is the key factor for transition from holoblastic to meroblastic cleavage patterns. Sturgeon (Acipenser) eggs are two times larger than those of the African clawed frog Xenopus laevis (amphibian); despite the varying size, sturgeon embryos retain nearly the same developmental characteristics as X. laevis. Comparatively, the fate of blastomeres derived from the vegetal pole (VP) of a sturgeon embryo is unspecified. Thus, the goal of this study was to determine whether the VP of the embryo contributes to embryonic development, or is simply extra-embryonic. This may also reveal whether the transition of the cleavage pattern (holoblastic to meroblastic) in the actinopterygian lineage is correlated with the egg size: yolk volume. Here, we found that sturgeon vegetal blastomeres formed only primordial germ cells, and the rest were made up of cellular yolk (yolk cells; YCs). Morphological and phenotypic characteristics revealed that after the 1 k-cell / mid-blastula transition, YCs became transcriptionally inactive and served only to provide nutrition to larvae as they developed. Furthermore, inhibition of vegetal blastomeres revealed that sturgeon can utilize their yolk in an acellular form, similar to teleosts, implying that meroblastic cleavage in the actinopterygians, like teleosts, might have evolved by the fusion of the vegetal blastomeres.
... S2, Supplementary Material online). These functions are critical to embryo compaction and the structural changes that occur during morula and blastocyst formation (Albertini et al. 1987;Vestweber et al. 1987;Lehtonen et al. 1988;Enders et al. 1990;Damsky et al. 1993;Koyama et al. 1994;Hardy et al. 1996;Fleming et al. 2000;Fleming 2001;Watson and Barcroft 2001;Eckert and Fleming 2008;Hasley et al. 2017;Aberkane et al. 2018;Lim and Plachta 2021); this suggests that ETCHbox genes are involved in coordinating the physical process of structural formation of the blastocyst. We note, however, that, despite GO analyses suggesting that this role may be conserved in humans, a similar response of "blastocyst genes" was not detected in experiments using human cells (Maeso et al. 2016). ...
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Eutherian Totipotent Cell Homeobox (ETCHbox) genes are mammalian-specific PRD-class homeobox genes with conserved expression in the preimplantation embryo but fast-evolving and highly divergent sequences. Here we exploit an ectopic expression approach to examine the role of bovine ETCHbox genes and show that ARGFX and LEUTX homeodomain proteins upregulate genes normally expressed in the blastocyst; the identities of the regulated genes suggest that, in vivo, the ETCHbox genes play a role in coordinating the physical formation of the blastocyst structure. Both genes also downregulate genes expressed earlier during development and genes associated with an undifferentiated cell state, possibly via the JAK/STAT pathway. We find evidence that bovine ARGFX and LEUTX have overlapping functions, in contrast to their antagonistic roles in humans. Finally, we characterise a mutant bovine ARGFX allele which eliminates the homeodomain and show that homozygous mutants are viable. These data support the hypothesis of functional overlap between ETCHbox genes within a species, roles for ETCHbox genes in blastocyst formation and the change of their functions over evolutionary time.
... In humans, paternally inherited centrioles (see MTOC in Glossary, Box 1) are present in the embryo (Avidor-Reiss, 2018;Fishman et al., 2018), but whether they actively participate in microtubule nucleation or organisation during interphase remains a point of contention. Similarly, in Xenopus laevis and Danio rerio (zebrafish) embryos, bridges composed of tubulin at the cleavage furrow initiate the formation of radial microtubules growing outwards into the cell periphery (Hasley et al., 2017;Eno et al., 2018) (Fig. 2B; Fig. 3B). These furrow-associated microtubule arrays are independent of the midzone and spindle microtubules, and are orthologous to the mammalian midbody associated with abscission (Danilchik et al., 1998;Jesuthasan and Stähle, 1997;Otegui et al., 2005). ...
With the advancement of cutting-edge live imaging technologies, microtubule remodelling has evolved as an integral regulator for the establishment of distinct differentiated cells. However, despite their fundamental role in cell structure and function, microtubules have received less attention when unravelling the regulatory circuitry of pluripotency. Here, we summarise the role of microtubule organisation and microtubule-dependent events required for the formation of pluripotent cells in vivo by deciphering the process of early embryogenesis: from fertilisation to blastocyst. Furthermore, we highlight current advances in elucidating the significance of specific microtubule arrays in in vitro culture systems of pluripotent stem cells and how the microtubule cytoskeleton serves as a highway for the precise intracellular movement of organelles. This Review provides an informed understanding of the intrinsic role of subcellular architecture of pluripotent cells and accentuates their regenerative potential in combination with innovative light-inducible microtubule techniques.
... egg centrifugation, bisection or shape modulations [7][8][9][10][11] . Second, maternal-effect genetic screens for cleavage defects have failed to identify general "organizers", and rather support that early embryo development relies on the proper functioning of division machineries, like centrosomes and microtubule asters, or blastomere adhesion and shape regulation 12,13 . And, third, many features of cleavages, including cell cycle progression, cell size regulation and division geometries may spontaneously emerge from egg cytoplasm extracts lacking gene expression or even DNA 14 . ...
Early cellular patterning is a critical step of embryonic development that determines the proper progression of morphogenesis in all metazoans. It relies on a series of rapid reductive divisions occurring simultaneously with the specification of the fate of different subsets of cells. Multiple species developmental strategies emerged in the form of a unique cleavage pattern with stereotyped division geometries. Cleavage geometries have long been associated to the emergence of canonical developmental features such as cell cycle asynchrony, zygotic genome activation and fate specification. Yet, the direct causal role of division positioning on blastomere cell behavior remain partially understood. Oriented and/or asymmetric divisions define blastomere cell sizes, contacts and positions, with potential immediate impact on cellular decisions, lineage specification and morphogenesis. Division positions also instruct daughter cells polarity, mechanics and geometries, thereby influencing subsequent division events, in an emergent interplay that may pattern early embryos independently of firm deterministic genetic programs. We here review the recent literature which helped to delineate mechanisms and functions of division positioning in early embryos.
... In the process of conjunction and the subsequent centering of pronuclei in the C. elegans zygote, the direction of movement of the male pronucleus changes, which is still waiting for an explanation. Simple phenomena, such as the accumulation of yolk at the vegetative pole of an egg, can lead to a change in the action vectors of various forces to the spindle of division and displace the spindle-and, after it, the division plane of the blastomeres [75]. Par1b/MARK kinase plays an important role in spindle positioning in the cell, regulating the interaction of microtubule ends with the cortex and balancing the pulling forces developed by dynein [25,76]. ...
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Centrosomes have a nonrandom localization in the cells: either they occupy the centroid of the zone free of the actomyosin cortex or they are shifted to the edge of the cell, where their presence is justified from a functional point of view, for example, to organize additional microtubules or primary cilia. This review discusses centrosome placement options in cultured and in situ cells. It has been proven that the central arrangement of centrosomes is due mainly to the pulling microtubules forces developed by dynein located on the cell cortex and intracellular vesicles. The pushing forces from dynamic microtubules and actomyosin also contribute, although the molecular mechanisms of their action have not yet been elucidated. Centrosomal displacement is caused by external cues, depending on signaling, and is drawn through the redistribution of dynein, the asymmetrization of microtubules through the capture of their plus ends, and the redistribution of actomyosin, which, in turn, is associated with basal-apical cell polarization.
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In eukaryotic cells, many macromolecules are organized as membraneless biomolecular condensates (or biocondensates). Liquid-liquid and liquid-solid phase transitions are the drivers of the condensation process. The absence of membrane borders makes biocondensates very flexible in their composition and functions, which vary in different cells and tissues. Some biocondensates are specific for germ line cells and are, thus, termed germ granules. This review summarizes the recent data on the composition of germ granules and their functions in gametes. According to these data, germ granules are involved in the determination of germline cells in some animals, such as Amphibia. In other animals, such as Mammalia, germ granules are involved in the processes of transposons inactivation and sequestration of mRNA and proteins to temporarily decrease their activity. The new data on germ granules composition and functions sheds light on germ cell differentiation and maturation properties.
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Cell division orientation is thought to result from a competition between cell geometry and polarity domains controlling the position of the mitotic spindle during mitosis. Depending on the level of cell shape anisotropy or the strength of the polarity domain, one dominates the other and determines the orientation of the spindle. Whether and how such competition is also at work to determine unequal cell division (UCD), producing daughter cells of different size, remains unclear. Here, we show that cell geometry and polarity domains cooperate, rather than compete, in positioning the cleavage plane during UCDs in early ascidian embryos. We found that the UCDs and their orientation at the ascidian third cleavage rely on the spindle tilting in an anisotropic cell shape, and cortical polarity domains exerting different effects on spindle astral microtubules. By systematically varying mitotic cell shape, we could modulate the effect of attractive and repulsive polarity domains and consequently generate predicted daughter cell size asymmetries and position. We therefore propose that the spindle position during UCD is set by the combined activities of cell geometry and polarity domains, where cell geometry modulates the effect of cortical polarity domain(s).
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Triggers and biological processes controlling male or female gonadal differentiation vary in vertebrates, with sex determination (SD) governed by environmental factors or simple to complex genetic mechanisms that evolved repeatedly and independently in various groups. Here, we review sex evolution across major clades of vertebrates with information on SD, sexual development and reproductive modes. We offer an up-to- date review of divergence times, species diversity, genomic resources, genome size, occurrence and nature of polyploids, SD systems, sex chromosomes, SD genes, dosage compensation and sex-biased gene expression. Advances in sequencing technologies now enable us to study the evolution of SD at broader evolutionary scales, and we now hope to pursue a sexomics integrative research initiative across vertebrates. The vertebrate sexome comprises interdisciplinary and integrated information on sexual differentiation, development and reproduction at all biological levels, from genomes, transcriptomes and proteomes, to the organs involved in sexual and sex-specific processes, including gonads, secondary sex organs and those with transcriptional sex-bias. The sexome also includes ontogenetic and behavioural aspects of sexual differentiation, including malfunction and impairment of SD, sexual differentiation and fertility. Starting from data generated by high-through- put approaches, we encourage others to contribute expertise to building understanding of the sexomes of many key vertebrate species. This article is part of the theme issue ‘Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part I)’.
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Cell division orientation is thought to result from a competition between cell geometry and polarity domains controlling the position of the mitotic spindle during mitosis. Depending on the level of cell shape anisotropy or the strength of the polarity domain, one dominates the other and determines the orientation of the spindle. Whether and how such competition is also at work to determine unequal cell division (UCD), producing daughter cells of different size, remains unclear. Here, we show that cell geometry and polarity domains cooperate, rather than compete, in positioning the cleavage plane during UCDs in early ascidian embryos. We found that the UCDs and their orientation at the ascidian third cleavage rely on the spindle tilting in an anisotropic cell shape, and cortical polarity domains exerting different effects on spindle astral microtubules. By systematically varying mitotic cell shape, we could modulate the effect of attractive and repulsive polarity domains and consequently generate predicted daughter cell size asymmetries and position. We therefore propose that the spindle position during UCD is set by the combined activities of cell geometry and polarity domains, where cell geometry modulates the effect of cortical polarity domain(s). Graphical abstract Highlight Spindle tilting in anisotropic cell shape induces unequal cell division Cortical polarity domain can exert attractive or repulsive effect on spindle Cell geometry and polarity domain cooperate to position the spindle Cell geometry modulates the effect of polarity domain
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Mouse studies have been instrumental in forming our current understanding of early cell-lineage decisions; however, similar insights into the early human development are severely limited. Here, we present a comprehensive transcriptional map of human embryo development, including the sequenced transcriptomes of 1,529 individual cells from 88 human preimplantation embryos. These data show that cells undergo an intermediate state of co-expression of lineage-specific genes, followed by a concurrent establishment of the trophectoderm, epiblast, and primitive endoderm lineages, which coincide with blastocyst formation. Female cells of all three lineages achieve dosage compensation of X chromosome RNA levels prior to implantation. However, in contrast to the mouse, XIST is transcribed from both alleles throughout the progression of this expression dampening, and X chromosome genes maintain biallelic expression while dosage compensation proceeds. We envision broad utility of this transcriptional atlas in future studies on human development as well as in stem cell research.
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Embryos from females homozygous for a recessive maternal-effect mutation in the gene aura exhibit defects including reduced cortical integrity, defective cortical granule (CG) release upon egg activation, failure to complete cytokinesis, and abnormal cell wound healing. Subcellular analysis shows that the cytokinesis defects observed in aura mutants are associated with aberrant cytoskeletal reorganization during furrow maturation, including abnormal F-actin enrichment and microtubule reorganization. Cortical F-actin prior to furrow formation fails to exhibit a normal transition into F-actin-rich arcs, and drug inhibition is consistent with aura function promoting F-actin polymerization and/or stabilization. In mutants, components of exocytic and endocytic vesicles, such as Vamp2, Clathrin and Dynamin, are sequestered in unreleased CGs, indicating a need for CG recycling in the normal redistribution of these factors. However, the exocytic targeting factor Rab11 is recruited to the furrow plane normally at the tip of bundling microtubules, suggesting an alternate anchoring mechanism independent of membrane recycling. A positional cloning approach indicates that the mutation in aura is associated with a truncation of Mid1 Interacting Protein 1L (Mid1ip1L), previously identified as an interactor of the X-linked Opitz G/BBB syndrome gene Mid1. A Cas9/CRISPR-induced mutant allele in mid1ip1L fails to complement the originally isolated aura maternal-effect mutation, confirming gene assignment. Mid1ip1L protein localizes to cortical F-actin aggregates, consistent with a direct role in cytoskeletal regulation. Our studies indicate that maternally provided aura/mid1ip1L acts during the reorganization of the cytoskeleton at the egg-to-embryo transition and highlight the importance of cytoskeletal dynamics and membrane recycling during this developmental period.
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Although time-lapse analysis of early embryo cleavage parameters (morphokinetics) predicts blastocyst development, it has not been definitively linked to establishing pregnancy and live birth. For example, a direct comparison of the developmental potential of embryos with optimal kinetic parameters compared to suboptimal kinetics has not been performed with human embryos. To ascertain whether such a linkage exists, we developed a mouse model of morphokinetic analysis of early embryo cleavage using time-lapse microscopy to predict blastocyst formation, and tested whether cleavage parameters predict pregnancy outcome by transferring morphokinetically optimal and suboptimal embryos into a single host. Using classification and regression trees (CART), we established that the timing of the 2nd and 3rd mitotic divisions (division from 2-3 and 3-4 cells, respectively) predicts blastocyst development in the mouse. Using this prediction model, we found that the incidence of sustained implantation at mid-gestation was significantly higher for the optimal compared to suboptimal embryos. In addition, the incidence of resorption among implanted embryos was significantly higher in the suboptimal compared to the optimal group. Transcript profiling of optimal and suboptimal embryos revealed minimal differences between the two groups, suggesting that time-lapse imaging of early embryo cleavage events provides additional information regarding developmental competence apart from gene expression.
Cytokinesis is the division of the cell body that follows the sorting and transport of chromosomes. This book traces the history of some of the major ideas in the field and gives an account of our current knowledge of animal cytokinesis. It contains descriptions of division in different kinds of cells and the proposed explanations of the mechanisms underlying the visible events. Experiments devised to test cell division theories are described and explained. The forces necessary for cytokinesis now appear to originate from the interaction of linear polymers and motor molecules that have roles in force production, motion and shape change that occur in other phases of the biology of the cell. The localization of the force-producing division mechanism to a restricted linear part of the subsurface is caused by the mitotic apparatus, the same cytoskeletal structure that ensures orderly mitosis.
Adherens junction formation is fundamental for compaction and trophectoderm differentiation during mammalian preimplantation development. We recently isolated an IQGAP-2 cDNA from a differential display-polymerase chain reaction screen of bovine preimplantation developmental stages. IQGAP-1 and -2 proteins mediate E-cadherin-based cell-to-cell adhesion through interactions with beta-catenin and the Rho GTPases, rac1 and cdc42. Our study demonstrates IQGAP-1,-2, rac-1 and cdc42 mRNAs are present throughout murine preimplantation development. IQGAP-1 and rac-1 protein distribution changes from predominantly plasma membrane associated to predominantly cytoplasmic as the embryo progresses through cleavage divisions and compaction to the blastocyst stage.
Chronology of three consecutive mitotic events in human pre-implantation embryos was examined by time-lapse imaging. In zygotes producing well-formed and pregnancy-yielding expanded blastocysts, uniform time-patterning of cleavage clusters (c) and interphases (i) was revealed: i2=11±1, i3=15±1, i4=23±1 h / c2=15±5, c3=40±10, c4=55±15 min. Oppositely, shortened or prolonged durations of one or more cell cycles were strongly predictive of poor implantation and development. Furthermore, trichotomic mitosis was discovered in 17 % of cases - zygotes cleaved into 3 blastomeres and 2-cell embryos into 5-6 cells (instead of normal 2 and 4). During conventional clinical assessment, such embryos are indistinguishable from normal, often considered just-in-course of the next cell cycle. Only detailed time-lapse monitoring paced at 10-minute intervals had proven all these embryos to be absolutely unviable, even in rare cases when they reduced their hypercellularity to normal cell counts via cell-cell fusion. Overall, we demonstrate that time-lapse embryo cleavage rating (ECR) as a standalone diagnostic procedure allows for effective identification of viable early embryos with 90 % specificity, while elimination of good-looking but unviable embryos can be assumed with a specificity of 100 %. Thus, making this non-invasive and contactless approach worth of addition to routine embryo screening in clinical IVF programs.
Growth rates of human preimplantation embryos fertilized in vitro were assessed and compared to the sex of the pregnancy outcome. The likelihood of a liveborn male was significantly greater than that of a female if the mean number of cells/embryo was four or greater at the scheduled time of transfer (odds ratio of 6: 1). This finding suggested that the Y chromosome expresses factors which influence embryonic growth rates immediately after fertilization.
Cleavage furrow in animal cell cytokinesis is formed by cortical constriction driven by contraction of an actomyosin network activated by Rho GTPase. Although the role of the mitotic apparatus in furrow induction has been well established, there remain discussions about the detailed molecular mechanisms of the cleavage signaling. While experiments in large echinoderm embryos highlighted the role of astral microtubules, data in smaller cells indicate the role of central spindle. Centralspindlin is a constitutive heterotetramer of MKLP1 kinesin and the non-motor CYK4 subunit and plays crucial roles in formation of the central spindle and recruitment of the downstream cytokinesis factors including ECT2, the major activator of Rho during cytokinesis, to the site of division. Recent reports have revealed a role of this centralspindlin-ECT2 pathway in furrow induction both by the central spindle and by the astral microtubules. Here, a unified view of the stimulation of cortical contractility by this pathway is discussed.