In metazoans, tissue maintenance and regeneration depend on
adult stem cells, which are characterized by their ability to self-
renew and generate differentiating progeny in response to the
needs of the tissues in which they reside. In the Drosophila
testis, germline and somatic stem cells are housed together in a
common niche, where they are regulated by local signals,
epigenetic mechanisms and systemic factors. These stem cell
populations in the Drosophila testis have the unique advantage
of being easy to identify and manipulate, and hence much
progress has been made in understanding how this niche
operates. Here, we summarize recent work on stem cells in the
adult Drosophila testis and discuss the remarkable ability of
these stem cells to respond to change within the niche.
Key words: Drosophila, JAK-STAT, Niche, Stem cell, Testis
Stem cells are undifferentiated cells with remarkable potential.
When a stem cell divides, each daughter cell can either remain a
stem cell (a process called self-renewal) or differentiate into a more
specialized type of cell. Stem cells reside in specific
microenvironments, called niches, which provide the molecular
signals that maintain stem cells and regulate their division. In adult
metazoans, the precise regulation of stem cells and their daughters
is crucial for tissue maintenance and repair. Among the best-
understood adult stem cell niches are those in the Drosophila testis
and ovary, which house the germline stem cells (GSCs) that give
rise to sperm or eggs. These anatomically simple niches contain
stem cells that are easier to identify, image and manipulate than
those in complex mammalian niches; therefore, they have become
two of the best models for studying the biology of adult stem cells
In this review, we focus on the stem cell niche of the adult
Drosophila testis. This is not intended to be a comprehensive review
but rather a sampling of recent findings, especially those that have
shed light on previously unexplored topics or that have challenged
our way of thinking about established topics. Comprehensive reviews
of the Drosophila testis include those by Fuller (Fuller, 1993) and
Davies and Fuller (Davies and Fuller, 2008). Recent reviews that
focus on specific topics relevant to Drosophila testis stem cells
include those on adhesion (Marthiens et al., 2010), asymmetric
division (Yamashita et al., 2010), aging (Wang and Jones, 2010) and
systemic regulation (Drummond-Barbosa, 2008; Jasper and Jones,
2010). For a comprehensive review of the Drosophila ovary stem cell
niche, see Xie et al. (Xie et al., 2008); a recent review by Fuller and
Spradling (Fuller and Spradling, 2007) compares and contrasts the
testis and ovary stem cell niches.
An overview of the Drosophila testis
Adult male Drosophila contain a pair of testes; each is a long
blind-ended tube that is coiled around a seminal vesicle. The
stem cell niche is located at the blind apical end of the testis.
Here, GSCs divide asymmetrically to generate one cell that
remains a stem cell and another, a gonialblast, that is displaced
away from the niche and differentiates (Fig. 1). Each gonialblast
is enveloped by two somatic cyst cells, which arise from cyst
stem cells (CySCs) that also divide asymmetrically to self-renew
and produce differentiating cyst cell daughters. A gonialblast
progresses through four rounds of transit-amplifying divisions to
produce a cluster of 16 spermatogonial cells; cytokinesis is
incomplete in each division and the 16 cells remain connected
by stable intercellular bridges called ring canals. These 16
spermatogonial cells progress through premeiotic S phase and
then switch to a spermatocyte program of growth and gene
expression; most of the gene products that are needed for the
development of spermatocytes and spermatids are transcribed at
this time (White-Cooper, 2010). GSCs, gonialblasts and
spermatogonia are almost identical morphologically, but
spermatocytes and spermatids undergo dramatic changes in both
size and shape. The two cyst cells that envelop the gonialblast
do not divide, but they continue to grow and encase the
gonialblast and its progeny throughout spermatogenesis. At the
end of spermatogenesis,
interconnections and become surrounded by individual plasma
membranes. Mature sperm are then released from the open end
of the testis into the seminal vesicle, where they are stored until
needed. Thus, the testis contains a gradient of developmental
stages, from stem cells in the niche at the apical end to mature
sperm at the basal end.
the spermatids lose their
Morphology and development of the testis niche
Many stem cells, including those of the Drosophila testis, reside in
stromal niches: the stem cells are anchored to specific stromal cells
that regulate their division and differentiation (Spradling et al.,
2008). At the apical tip of the testis, adjacent to the basement
membrane, is a group of ~10-15 non-dividing stromal cells called
the hub (Hardy et al., 1979) (Fig. 1). These hub cells are small and
closely packed and they are arranged in a distinctive dome-shaped
structure that protrudes into the testis. Surrounding the hub are
GSCs; the number of GSCs can vary widely from one strain to
another, but typically there are 6-9 GSCs per testis. GSCs are
shaped like spheres but are flattened where they make broad
contact with the hub. Each GSC is flanked by two CySCs; the
number of CySCs per testis is therefore about twice the number of
GSCs. CySCs also contact the hub, but their nuclei are located
farther from the hub than those of the GSCs and they make small
regions of contact with the hub via thin cytoplasmic extensions
(Hardy et al., 1979). The CySCs and cyst cells completely encase
their associated germ cells and isolate them from one another; thus,
the only germ cells that contact each other are those that are
connected by ring canals.
Development 138, 2861-2869 (2011) doi:10.1242/dev.056242
© 2011. Published by The Company of Biologists Ltd
The stem cell niche: lessons from the Drosophila testis
Margaret de Cuevas* and Erika L. Matunis*
Department of Cell Biology, Johns Hopkins School of Medicine, 725 N. Wolfe Street,
Baltimore, MD 21205, USA.
*Authors for correspondence (firstname.lastname@example.org; email@example.com)
The male gonad forms in mid-embryogenesis, when germ cells
and somatic gonadal precursor cells (SGPs) coalesce to form a
spherical gonad, and by the end of embryogenesis both hub cells
and GSCs can be distinguished (Le Bras and Van Doren, 2006;
Sheng et al., 2009b). Because the hub is not visible earlier, hub cell
specification was thought to occur late in embryogenesis. However,
recent work suggests that hub cells are specified much earlier in
development, prior to gonad coalescence. Cells in the posterior
midgut produce the ligand Delta, which activates the Notch
signaling pathway in a subset of SGPs to specify hub cell fate
(Okegbe and DiNardo, 2011). Epidermal growth factor receptor,
which represses hub cell formation, is activated in posterior SGPs
and restricts hub cell formation to the anterior of the gonad
(Kitadate and Kobayashi, 2010). Hub cell specification also
requires the gene bowl, which encodes a transcription factor
(DiNardo et al., 2011). CySCs are also formed from SGPs, and the
lines gene, which encodes an antagonist of Bowl, is required to
prevent CySCs from expressing markers of hub cell fate (Hatini et
al., 2005; DiNardo et al., 2011). Taken together, these studies
suggest that CySCs and hub cells are derived from a common pool
of precursor cells in the embryo, and that signaling through
multiple pathways is required to specify the appropriate number of
each cell type. The ability to follow its development at this level of
detail makes the Drosophila male gonad one of the best models for
understanding the process of niche formation.
Cellular mechanisms that regulate the Drosophila
Stem cell niches provide the local signals that maintain stem cell
fate. When stem cells divide, daughters that remain in the niche
continue to receive these signals and self-renew, whereas daughters
that are displaced from the niche no longer receive these signals
and differentiate. Therefore, to maintain tissue homeostasis, the
cells that comprise the niche must be maintained, and the number
of stem cell daughters that remain in the niche, as well as the
number of those that differentiate, must be regulated. In the
Drosophila testis niche, both GSCs and CySCs adhere to the hub,
and their divisions are precisely oriented to balance self-renewal
Asymmetrically oriented GSC divisions rely on cell polarity
Testis GSCs, which are all mitotically active (Wallenfang et al.,
2006), normally divide asymmetrically: one daughter cell stays in
contact with the hub and retains the stem cell fate, whereas the
other is displaced away from the niche and differentiates. This
pattern of division results from the stereotypical orientation of
centrosomes and spindles in GSCs (Hardy et al., 1979; Yamashita
et al., 2003). During early interphase, GSCs contain a single
centrosome located at the proximal end of the cell, next to the hub-
GSC interface. Later in interphase, when the duplicated
centrosomes separate, one centrosome remains anchored at the hub
while the other moves to the distal end of the cell. Differential
labeling of mother and daughter centrosomes in living testes has
shown that the centrosome retained at the hub is the mother,
whereas the daughter centrosome moves away (Yamashita et al.,
2007). Both centrosomes keep their positions for the rest of the cell
cycle; thus, in mitosis, the spindle is oriented perpendicular to the
hub-GSC interface and the mother centrosome is retained in the
daughter cell that remains at the hub. CySCs also divide
asymmetrically but use a mechanism that is strikingly different
from that used by GSCs (Cheng et al., 2011). The mitotic spindle
in CySCs forms in a random orientation but then repositions in
anaphase, when one spindle pole moves to the hub-CySC interface.
Thus, as with GSCs, one daughter CySC remains attached to the
hub while the other is displaced.
The mechanism controlling centrosome orientation in GSCs is
intracellular and depends on polarity cues from the hub-GSC
interface. Ultrastructural analysis of wild-type GSCs has shown
that mother centrosomes are located near adherens junctions at the
hub-GSC interface and are associated with a robust array of
microtubules (Yamashita et al., 2007). Centrosomin, a centrosomal
protein that tethers centrosomes to astral microtubules, and
Adenomatous polyposis coli 2 (Apc2), which is thought to link
astral microtubules to adherens junctions, are both required to
anchor the mother centrosome to the hub-GSC interface; in GSCs
lacking either protein, centrosomes are often misoriented, with
neither located next to the hub (Yamashita et al., 2003; Inaba et al.,
2010). Therefore, in wild-type GSCs it is likely that mother
centrosomes are anchored by astral microtubules to adherens
junctions at the hub-GSC interface. By contrast, new daughter
centrosomes associate with very few microtubules, which might
explain how they are able to move away from the hub. The polarity
cue that positions the mother centrosome at the hub-GSC interface
is likely to be the adhesion protein E-cadherin (Shotgun –
FlyBase). E-cadherin is located exclusively at the hub-GSC
interface, as is Apc2 (Yamashita et al., 2003; Inaba et al., 2010).
However, when E-cadherin is expressed ectopically throughout the
GSC cortex, Apc2 is also dispersed, and this dispersal of Apc2
results in a high frequency of misoriented centrosomes (Inaba et
al., 2010). E-cadherin is therefore an important polarity cue for
orienting centrosomes in GSCs.
In wild-type testes, GSCs with misoriented centrosomes are
found occasionally, but misoriented spindles are almost never seen.
What happens to GSCs with misoriented centrosomes? Time-lapse
imaging of cultured live testes suggests that GSCs have a
checkpoint mechanism for sensing and restoring centrosome
orientation: GSCs with misoriented centrosomes do not divide, but
instead pause until the correct orientation is restored and then
continue dividing (Cheng et al., 2008). Thus, GSCs have robust
mechanisms for ensuring that spindles are always oriented
perpendicular to the hub, resulting in an asymmetric division.
Surprisingly, GSCs are maintained and divide with correctly
Development 138 (14)
Fig. 1. The Drosophila testis stem cell niche. Stromal hub cells
(green) adhere to the apical tip of the testis. Surrounding the hub are
germline stem cells (GSCs, yellow) and somatic cyst stem cells (CySCs,
blue), which share the niche. GSCs and CySCs divide and produce
daughter cells that remain in the niche (self-renewal) or leave the niche
and differentiate. GSCs give rise to spermatogonia (light yellow), which
ultimately develop into sperm; CySCs give rise to cyst cells (light blue),
which encase the developing spermatogonia.
Development 138 (14)
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