Does cell lineage in the developing cerebral cortex contribute to its columnar organization?
ABSTRACT Since the pioneer work of Lorente de Nó, Ramón y Cajal, Brodmann, Mountcastle, Hubel and Wiesel and others, the cerebral cortex has been seen as a jigsaw of anatomic and functional modules involved in the processing of different sets of information. In fact, a columnar distribution of neurons displaying similar functional properties throughout the cerebral cortex has been observed by many researchers. Although it has been suggested that much of the anatomical substrate for such organization would be already specified at early developmental stages, before activity-dependent mechanisms could take place, it is still unclear whether gene expression in the ventricular zone (VZ) could play a role in the development of discrete functional units, such as minicolumns or columns. Cell lineage experiments using replication-incompetent retroviral vectors have shown that the progeny of a single neuroepithelial/radial glial cell in the dorsal telencephalon is organized into discrete radial clusters of sibling excitatory neurons, which have a higher propensity for developing chemical synapses with each other rather than with neighboring non-siblings. Here, we will discuss the possibility that the cell lineage of single neuroepithelial/radial glia cells could contribute for the columnar organization of the neocortex by generating radial columns of sibling, interconnected neurons. Borrowing some concepts from the studies on cell-cell recognition and transcription factor networks, we will also touch upon the potential molecular mechanisms involved in the establishment of sibling-neuron circuits.
- SourceAvailable from: med.nyu.edu[show abstract] [hide abstract]
ABSTRACT: The contribution of early cell lineage to regional fate in the mammalian forebrain remains poorly understood. Previous lineage-tracing studies using retroviral methods were only begun at mid-neurogenesis and have suffered from region-specific retroviral silencing. We have been able to study cell lineage in the telencephalon from the onset of neurogenesis by using ultrasound backscatter microscopy to label the forebrain neuroepithelium and a modified retroviral lineage library to overcome regional silencing. Our studies suggest that by embryonic day 9.5, forebrain clones are primarily restricted to territories within anatomically demarcated regional boundaries, such as the cortex, striatum and hypothalamus. In addition, we observed a subset of clones that appeared to be composed entirely of glia. These observations suggest that both regional and cell-type restrictions exist within progenitor populations before the first forebrain cells become postmitotic.Journal of Neuroscience 10/2001; 21(17):6772-81. · 6.91 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: The massive migration of neuroblasts and young neurons through the anterior extension of the postnatal subventricular zone (SVZ), known as the rostral migratory stream (RMS) is still poorly understood on its molecular basis. In this work, we investigated the involvement of gap junctional communication (GJC) in the robust centrifugal migration from SVZ/RMS explants obtained from early postnatal (P4) rats. Cells were dye-coupled in homocellular and heterocellular pairings and expressed at least two connexins, Cx 43 and 45. Treatment with the uncoupler agent carbenoxolone (CBX, 10-100 microM) reversibly reduced outgrowth from SVZ explants, while its inactive analog, glycyrhizinic acid (GZA), had no effect. Consistent with a direct effect on cell migration, time-lapse video microscopy show that different pharmacological uncouplers cause an abrupt and reversible arrest of cell movement in explants. Our results indicate that GJC is positively involved in the migration of neuroblasts within the SVZ/RMS.Developmental Neurobiology 07/2009; 69(11):715-30. · 4.42 Impact Factor
- [show abstract] [hide abstract]
ABSTRACT: Pathfinding by growing axons in the developing nervous system is guided by diffusible or bound factors that attract or repel the axonal growth cone. The cytoplasmic signalling mechanisms that trigger the responses of the growth cone to guidance factors are mostly unknown. Previous studies have shown that the level and temporal patterns of cytoplasmic Ca2+ can regulate the rate of growth-cone extension in vitro and in vivo. Here we report that Ca2+ also mediates the turning behaviour of the growth cones of cultured Xenopus neurons that are induced by an extracellular gradient of netrin-1, an established diffusible guidance factor in vivo. The netrin-1-induced turning response depends on Ca2+ influx through plasma membrane Ca2+ channels, as well as Ca2+-induced Ca2+ release from cytoplasmic stores. Reduction of Ca2+ signals by blocking either of these two Ca2+ sources converted the netrin-1-induced response from attraction to repulsion. Activation of Ca2+-induced Ca2+ release from internal stores with a gradient of ryanodine in the absence of netrin-1 was sufficient to trigger either attractive or repulsive responses, depending on the ryanodine concentration used. These results support the model that cytoplasmic Ca2+ signals mediate growth-cone guidance by netrin-1, and different patterns of Ca2+ elevation trigger attractive and repulsive turning responses.Nature 02/2000; 403(6765):93-8. · 38.60 Impact Factor
Frontiers in Neuroanatomy www.frontiersin.org June 2010 | Volume 4 | Article 26 | 1
published: 28 June 2010
Within a given cortical column, discrete clusters of neurons
project to a limited number of sites and tend to link columns of
common functional properties (Mountcastle, 1997). The radial dis-
persion of these clusters is about 400 μm, similar to the spread of the
dendritic arbor. Therefore, the cerebral cortex can be seen as a jigsaw
of local neuronal microcircuits, which are interconnected by small
subsets of neurons. Approaching the developmental building blocks
of these microcircuits is an important step towards understanding
the emergence of functional properties of cortical columns.
Is the development of cortIcal columns Influenced
by molecular cues IntrInsIc to the developIng
It has long been though that both development and plasticity of
cortical columns rely exclusively on activity-dependent mecha-
nisms. Indeed, the development of ocular dominance columns
is highly dependent on visual experience, as clearly evidenced by
the physiological and anatomical shifts caused by monocular eye
closure during the critical period (Wiesel and Hubel, 1963, 1965;
LeVay et al., 1978, 1980). However, there has also been accumulat-
ing evidence indicating that the initial establishment of cortical
columns may take place before the critical period. For instance,
it has been shown that the basic structure of segregated lateral
geniculate nucleus (LGN) afferents in the primary visual cortex
(V1) of macaque monkeys is formed before birth (Rakic, 1976).
anatomIc and functIonal organIzatIon of the
mammalIan cerebral cortex
The cerebral cortex consists of distinct cytoarchitectonic areas, each
serving a function ranging from sensory perception and motor
control to symbolic thinking and language in humans. In fact, the
anatomical observation of discontinuous architectural features of
cerebral cortex uncovered very early by Ramón y Cajal and Lorente
de Nó was followed by the realization that a functional architecture
was also present probably as an emerging property of the underly-
ing anatomical architecture. A striking feature of the neocortex, first
unraveled in the work by Mountcastle (1957) describing the corti-
cal representation of somatosensory perception, is its organization
into functional columns.
The columnar organization of the cerebral cortex is a broadly
documented principle of design preserved throughout mamma-
lian evolution (Mountcastle, 1997), which has been proposed to
be important to allow a large number of neurons to be connected
without a significant increase in cortical volume. Indeed, it has
been estimated that fusing 100 cortical columns would lead to a
10-fold increase in cortical volume (Mitchison, 1992). The explana-
tion for such increase comes from the fact that neurons are locally
connected within cortical columns and only restricted subsets of
neurons are involved in long distance connections. Consequently,
the length of axons that interconnect neurons is shortened, reduc-
ing also the cortical volume.
Does cell lineage in the developing cerebral cortex contribute
to its columnar organization?
Marcos R. Costa1,2* and Cecilia Hedin-Pereira3,4
1 Edmond and Lily Safra International Institute of Neuroscience of Natal, Natal, Rio Grande do Norte, Brazil
2 Universidade Federal do Rio Grande do Norte, Natal, Rio Grande do Norte, Brazil
3 Instituto de Biofísica Carlos Chagas Filho, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
4 Laboratório de Neuroanatomia Celular, Instituto de Ciências Biomédicas, Universidade Federal do Rio de Janeiro, Rio de Janeiro, Brazil
Since the pioneer work of Lorente de Nó, Ramón y Cajal, Brodmann, Mountcastle, Hubel and
Wiesel and others, the cerebral cortex has been seen as a jigsaw of anatomic and functional
modules involved in the processing of different sets of information. In fact, a columnar distribution
of neurons displaying similar functional properties throughout the cerebral cortex has been
observed by many researchers. Although it has been suggested that much of the anatomical
substrate for such organization would be already specified at early developmental stages, before
activity-dependent mechanisms could take place, it is still unclear whether gene expression in
the ventricular zone (VZ) could play a role in the development of discrete functional units, such
as minicolumns or columns. Cell lineage experiments using replication-incompetent retroviral
vectors have shown that the progeny of a single neuroepithelial/radial glial cell in the dorsal
telencephalon is organized into discrete radial clusters of sibling excitatory neurons, which have a
higher propensity for developing chemical synapses with each other rather than with neighboring
non-siblings. Here, we will discuss the possibility that the cell lineage of single neuroepithelial/
radial glia cells could contribute for the columnar organization of the neocortex by generating
radial columns of sibling, interconnected neurons. Borrowing some concepts from the studies
on cell–cell recognition and transcription factor networks, we will also touch upon the potential
molecular mechanisms involved in the establishment of sibling-neuron circuits.
Keywords: cortical columns, sister neurons, cell lineage, transcription factors
Javier DeFelipe, Cajal Institute, Spain
Monique Esclapez, Institut National de
la Santé et de la Recherche Médicale,
George W. Huntley, Mt Sinai School of
Marcos R. Costa, Edmond and Lily
Safra International Institute of
Neuroscience of Natal, Rua Prof.
Francisco Luciano de Oliveira, 2460
Natal, Rio Grande do Norte 59066-060,
Frontiers in Neuroanatomy www.frontiersin.org June 2010 | Volume 4 | Article 26 | 2
Costa and Hedin-Pereira Neuronal clones as functional units
cortical VZ grow an axon directed towards the white matter and
later an apical dendrite that ramifies in layer I. The sterotypical
polarity of pyramidal neurons, an important feature of columnar
organization appears to partially derive from the phosphorylation
of neurogenin 2 (NGN2), a proneural gene, which has a double
function – as an important pyramidal neuron phenotype determi-
nant and as a cytoskeleton organizer for the emergence of dendrites
in the apical portion of these cells (Schuurmans et al., 2004; Hand
et al., 2005). These functions appear to be independent. Molecules
that guide migration and axon growth such as semaphorins may
also be important to orient axon-dendrite polarization (Polleux
et al., 2000) influencing each of these components in a differential
manner (repulsing axons and attracting dendrites).
Recently, it has also been shown that the radial distribution of
clonally related neurons in the developing cerebral cortex depends
on the expression levels of Ephrin A receptors (EphA) and ephrin-As
(Torii et al., 2009). By using loss- and gain-of-function strategies
to manipulate ephrin-As and EphA7 expression, Torii et al. (2009)
have provided compelling evidence indicating that EphAs and
ephrin-As signaling controls lateral dispersion of cortical excitatory
neurons, likely contributing to the generation of cortical columns.
Interestingly, ephrins and Eph receptors have also been involved
in the establishment of other maps in the cerebral cortex, such as
retinotopy (Flanagan and Vanderhaeghen, 1998).
clones of excItatory neurons dIsperse Into
IndIvIdual cortIcal columns
The lateral dispersion of clonally related neurons could also con-
tribute to the early specification of cortical columns by generating
spatially restricted radial clusters of neurons, which would later
receive afferent connections and become involved in specific tasks.
According to this scenario, we could expect that neurons generated
from the same progenitor would disperse into unique functional
cortical columns. To the best of our knowledge, this possibility has
not been directly tested. However, there is accumulating evidence
indicating that sibling neurons keep spatial relationships and dis-
play connection preferences at least compatible with their tentative
role in the organization of cortical columns.
In order to investigate the degree of neuronal dispersion during
development, a technique has been used which allows the infection
of few progenitor cells at early developmental stages using a replica-
tion incompetent retroviral vector carrying a reporter gene (Luskin
et al., 1988, 1993; Price and Thurlow, 1988; Walsh and Cepko, 1988,
1992, 1993; Parnavelas et al., 1991; Luskin and McDermott, 1994;
Mione et al., 1994, 1997; Kornack and Rakic, 1995; Reid et al., 1995;
Gaiano et al., 1999; McCarthy et al., 2001; Reid and Walsh, 2002;
Costa et al., 2009). Later, the progeny of those few infected progeni-
tors can be identified by the expression of the reporter gene, allow-
ing measuring the dispersion of neuronal siblings. These studies
have been termed “cell lineage” or “clonal analysis” studies and we
may use both terms interchangeably.
The first cell lineage studies have found controversial results
regarding the radial distribution of sibling neurons (Luskin et al.,
1988; Price and Thurlow, 1988; Walsh and Cepko, 1988). While
Luskin and co-workers suggested that clonally related neurons
occur in columns, the works by Price and Walsh suggested that
clones of neurons do not form radial columns, but rather dis-
Similarly, injection of anterograde tracers in the LGN of ferrets
2–3 weeks prior to the onset of the critical period reveals a clear
ocular dominance segregation of the afferents (Crowley and Katz,
2000), indicating that molecular cues, intrinsic to the develop-
ing thalamocortical system, may be involved in the establishment
of columns. Likewise, other systems which form discontinuous
projections such as the interhemispheric cortico-cortical connec-
tions have also shown topographical precision in their innervation
from the out start (Aggoun-Aouaoui et al., 1996; Hedin-Pereira
et al., 1999). Moreover, laser-scanning photostimulation in brain
slices combined with morphological analysis of axonal arbors has
revealed that connections between layer 4 and layers 2/3 neurons
develop with great specificity and without detectable pruning at
the level of the cortical columns (Bureau et al., 2004).
These data prompt the question of which are the mechanisms
governing this early columnar organization of neurons. As briefly
noted above, cortical columns have been characterized by the exist-
ence of neurons sharing similar electrophysiological properties,
involved in the processing of particular stimuli and distributed in
discrete horizontal clusters along the cerebral cortex. Thus, the very
first pre-requisite to link a given factor to the columnar organiza-
tion of the cerebral cortex would be the capacity of this factor to
organize neurons in discrete radial columns of neurons and to favor
Gap junctions have been recurrently implicated as players in
the establishment of functional units (Yuste et al., 1992, 1995;
Kandler and Katz, 1995). The coordinated calcium fluctuation pat-
terns underlying gap junctional mediated communication were
suggested to form the basis of functional cell assemblies in post-
natal cerebral cortex. Blocking activity did not eliminate calcium
functional domains suggesting that gap junctions may promote
metabolic rather than activity related assemblies (Kandler and Katz,
1998). Recently, it has been shown that glial cells in layer IV of
the somatosensory cortex form gap-junction coupled ensembles
correlated to barrels (anatomical structures that display eletro-
physiological responses to individual whiskers) showing that non
neuronal cells may also be important players in the formation of
cortical units (Houades et al., 2008).
It has been shown previously that since very early, during embry-
onic neurogenesis, cells in different stages of the cell cycle (mainly
S and G2 but also G1) are gap junctionally coupled with radial glia
forming columnar functional units with 15 to 20 cells (Bittman et al.,
1997). It is possible that these cells belong to the same clone and their
functional and metabolic coupling at this stage could be important
for the later establishment of connections among themselves (see
discussion about neuronal clones in the following section). It is also
known that gap junctions regulate neuronal migration (Elias et al.,
2007; Marins et al., 2009) and adhesive connections formed by gap
junctions between migrating neurons and radial glia were shown to
be important for gliophilic migration. Thus, gap junctions and other
molecules that regulate migration may be important at later stages
helping connect columnar neuronal networks whose identity may
have been primed early within the VZ.
An important aspect of the formation of columnar structures is
the radial axon-dendrite polarity established from the very begin-
ning by neurons migrating attached to radial glia with their leading
and trailing processes. During migration neurons derived from the
Frontiers in Neuroanatomy www.frontiersin.org June 2010 | Volume 4 | Article 26 | 3
Costa and Hedin-Pereira Neuronal clones as functional units
et al., 2008). Therefore, it is possible that much of the tangential
dispersion observed in early cell lineage studies was a consequence
of tangential dispersion of GABAergic neurons (Walsh and Cepko,
1988, 1992, 1993; Tan et al., 1995).
At this point, it is important to cite that fate-mapping studies
in the rodent cerebral cortex have provided compelling evidence
indicating that glutamatergic and GABAergic neurons are indeed
derived from separate pools of progenitors in the dorsal and ventral
telencephalon, respectively. Using the Cre-lox system (Orban et al.,
1992; Sauer, 1998), it has been shown that progenitors located in
the dorsal telencephalon express the TF Emx1 and generate exclu-
sively cortical excitatory neurons (Gorski et al., 2002), whereas those
located in the ventral telencephalon express Nkx2.1, Lhx6, Gsh2
or Nkx6.2 and generate different subtypes of cortical interneurons
(Fogarty et al., 2007; Xu et al., 2008). Thus, genetic fate-mapping
studies indicate that the progeny of a single progenitor will com-
prise either glutamatergic or GABAergic neurons. Thus, these two
types of neurons are very unlikely to be clonally related, at least in
the rodent cerebral cortex.
Having that in mind, we have recently readdressed the issue of
cell lineage in the developing cerebral cortex by using a combination
of two silencing-resistant vectors carrying different reporter genes
(Costa et al., 2009). We analyzed exclusively clones comprising spiny
pyramidal neurons, so that the radial dispersion within clones of
glutamatergic neurons, i.e. derived from dorsal telencephalic progen-
itors, could be accurately measured. Our results indicate that clonally
related glutamatergic neurons generated from E13 progenitors do not
disperse further than 280 μm in the adult cerebral cortex (Figure 1),
what could very well be explained by the horizontal growth of the
brain between the time of injection and analysis (Costa et al., 2009).
perse several hundreds of micrometers of cortex in the horizontal
dimension. Several factors may have contributed for these divergent
interpretations, such as the definition of clones based solely on
the anatomical dispersion of cells (Luskin) or the expression of
individual genetic tags by polymerase chain reaction (Walsh). For
the reader interested in how these two variables may affect the
conclusions of the cell lineage studies, we refer to a recent paper by
Costa et al. (2009) where these issues were addressed.
Importantly, primary cell lineage studies were performed before
the discovery of the massive tangential migration of GABAergic
neurons in the developing brain (Marin and Rubenstein, 2001),
what per se may have lead to misinterpretations about clonal rela-
tionship between cells. In fact, while the original population studies
using tritiated thymidine to label post-mitotic cells have suggested
that radial cell dispersion would be the primary mechanisms by
which neurons could arrive to their final destination in the cerebral
cortex (Angevine and Sidman, 1961; Rakic, 1974), it was only in the
late 90s that tangential neuronal migration has been recognized as
an important mechanism for settling GABAergic neurons in the
cerebral cortex (Anderson et al., 1997). Nowadays, it is broadly
accepted that, in the developing cerebral cortex of rodents, gluta-
matergic neurons migrate radially towards their final position in
the cerebral cortex, whereas GABAergic neurons migrate tangen-
tially (Marin and Rubenstein, 2003). Consequently, whilst radially
migrating neurons could be distributed in arrays perpendicular
to the pial surface, tangentially migrating ones disperse across
different areas and no topographical relation has been detected
between the ganglionic eminence VZ and the cortical destination
of these cells. Rather the GE ventricular surface has been corre-
lated to different types of interneurons (Fogarty et al., 2007; Xu
Adult Cerebral Cortex
Figure 1 | Columnar distribution of sister neurons in the cerebral cortex.
(A) Schematic representation of a single progenitor cell transfected by a
retroviral vector and its subsequent progeny (green). (B) Coronal section of the
adult mouse cerebral cortex labeled for GFP (green) and DAPI (blue) where two
neuronal siblings can be observed. For this experiment, the retrovirus carrying
the gene for the protein GFP was injected into the lateral ventricle of an E13
animal. Abbreviations: VZ, ventricular zone; CAG-GFP , green fluorescent protein
encoding plasmid; L, layer. Calibration bar: 100 μm.
Frontiers in Neuroanatomy www.frontiersin.org June 2010 | Volume 4 | Article 26 | 4
Costa and Hedin-Pereira Neuronal clones as functional units
for the wiring of neurons in the central nervous system. In fact,
several groups have identified cell surface molecules involved in
the patterning of neural circuits (Song and Poo, 2001). Yet, one
important criticism to Sperry’s theory has been the fact that the
number of neurons and connections in the brain (about 1012
and 1015, respectively, in humans) is far higher than the number
of genes encompassed in the whole genome and, therefore, the
number of different molecules would not suffice to specify all
the neuronal connections. One possible solution envisioned by
Sperry (1963) was the graded expression of cell surface molecules
and their receptors, which has also been validated by recent find-
ings (O’Leary et al., 1994). Such graded expression would lead
to much higher combinatorial possibilities of cell responses than
could be predicted by the number of signaling molecules and
receptors. Indeed, it has been shown that a given ligand can elicit
different responses in growing axons depending on the recep-
tor complexes expressed in the target cell (Hong et al., 1999).
Additional complexity is also added to the system when we con-
sider the metabolic state of the axon. For example, the intracellular
concentration of calcium can determine whether some axons are
repelled or attracted by a given molecule (Hong et al., 2000). Thus,
the number of genes in the cell genome clearly underestimates
the repertoire of molecular combinations capable of dictating
distinct cellular behaviors.
Furthermore, a large number of cell surface molecules can be
generated from a limited number of genes in the nervous system
through genetic rearrangements, such as the alternative splic-
ing observed in the Drosophila gene Dscam1 (Down syndrome
cell adhesion molecule) (Schmucker et al., 2000; Wojtowicz et al.,
In fact, we found that most neuronal clones derived from E13 pro-
genitors span 150–250 μm in the horizontal axis and contribute to all
cortical layers generated after that embryonic stage, namely layers V,
IV, and II/III. Mathematical extrapolations for injections performed
at the onset of neurogenesis in the cerebral cortex (E10-11) suggest
that neuronal siblings would not disperse more than 400–500 μm.
Thus, both the radial and horizontal dispersion of excitatory neuro-
nal clones fits well with the possibility that they could help to create
a structural basis for the future specification of columns.
Concurrently, it has also been suggested that excitatory neurons
generated from the same progenitor are more likely to establish
synaptic connections than non-sibling neurons (Yu et al., 2009).
By injecting EGFP-expressing retroviruses through the uterus into
the lateral ventricle of mouse embryos at early neurogenesis, the
authors were able to identify individual clones of pyramidal neu-
rons, similar to the cells shown in Figure 1. Next, they performed
simultaneous whole-cell recordings on two EGFP-expressing sis-
ter neurons and observed that these cells displayed unidirectional
synaptic connections in 35% of pairs. In contrast, less than 7%
of radially situated non-sister excitatory neurons were connected
(Yu et al., 2009).
Taken together, these new lineage studies indicate that clonally
related excitatory neurons not only keep a tight spatial relationship
but are also capable of recognizing their siblings, either chemically
or electrically, and establish functional synaptic connections.
the lInk between progenItors and post-mItotIc
neurons as the cellular basIs for the generatIon of
Based on these findings, we would like to put forward a more tempt-
ing hypothesis, namely that transcriptional networks in cortical
progenitors may help to establish functional units throughout the
cerebral cortex by enabling these progenitors to generate neurons
with similar electrochemical properties and high connectivity.
According to this hypothesis, different levels and combinations of
TFs expressed by discrete pools of progenitors would be responsible
for the generation of individual microcircuits of sibling neurons,
which would be able to recognize each other and establish synaptic
connections. In other words, gene expression in individual cortical
progenitors could influence the development of functional units
throughout the cerebral cortex by generating small radial clusters of
interconnected neurons (Figure 2), which in turn could be assem-
bled together to generate functional minicolumns and columns.
The rationale behind our hypothesis is that neurons derived
from the same progenitor are more likely to display similar chemi-
cal and physical properties, due to their genetic inheritance. Thus,
sibling neurons would be more likely to recognize and respond
stereotypically to the same molecular cues that could influence
the early arrangement, metabolic coupling, and interconnectiv-
ity of those neurons within a single column. This ability is likely
to rely on the expression of a similar set of surface molecules in
sister neurons, which in turn could be controlled by TF networks
operating in cortical progenitor cells.
But how molecules could contribute to specify the connec-
tion between sibling neurons? Sperry’s theory (1963), known as
chemoaffinity, proposed that molecules would be responsible
Adult Cerebral Cortex
Figure 2 | Hypothetic model for the generation of functional units
from individual progenitors. Schematic drawing showing three progenitor
cells in the embryonic ventricular zone (VZ) expressing different sets/levels
of transcription factors, labeled in red, blue, and green. Each of these cells
generates a clone of pyramidal neurons that inherit analogous genetic
information from the founder progenitor and are organized in discrete radial
arrays in the adult cerebral cortex. The similar genetic pedigree of sibling
neurons allows their recognition and establishment of synaptic
connections, creating a microcircuit of clonally related glutamatergic
Frontiers in Neuroanatomy www.frontiersin.org June 2010 | Volume 4 | Article 26 | 5
Costa and Hedin-Pereira Neuronal clones as functional units
2004) and the mammal protocadherin family (Kohmura et al.,
1998; Wu and Maniatis, 1999). Indeed, it has been proposed that
Dscam1 gene gives rise to 18,048 proteins that could control self-
avoidance between neurites through isoform-specific homophilic
binding (Wojtowicz et al., 2007), what clearly indicates that a
limited number of genes may generate a far broader variety of
molecular tags responsible for cell-to-cell specific recognition in
the nervous system. Therefore, it is not entirely absurd to suggest
that the connectivity preference between sister neurons may be
mediated by the expression of a given set of proteins involved in
But now, how could gene expression in progenitor cells influ-
ence that? One possibility is that different expression levels of TFs
could control the expression of distinct sets of surface molecules
allowing the recognition of sibling neurons. Although we are just
beginning to understand this phenomenon, increasing evidence
support the notion that (i) distinct expression levels of a given
TF; (ii) combination of TFs; or (iii) interactions between TF and
its cofactors in the same cell type can in fact lead to completely
different biological outcomes, likely reflecting differential gene
expression induced by the TF.
For instance, it has been shown that the expression level Pax6
in the developing cerebral cortex is essential for controlling the
balance between proliferation and differentiation (Sansom
et al., 2009). By using Pax6 gain- and loss-of-function strate-
gies, Sansom and colleagues have shown that the Pax6-regulated
networks operating in cortical progenitors are highly dosage
sensitive, so that relative levels of Pax6 are key determinants
for controlling whether VZ progenitors will self-renew, gener-
ate neurons or basal progenitors. Therefore, it is not entirely
absurd to suggest that different levels of Pax6 (or any other TF)
within cortical progenitors might also kick off individual genetic
programs by their neuronal lineage, leading to the expression
of molecules responsible for the recognition and connectivity
of these neurons.
Another example of such diversity in cell-response to a single
TF is the activation of specific target genes by REST (repres-
sor element-1 silencing transcription factor) during neuronal
subtype specification (Abrajano et al., 2009). In this study, the
authors have shown by chromatin immunoprecipitation on chip
(Chip-chip) that REST and its cofactor CoREST (corepressor for
element-1 silencing transcription factor) modulate the expres-
sion of largely distinct gene profiles responsible for inducing
and maintaining different neuronal subtype identities, such as
cholinergic, GABAergic, glutamatergic, and medium spiny neu-
rons. These data clearly indicate that the balance between a single
TF and its corepressor can regulate complex and distinct gene
networks underlying important cell behaviors, such as neuronal
Collectively, these data support the idea that TF networks could
modulate the expression of genes encoding proteins involved in
cell–cell recognition and, consequently, contribute for the capacity
of sibling neurons to recognize each other and establish synaptic
connections. In the future, it will be interesting to investigate
which genes and molecules subsidize the high probability of
connection between sister neurons. Notably, we have observed a
similar phenomenon in vitro, further suggesting that the capac-
ity of clonally related neurons to recognize each other is a cell-
It has been suggested that one important phenomenon for the
increased cerebral complexity during evolution may be the mul-
tiplication of neuronal columns throughout the cerebral cortex
(Rakic and Caviness, 1995). Here, we further refine this conjec-
ture by suggesting that discrete alterations in the gene expression
pattern during development may support this phenomenon by
allowing a larger number of individual progenitor cells to generate
individual and highly interconnected neuronal clones. In other
terms, neuronal clones could be seen as fundamental blocks in
the construction of brain circuits, upon which later influences
brought by axonal growth, synaptogenesis and activity will act
to establish the functional anatomy of the cerebral cortex (Sur
and Rubenstein, 2005). These fundamental blocks could also be
influenced by earlier factors, such as incoming afferent systems
to cerebral cortex that have been shown to regulate the cell cycle
length in the germinal zone and contribute to generate areal dif-
ferences in the germinal zones (Dehay and Kennedy, 2007). Also
in that direction, recent work has shown that horizontal intercon-
nectivity between columns is important to stabilize columnar size
(Kaschube et al., 2009). Therefore, although lineage relationship
could be at the base of the columnar organization of the cortex,
several environmental factors are able to regulate column size
and determine the properties that will be processed by the func-
As can be deduced from our previous discussion about the origin
of glutamatergic and GABAergic neurons, our hypothesis apply
exclusively to the generation of glutamatergic neuronal clones. In
fact, there is no evidence supporting the notion that tangentially
migrating GABAergic neurons would settle in the cerebral cor-
tex in an orderly manner, reflecting their original position in the
VZ of the ventral telencephalon. Further support to this notion
comes also from recent transplantation experiments indicating that
GABAergic neurons are rather plastic and may develop functional
inhibitory circuits in the visual primary cortex despite their site of
origin (Southwell et al., 2010).
Concluding, we suggest here that the development of individual
clones of glutamatergic neurons is a fundamental step for the par-
cellation of the cerebral cortex. These individual clones could be
seen as singular functional units, which will be assembled into
more complex parcels, such as minicolumns or columns, under
the influence of intrinsic and extrinsic signals. According to this
view, the number of independent functional units throughout the
cerebral cortex would be increased not only by the enlargement
of progenitor pools (Caviness et al., 1995), but also by discrete
changes in the combinatory levels of TFs expressed in the progeni-
tor cells. Consequently, this transcription network would represent
an important target in brain evolution.
Work by Marcos R. Costa and Cecilia Hedin-Pereira is supported
by CNPq, FAPERN and FAPERJ.
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Conflict of Interest Statement: The
authors declare that the research
was conducted in the absence of any
commercial or financial relationships that
could be construed as a potential conflict
Received: 01 March 2010; paper pend-
ing published: 19 March 2010; accepted:
26 May 2010; published online: 28 June
Citation: Costa MR and Hedin-Pereira C
(2010) Does cell lineage in the developing
cerebral cortex contribute to its columnar
organization? Front. Neuroanat. 4:26. doi:
Copyright © 2010 Costa and Hedin-
Pereira. This is an open-access article subject
to an exclusive license agreement between
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