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Cerebral Cortex: Symmetric vs. Asymmetric Cell Division
Cerebral Cortex: Symmetric vs. Asymmetric Cell Division 785
Cerebral Cortex: Symmetric vs. Asymmetric Cell Division G Fishell and C Hanashima, New York University Medical Center, New York, NY, USA ã 2009 Elsevier Ltd. All rights reserved.
Introduction The human cerebral cortex contains approximately 1010 cells that exhibit a wide array of physiological properties; these cells are wired together by synaptic connections that show remarkable specificity. This highly ordered structure is organized along two axes. Vertically, the cerebral cortex can be divided into six discrete laminae, each of which displays distinct afferent and efferent connections. Horizontally, cortical columns within each region of the cortex subserve specific aspects of cortical function, such as processing somatosensory or visual information or coordinating motor outputs. Despite this impressive architecture, cortical neurons can be grossly subdivided into two major classes, the excitatory pyramidal or principal neurons (80%) that are glutamatergic and provide the major structural inputs and outputs of the cortex, and the inhibitory GABAergic interneurons (20%) that regulate cortical function through local connections. Pyramidal neurons with characteristic axonal connections, dendritic morphologies, and physiological properties are segregated in each of the different cortical layers. For instance, morphologically, deep-layer (V/VI) neurons have a bigger soma compared to the superficial-layer (II/III) neurons, which typically contain smaller cell bodies. Moreover, a majority of layer V/VI pyramidal neurons project to subcortical structures, including the thalamus, basal ganglia, and spinal cord, whereas layer II/III neurons project intracortically to other cortical areas. Similarly, cortical interneurons at least in some cases are enriched within specific cortical laminae. While the cell bodies of the Martinotti interneurons are found predominantly in the deep layers of the cortex, the neuroglioform interneurons are preferentially found in the superficial cortex. A similar subdivision of both of these populations occurs within the horizontal plane, as best shown by the expansion of input cortical layers (layers I and IV) in sensory regions such as visual cortex and enlargement of output layers of the cortex (layers V and VI) in motor regions. Understanding how such exquisite organization is established during development is one of the major challenges in developmental neurobiology. Efforts to understand how cortical cellular diversity arises dates back to the late ninteenth century. The first systematic attempts to explore the organization of the developing brain came from the study of early
neurogenesis, a time when epithelial cells span their fibers across the entire cortical wall. These epithelial cells, which appear to be equivalent to what we today call ‘radial glial cells,’ were referred to by His as ‘spongioblasts’ based on their elongated spongelike appearance and were thought to give rise solely to glial cells. The other ‘cell type’ observed by His was the rounded ‘germinal cells’ that reside at the apical surface of the neural tube, which he proposed were the mitotically active neuronal progenitors. This view was challenged by Magini, who observed dramatic morphological variations within the spongioblasts, and suggested that the spongioblasts and germs cells were simply the same population at different points in the cell cycle, an idea that was later confirmed by Sauer. Sauer reached this conclusion through the observation that cell nuclei at the basal side of the neural tube contain an increased complement of DNA (i.e., S phase), while those adjacent to the ventricle were tetraploid (i.e., M phase). This suggested that neural progenitors undergo ‘interkinetic nuclear migration’ as they progress through the cell cycle and concurrently undergo dynamic changes in their cellular morphology and nuclear position. That this occurred was definitively shown in 1959 where the movements of dividing cells were followed using post hoc analysis of cells previously labeled with [3H]thymidine. This autoradiographic method revealed that cortical progenitor cells indeed undergo DNA replication at a distance from the ventricular surface, and then subsequently translocate to the apical surface, where the cells round up and undergo mitosis. From these divisions arise daughter cells that either terminally differentiate into neurons and migrate to the pia or undergo a second cycle of proliferation. Although nuclei prior to neurogenesis translocate across the entire width of the neural tube, subsequent to the onset of neurogenesis, interkinetic migration is confined to a region adjacent to the lateral ventricle. The geographical restriction of cells undergoing interkinetic nuclear migration led to this region being designated as the ‘ventricular zone.’ Within the cortex, postmitotic cells exit the ventricular zone and migrate along radial glia toward the pial surface. Surprisingly, the structural role of radial glial cells in guiding migration in the cortex was established long before it was appreciated that the radial glia were themselves the progenitor population that gave rise to newborn neurons. Indeed, for the three decades after radial glial cells were known to support neuronal migration, the radial glia and cortical neuronal progenitors were considered to be two distinct cell lineages, with the former solely
786 Cerebral Cortex: Symmetric vs. Asymmetric Cell Division Pia
Basal SVZ Radial glial progenitor VZ Apical G1
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G2
Postmitotic neuron
M SVZ progenitor Pia VZ:
ventricular zone
SVZ:
subventricular zone
Basal SVZ
VZ Apical G1
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M
Figure 1 Asymmetric and symmetric cell division in the cerebral cortex. Top panel: Radial glial progenitors undergo interkinetic nuclear migration during the cell cycle. One asymmetric cell division results in the production of two daughter cells, one of which remains in the VZ and inherits the radial fiber and the other of which translocates outward toward the pia along the radial fiber to become a postmitotic neuron. Bottom panel: Alternatively, SVZ progenitors arise from the VZ and subsequently undergo symmetric divisions to produce two postmitotic neurons. Adapted from Noctor SC, Martinez-Cerdeno V, Ivic L, et al. (2004) Cortical neurons arise in symmetric and asymmetric division zones and migrate through specific phases. Nature Neuroscience 7: 136–144.
producing astrocytes. However, the seminal paper by Noctor and Kriegstein used time-lapse imaging of retrovirally infected cortical progenitors to demonstrate that radial glial cells, in addition to supporting the migration of nascent neurons, gave rise to them. These studies revealed that neurons assembled in close radial units are not only clonally related, but also that radial glial cells can simultaneously give rise to both differentiated neurons and progenitor cells (Figure 1). This once and for all established that radial glial cells are the major and perhaps sole progenitor for cortical pyramidal neurons. Moreover, although previous analysis suggested that asymmetric cell divisions occurred in the cerebral cortex, this analysis provided the first definitive evidence establishing the existence of this phenomenon.
Cell-Type Specification in the Neocortex Temporal Determinants: Birthdate and Laminar Fate
How is the diversity of distinct layer neuronal subtypes created from a restricted progenitor pool within the VZ? The VZ appears to utilize temporal cues to sequentially produce different subclasses of neurons, rather than simultaneously generating multiple
progenitor pools, each of which gives rise to particular cell type. Studies examining the origin of distinct neuronal subtypes indicate specific neuronal populations arise at precise times during development, presumably in response to changing temporal determinants. The first evidence that birthdate was a strong predictor of neuronal fate in the cerebral cortex came from [3H] thymidine studies in rodents; it was revealed that the administration of a tritiated thymidine injection at distinct times during neurogenesis resulted in the labeling of neurons within particular cortical laminae. Moreover, this occurred in an inside-out fashion, where early-born neurons occupy the deepest position within the cerebral cortex, while later-born cells migrate past their ascendants, to take up more superficial positions. Hence, in the mature cortex, layer VI neurons are born first and layer II neurons are born last. The only exception to this rule is the earliest-born neurons, the Cajal–Retzius (RC) cells that transiently reside in layer I. The inside-out relation of laminar position to birthdate, although evident in other mammals, is more precise in higher species, such as primates. This has been interpreted as suggesting that the progenitors within the ventricular zone undergo multiple cell cycle divisions, resulting in the production of neurons destined for sequentially more superficial positions.
Cerebral Cortex: Symmetric vs. Asymmetric Cell Division 787
Direct evidence for the sequential production of cortical neurons came from both static and liveimaging clonal analysis of ventricular progenitors using retroviruses. These studies revealed that individual cortical progenitors produced multiple subtypes over a prolonged period during cortical development. These clonal and birthdating studies raised the question as to how neural progenitors in the ventricular zone change their distinct laminar fate according to temporal cues. One possibility is that extrinsic cues surrounding the ventricular zone change over time to influence the fate of the progenitor cells. Alternatively, it was proposed that progenitors cells might be intrinsically programmed to sequentially produce distinct neuron types at particular developmental timepoints. To test the relative contribution of extrinsic and intrinsic factors to temporal cortical cell-fate determination, classical transplantation experiments in ferrets were carried out to heterochronically challenge ventricular zone progenitors by transplanting them reciprocally to earlier or later developmental timepoints. Specifically, in these experiments early-born progenitors fated for deep layers of cortex (layer VI) were transplanted to periods of development when late-born neurons within layers II/III were being generated. Conversely, cortical progenitors that would normally produce layers II/III were transplanted into host cortical ventricular zone at a time when layer V was being generated. The results revealed a forward ratcheting in the potential of progenitors as development progressed. While early progenitors were found to be competent to adopt later fates when transplanted to older hosts (i.e., progenitors normally fated to produce layer V neurons upon transplantation to later stages of development could produce layer II/III neurons), the converse was not true (i.e., progenitors from later timepoints in development did not produce earlier-born populations of neurons even when transplanted to host animals where earlier populations of neurons are being produced). These results highlight that the cell fate in the cerebral cortex is largely established by intrinsic determinants and that progenitor potential becomes progressively restricted as development progresses. Interestingly, while the molecular mechanism governing these fate restrictions is not well understood, timed transplants of early-born cells to late donors revealed that the fate of neurons is set within 6 h of their final S phase. Asymmetric and Symmetric Division of Cortical Progenitors Function in the Generation of Cortical Cell Diversity
What is the mechanism by which a common pool of cortical progenitors within the germinal zone gives rise to a broad diversity of neurons over a prolonged
period of time? It has been postulated that the mode of cellular division utilized by the progenitor is central to this process. During early periods of neurogenesis, a majority of cortical progenitors undergo symmetric divisions that produce two progenitor cells, in order to expand the cortical progenitor pool. However, that a common progenitor produces distinct types of neurons over multiple cell divisions also implies that later there is a second process by which two daughter cells can assume distinct cell fates. Given that postmitotic daughter cells are produced sequentially, it suggests the existence of asymmetric divisions where one daughter cell reenters the cell cycle, while the other exits to differentiate into a mature neuron. That this can occur hints that during an asymmetric division the two daughter cells adopt fundamentally distinct intrinsic cell-fate programs, where asymmetric divisions unequally bestow cell-fate determinants into only one of the two daughter cells. Considerable efforts have been made to identify these ‘determinants.’ Drosophila neurogenesis has provided a perfect context to study this question, as neuroblasts in the central nervous system (CNS) as well as sensory organ precursor cells in the PNS undergo asymmetric divisions, depending on the precise orientation of their cleavage plane. Notably, the protein Numb, together with Prospero and the adaptor proteins Miranda and Pon, are asymmetrically localized to the basal surface of neuroblasts during metaphase. Therefore, when the orientation of the cleavage plane is horizontal to the apical surface, the daughter cell on the basal side inherits the majority of determinants, such as Numb protein. Thus, asymmetrical cell division requires two events, establishing the cell polarity by regulating the orientation of the mitotic spindle along the apical–basal axis, and targeting Numb and associated proteins to the basal side. Identification of asymmetric cell-fate determinants has come from analysis of the aberrations in the neural lineages of mutant flies, where one of the critical determinants, such as Numb or Prospero, is lost. The loss of these results in a randomized spindle orientation and a corresponding perturbed cell fate. Although the precise function of asymmetric localization of Numb is not clear, it is likely that Numb functions as an inhibitor of Notch signaling, which is required for maintaining neuroblasts in a progenitor state. In the cerebral cortex, it has been suggested that the plane of cell cleavage may also determine the fate of the resulting daughter cells. Examination of the ferret cortex has shown that around 15% of the mitosis in the ventricular zone has a horizontally oriented cleavage plane, where, following division, one daughter cell retains the apical contact and remains in the ventricular zone, while the basal daughter cell migrates away from the ventricular
788 Cerebral Cortex: Symmetric vs. Asymmetric Cell Division
zone and becomes a postmitotic neuron. Interestingly, the mouse homolog of Drosophila Numb, m-Numb, is asymmetrically localized to the apical membrane of the dividing progenitors in the ventricular zone, suggesting a conserved function of Numb in determining progenitor outcome in mammals. However, a majority of the cell divisions exhibit a vertical or randomly oriented cleavage plane. Thus, the cleavage orientation is not clearly bifurcated into distinct division modes as is the case in flies. Hence, as the 15% of the horizontal cleavage cannot account for all neuronal production during peak neurogenesis period, the relationship between the cleavage plane and asymmetric division, and its contribution to developmental fate in the cerebral cortex, need to be clarified. What, then, is the function of Numb and Notch proteins in the cerebral cortex? In mammals, the interpretation of loss-of-function of either of the genes is not straightforward, as there are multiple homologs of both genes that appear to functionally compensate for each other. In mouse two numb homologs exist, mouse numb (m-numb) and numblike (nbl). Mice that lack m-numb have highly perturbed development and die prior to E11.5. Although the nbl –/– mutant alone does not exhibit an apparent neural developmental phenotype, this is likely a result of functional compensation between these two genes, as double mutants demonstrate a more severe phenotype and die around E8.5. To circumvent this early lethality, a conditional allele of m-numb was generated to test the requirement of this gene during the neurogenic period. The outcome of these experiments suggested that Numb has pleiotropic roles during the corticogenesis period, as distinct phenotypes were observed, depending on when the gene was removed. Conditionally ablating m-numb and nbl with Crerecombinase driven under the Nestin promoter, which specifically removes genes starting around E8.5, resulted in an early depletion of the cortical progenitor pool. Similarly, compound removal of m-numb and nbl during later corticogenesis using a late Cre driver, D6-Cre (which becomes active only after E10.5), also suggested that both genes are required to maintain the progenitor cells in an undifferentiated state. However, removal of m-numb/nbl gene function using an Emx1-Cre (which becomes active at E9.5 and results in complete recombination within the cerebral cortex by E12.5) resulted in hyperproliferation of the cortical progenitors as well as inhibition of neuronal differentiation, suggesting that in this context Numb acts to direct progenitors to a neuronal fate. Taken together it is apparent that further work is needed to assess why the function of Numb is diametrically opposite, depending on the context of its removal.
More clear-cut is the function of Notch signaling during neurogenesis. Loss of Notch signaling results in the precocious differentiation of neural progenitors, while overexpressing a constitutively active form of Notch (caNotch) results in inhibition of neurogenesis and promotion of a radial glia cell fate. It is notable that the transient ectopic activation of Notch signaling early in neurogenesis during the period when deep-layer neurons are produced does not disrupt the normal temporal progression of neurogenesis. Upon removal of the ectopic Notch activation, upper-layer corticogenesis resumes normally. Hence even though the transient activation of Notch prevents progenitors from differentiating, on removal of Notch signaling the normal temporal progression of neurogenesis resumes. This implies that the primary function of Notch in cortical development is to maintain the progenitors in an undifferentiated state, but it does so without disrupting the developmental clock that controls the competence of progenitors. In the future, better understanding of the precise roles of Numb and Notch in cortical development, as well as their genetic and biochemical interactions, should prove informative. In addition, whether asymmetrical cell division determines cell fate or merely controls the length of corticogenesis as a means of expanding the number of neural progenitors needs to be clarified. Does the Segregation of Progenitors to the Ventricular Zone and Subventricular Zone Represent a Bifurcation of Cell Fates?
The types of neurons produced in the cerebral cortex are clearly restricted by temporal cues within the ventricular zone; however, the mechanisms that determine layer neuron specification may not solely depend on temporal changes in progenitor competence. Recent studies have suggested that the restriction in neuronal class production can occur by a spatial segregation of cortical progenitors. In this regard, the emergence of the secondary germinal zone in the neocortex, the subventricular zone (SVZ), may play an important role in determining the identity of upper-layer neurons in the cerebral cortex. The SVZ is a transient zone that is juxtapositioned between the ventricular zone and the intermediate zone. It was traditionally defined by those progenitors that do not undergo interkinetic nuclear migration as they transit through the cell cycle. The SVZ is also distinguishable from the ventricular zone by its lack of pseudostratified epithelial morphology. It has been postulated that since SVZ develops later than the VZ, SVZ progenitors predominantly give rise to the glial cells at the end of corticogenesis. However, since the peak neurogenesis period of upper-layer (layer II–IV) neurons overlaps with the
Cerebral Cortex: Symmetric vs. Asymmetric Cell Division 789
expansion of SVZ progenitors, the alternative view is that the spatial segregation between SVZ and VZ may directly restrict the fate of distinct layer neurons. This hypothesis is further supported by the strong correlation in genes expressed in both SVZ and upper-layer neurons. Two genes, Svet1 and Cux2, specifically label mitotic cells in the SVZ. Notably, the level of expression in the SVZ population declines as the progenitors in this pool are depleted. In turn, Svet1 and Cux2 are expressed in postmitotic neurons that comprise neurons in layers II–IV. Beyond this simple correlation in gene expression, mutants that have abnormal development in SVZ also show abnormalities in the differentiation of upper layers of cortex. In Pax6 mutants, the expression of Svet1 and Cux2 is downregulated in SVZ progenitors, and the upper-layer neurons fail to be properly established. Furthermore, expression of both Svet1 and Cux2 is inverted in reeler mutants, suggesting that these genes define the intrinsic properties of upper-layer neurons rather than their laminar position or migratory patterns. It is notable that progenitors change their mode of cell division as they transit from the VZ to the SVZ. During corticogenesis, a majority of VZ progenitors undergo asymmetric divisions, in which the two daughter cells adopt distinct fates to remain a progenitor or become a postmitotic neuron (Figure 1, top panel). However, SVZ progenitors are more likely to follow a symmetric pattern of division, where two daughter cells either become a pair of postmitotic neurons or together remain as progenitors (Figure 1, bottom panel). It is worth mentioning that not all the progenitors undergo mitosis in the SVZ, even during the peak neurogenesis of upper-layer neurons, and it remains possible that within the VZ and the SVZ there exists greater heterogeneity of progenitors than has yet to be appreciated. In this regard it is notable that in the primate neocortex studies of neurogenesis have subdivided the SVZ into inner and outer zones, the latter of which has been described to be another major source of cortical neurons. The emergence of SVZ during evolution may be the consequence of an evolutional strategy of expanding the diversity or numbers of cortical neurons that can be produced at a given time. Hence during evolution, in addition to the size and complexity of the architecture of the cerebral cortex, the cellular diversity may also have increased. Intrinsic Factors of Cortical Cell-Fate Specification
Fundamental to cortical neurogenesis is the prolonged production of different siblings from a common pool of progenitors within the VZ and SVZ. This
almost certainly depends on intrinsic cues to direct distinct neuronal fates. This process allows the progenitor pool to expand and simultaneously produce different cell types at precise times during neural development. What, then, is the identity of temporal determinants that alter the fate of the neuron types produced? As previously mentioned, cortical progenitors appear to utilize a ratcheting mechanism by which the neuronal types produced over time become progressively restricted. Moreover, later-born progenitors retain a limited ability to differentiate into neuron types prior to their own birthdate. In rodents, the molecular identity of this progressive restriction is best understood in the switch from generating the earliest-born CR cells to deep-layer projection neurons. This process is regulated by the transcriptional repressor Foxg1. In the cerebral cortex, Foxg1 is expressed in the majority of the cortical projection neurons, with the exception of layer I CR neurons. Moreover, in the absence of Foxg1, cortical progenitors fail to generate later-born neurons and instead continue to produce the earliest-born CR neurons. In order to determine the cell-autonomous function of Foxg1 in cell-fate specification, Foxg1 was conditionally inactivated in deep-layer progenitors. Interestingly, removal of Foxg1 at E13 (the birthdate of deep-layer neurons) results in the resumption of CR cell production in the cerebral cortex. By labeling the progenitors that were born subsequent to the removal of Foxg1 with BrdU, it was confirmed that the cells that were normally destined to become deep-layer neurons instead adopted an earliest-cell fate. This work has recently been replicated in vitro, where the fate of individual neuroblasts can be directly followed. These studies suggest the early cell-fate specification in the mammalian cerebral cortex is determined by an active mechanism whereby a transcriptional repressor prevents the later-born neurons from adopting an earlier fate. How is the cortical cell fate established during the subsequent corticogenesis period? It has been reported that a zinc-finger-containing transcription factor, Fezl, which is specifically expressed in corticospinal motor neurons in layer V, directs both the specification and identity of these cell types. Loss of Fezl results in both the loss of corticospinal neurons and the failure of subcortical projections. Furthermore, overexpression of Fezl near the end of layer V genesis can partially override the transition of cortical progenitors to produce neurons that migrate to layer IV, which instead send a projection through the internal capsule. It remains to be explored whether the competence window of cortical progenitors responding to either Foxg1 or Fezl is limited. Beyond this, it seems clear that a great deal
790 Cerebral Cortex: Symmetric vs. Asymmetric Cell Division
more remains to be learned concerning the intrinsic mechanisms by which temporal competence is controlled within the cerebral cortex. A Cascade of Intrinsic Determinants Controls the Production of Neurons in Drosophila
The progressive restriction of progenitors appears to be central for the specification of distinct cell types in a variety of tissues, such as in the retina and hematopoietic systems. This mechanism appears to be evolutionarily conserved, as similar rules appear to underlie the specification of cells in the nerve cord of invertebrates. In particular, work in Drosophila has begun to elucidate the molecular determinants controlling the precisely orchestrated production of different neuronal subtypes during development. In flies, the cell-lineage relationships of distinct neuron types have been well characterized and transcription factors that are specifically expressed in neuroblasts at distinct times have been shown to be required for the acquisition of specific neuronal identities. For instance, the well-characterized NB7–1 lineage consists of 30þ neurons, including 5 Eveþ neurons (U1–U5) and their siblings, and subsequently 20þ interneurons. Analogous to the mammalian cortical neurons, the earliest-born U1 neurons reside in the deepest position and project a longest distance to their target muscle, whereas the latest-born U5 neurons reside superficially and send their processes only proximally. The neuroblasts that produce the 5 Eveþ distinct types express Hunchback (Hb), Hb/Kruppel (Kr), Kr, Pdm, and Pdm/Castor (Cas). It has been demonstrated by removing the function or overexpressing these genes that both Hb and Kr are necessary and sufficient to determine the first-born and second-born cell fates, respectively. Furthermore, overexpression of Hb induces Kr, and similarly expression of Kr induces Pdm, showing that sequential expression of these transcription cascades is tightly regulated to produce these distinct neuron types in an invariant order. It has also been demonstrated that the sequential expression of fate-determining genes is largely cell intrinsic, as the expression occurs in isolated neuroblasts in culture and even can partially progress in the absence of cell cycle progression (Kr-Pdm-Cas). Although corresponding mammalian homologs of Hb (Ikaros family genes), Pdm (SCIP/ Oct-6), and Cas (Casz1) exist, the contribution of these factors to temporal lineage progression is not equivalent (SCIP) or has not been addressed (Ikaros, Casz1). It is also noteworthy that in Drosophila, the temporal progression in neuroblast identity is dependent on transcriptional factors that positively
promote neuroblast identity. This is in contrast with the model in the mammalian cerebral cortex, where at least during the earliest corticogenesis, cell fate is regulated by a repressive mechanism by which a transcriptional repressor, Foxg1, prevents deep-layer neurons from adopting an earliest-born CR cell fate. The extent to which future analysis will reveal further similarities between mammalian cortical neurogenesis and that seen in the Drosophila nerve cord is presently unclear. Regardless of the precise details, it seems likely that the study of Drosophila neuroblasts will prove informative in helping us understand how cortical progenitors generate the diversity seen in the mature cortex. Furthermore, it seems inevitable that considerable progress will be forthcoming concerning the specific transcription cascades involved in generating cerebral cortex neuron diversity, as well as with regard to the mechanisms that ensure symmetric and asymmetric cell divisions that allow the production of the appropriate numbers of neurons at the appropriate times. See also: Cerebral Cortex: Inhibitory Cells; Cerebral Cortex; Memory Consolidation: Cerebral Cortex; Neocortex: Origins; Neurogenesis in the Intact Adult Brain; Neurogenesis and Neural Precursors, Progenitors, and Stem Cells in the Adult Brain; Radial Glial Cells: Brain Functions.
Further Reading Chenn A and McConnell SK (1995) Cleavage orientation and the asymmetric inheritance of Notch1 immunoreactivity in mammalian neurogenesis. Cell 82: 631–641. Desai AR and McConnell SK (2000) Progressive restriction in fate potential by neural progenitors during cerebral cortical development. Development 127: 2863–2872. Fishell G and Kriegstein AR (2003) Neurons from radial glia: The consequences of asymmetric inheritance. Current Opinion in Neurobiology 13: 34–41. Hanashima C, Li SC, Shen L, et al. (2004) Foxg1 suppresses early cortical cell fate. Science 303: 56–59. Lu B, Jan L, and Jan YN (2000) Control of cell divisions in the nervous system: Symmetry and asymmetry. Annual Review of Neuroscience 23: 531–556. McConnell SK and Kaznowski CE (1991) Cell cycle dependence of laminar determination in developing neocortex. Science 254: 282–285. Mizutani K and Saito T (2005) Progenitors resume generating neurons after temporary inhibition of neurogenesis by Notch activation in the mammalian cerebral cortex. Development 132: 1295–1304. Molyneaux BJ, Arlotta P, Hirata T, et al. (2005) Fez1 is required for the birth and specification of corticospinal motor neurons. Neuron 47: 817–831. Noctor SC, Flint AC, Weissman TA, et al. (2001) Neurons derived from radial glial cells establish radial units in neocortex. Nature 409: 714–720.
Cerebral Cortex: Symmetric vs. Asymmetric Cell Division 791 Noctor SC, Martinez-Cerdeno V, Ivic L, et al. (2004) Cortical neurons arise in symmetric and asymmetric division zones and migrate through specific phases. Nature Neuroscience 7: 136–144. Pearson BJ and Doe CQ (2004) Specification of temporal identity in the developing nervous system. Annual Review of Cell and Developmental Biology 20: 619–647. Reid CB, Tavazoie SF, and Walsh CA (1997) Clonal dispersion and evidence for asymmetric cell division in ferret cortex. Development 124: 2441–2450.
Tarabykin V, Stoykova A, Usman N, et al. (2001) Cortical upper layer neurons derive from the subventricular zone as indicated by Svet1 gene expression. Development 128: 1983–1993. Zhong W, Feder JN, Jiang MM, et al. (1996) Asymmetric localization of a mammalian numb homolog during mouse cortical neurogenesis. Neuron 17: 43–53. Zimmer C, Tiveron MC, Bodmer R, et al. (2004) Dynamics of Cux2 expression suggests that an early pool of SVZ precursors is fated to become upper cortical layer neurons. Cerebral Cortex 14(12): 1408–1420.
Cerebral Cortex: Symmetric vs. Asymmetric Cell Division G Fishell and C Hanashima, New York University Medical Center, New York, NY, USA ã 2009 Elsevier Ltd. All rights reserved.
Introduction The human cerebral cortex contains approximately 1010 cells that exhibit a wide array of physiological properties; these cells are wired together by synaptic connections that show remarkable specificity. This highly ordered structure is organized along two axes. Vertically, the cerebral cortex can be divided into six discrete laminae, each of which displays distinct afferent and efferent connections. Horizontally, cortical columns within each region of the cortex subserve specific aspects of cortical function, such as processing somatosensory or visual information or coordinating motor outputs. Despite this impressive architecture, cortical neurons can be grossly subdivided into two major classes, the excitatory pyramidal or principal neurons (80%) that are glutamatergic and provide the major structural inputs and outputs of the cortex, and the inhibitory GABAergic interneurons (20%) that regulate cortical function through local connections. Pyramidal neurons with characteristic axonal connections, dendritic morphologies, and physiological properties are segregated in each of the different cortical layers. For instance, morphologically, deep-layer (V/VI) neurons have a bigger soma compared to the superficial-layer (II/III) neurons, which typically contain smaller cell bodies. Moreover, a majority of layer V/VI pyramidal neurons project to subcortical structures, including the thalamus, basal ganglia, and spinal cord, whereas layer II/III neurons project intracortically to other cortical areas. Similarly, cortical interneurons at least in some cases are enriched within specific cortical laminae. While the cell bodies of the Martinotti interneurons are found predominantly in the deep layers of the cortex, the neuroglioform interneurons are preferentially found in the superficial cortex. A similar subdivision of both of these populations occurs within the horizontal plane, as best shown by the expansion of input cortical layers (layers I and IV) in sensory regions such as visual cortex and enlargement of output layers of the cortex (layers V and VI) in motor regions. Understanding how such exquisite organization is established during development is one of the major challenges in developmental neurobiology. Efforts to understand how cortical cellular diversity arises dates back to the late ninteenth century. The first systematic attempts to explore the organization of the developing brain came from the study of early
neurogenesis, a time when epithelial cells span their fibers across the entire cortical wall. These epithelial cells, which appear to be equivalent to what we today call ‘radial glial cells,’ were referred to by His as ‘spongioblasts’ based on their elongated spongelike appearance and were thought to give rise solely to glial cells. The other ‘cell type’ observed by His was the rounded ‘germinal cells’ that reside at the apical surface of the neural tube, which he proposed were the mitotically active neuronal progenitors. This view was challenged by Magini, who observed dramatic morphological variations within the spongioblasts, and suggested that the spongioblasts and germs cells were simply the same population at different points in the cell cycle, an idea that was later confirmed by Sauer. Sauer reached this conclusion through the observation that cell nuclei at the basal side of the neural tube contain an increased complement of DNA (i.e., S phase), while those adjacent to the ventricle were tetraploid (i.e., M phase). This suggested that neural progenitors undergo ‘interkinetic nuclear migration’ as they progress through the cell cycle and concurrently undergo dynamic changes in their cellular morphology and nuclear position. That this occurred was definitively shown in 1959 where the movements of dividing cells were followed using post hoc analysis of cells previously labeled with [3H]thymidine. This autoradiographic method revealed that cortical progenitor cells indeed undergo DNA replication at a distance from the ventricular surface, and then subsequently translocate to the apical surface, where the cells round up and undergo mitosis. From these divisions arise daughter cells that either terminally differentiate into neurons and migrate to the pia or undergo a second cycle of proliferation. Although nuclei prior to neurogenesis translocate across the entire width of the neural tube, subsequent to the onset of neurogenesis, interkinetic migration is confined to a region adjacent to the lateral ventricle. The geographical restriction of cells undergoing interkinetic nuclear migration led to this region being designated as the ‘ventricular zone.’ Within the cortex, postmitotic cells exit the ventricular zone and migrate along radial glia toward the pial surface. Surprisingly, the structural role of radial glial cells in guiding migration in the cortex was established long before it was appreciated that the radial glia were themselves the progenitor population that gave rise to newborn neurons. Indeed, for the three decades after radial glial cells were known to support neuronal migration, the radial glia and cortical neuronal progenitors were considered to be two distinct cell lineages, with the former solely
786 Cerebral Cortex: Symmetric vs. Asymmetric Cell Division Pia
Basal SVZ Radial glial progenitor VZ Apical G1
S
G2
Postmitotic neuron
M SVZ progenitor Pia VZ:
ventricular zone
SVZ:
subventricular zone
Basal SVZ
VZ Apical G1
S
G2
M
Figure 1 Asymmetric and symmetric cell division in the cerebral cortex. Top panel: Radial glial progenitors undergo interkinetic nuclear migration during the cell cycle. One asymmetric cell division results in the production of two daughter cells, one of which remains in the VZ and inherits the radial fiber and the other of which translocates outward toward the pia along the radial fiber to become a postmitotic neuron. Bottom panel: Alternatively, SVZ progenitors arise from the VZ and subsequently undergo symmetric divisions to produce two postmitotic neurons. Adapted from Noctor SC, Martinez-Cerdeno V, Ivic L, et al. (2004) Cortical neurons arise in symmetric and asymmetric division zones and migrate through specific phases. Nature Neuroscience 7: 136–144.
producing astrocytes. However, the seminal paper by Noctor and Kriegstein used time-lapse imaging of retrovirally infected cortical progenitors to demonstrate that radial glial cells, in addition to supporting the migration of nascent neurons, gave rise to them. These studies revealed that neurons assembled in close radial units are not only clonally related, but also that radial glial cells can simultaneously give rise to both differentiated neurons and progenitor cells (Figure 1). This once and for all established that radial glial cells are the major and perhaps sole progenitor for cortical pyramidal neurons. Moreover, although previous analysis suggested that asymmetric cell divisions occurred in the cerebral cortex, this analysis provided the first definitive evidence establishing the existence of this phenomenon.
Cell-Type Specification in the Neocortex Temporal Determinants: Birthdate and Laminar Fate
How is the diversity of distinct layer neuronal subtypes created from a restricted progenitor pool within the VZ? The VZ appears to utilize temporal cues to sequentially produce different subclasses of neurons, rather than simultaneously generating multiple
progenitor pools, each of which gives rise to particular cell type. Studies examining the origin of distinct neuronal subtypes indicate specific neuronal populations arise at precise times during development, presumably in response to changing temporal determinants. The first evidence that birthdate was a strong predictor of neuronal fate in the cerebral cortex came from [3H] thymidine studies in rodents; it was revealed that the administration of a tritiated thymidine injection at distinct times during neurogenesis resulted in the labeling of neurons within particular cortical laminae. Moreover, this occurred in an inside-out fashion, where early-born neurons occupy the deepest position within the cerebral cortex, while later-born cells migrate past their ascendants, to take up more superficial positions. Hence, in the mature cortex, layer VI neurons are born first and layer II neurons are born last. The only exception to this rule is the earliest-born neurons, the Cajal–Retzius (RC) cells that transiently reside in layer I. The inside-out relation of laminar position to birthdate, although evident in other mammals, is more precise in higher species, such as primates. This has been interpreted as suggesting that the progenitors within the ventricular zone undergo multiple cell cycle divisions, resulting in the production of neurons destined for sequentially more superficial positions.
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Direct evidence for the sequential production of cortical neurons came from both static and liveimaging clonal analysis of ventricular progenitors using retroviruses. These studies revealed that individual cortical progenitors produced multiple subtypes over a prolonged period during cortical development. These clonal and birthdating studies raised the question as to how neural progenitors in the ventricular zone change their distinct laminar fate according to temporal cues. One possibility is that extrinsic cues surrounding the ventricular zone change over time to influence the fate of the progenitor cells. Alternatively, it was proposed that progenitors cells might be intrinsically programmed to sequentially produce distinct neuron types at particular developmental timepoints. To test the relative contribution of extrinsic and intrinsic factors to temporal cortical cell-fate determination, classical transplantation experiments in ferrets were carried out to heterochronically challenge ventricular zone progenitors by transplanting them reciprocally to earlier or later developmental timepoints. Specifically, in these experiments early-born progenitors fated for deep layers of cortex (layer VI) were transplanted to periods of development when late-born neurons within layers II/III were being generated. Conversely, cortical progenitors that would normally produce layers II/III were transplanted into host cortical ventricular zone at a time when layer V was being generated. The results revealed a forward ratcheting in the potential of progenitors as development progressed. While early progenitors were found to be competent to adopt later fates when transplanted to older hosts (i.e., progenitors normally fated to produce layer V neurons upon transplantation to later stages of development could produce layer II/III neurons), the converse was not true (i.e., progenitors from later timepoints in development did not produce earlier-born populations of neurons even when transplanted to host animals where earlier populations of neurons are being produced). These results highlight that the cell fate in the cerebral cortex is largely established by intrinsic determinants and that progenitor potential becomes progressively restricted as development progresses. Interestingly, while the molecular mechanism governing these fate restrictions is not well understood, timed transplants of early-born cells to late donors revealed that the fate of neurons is set within 6 h of their final S phase. Asymmetric and Symmetric Division of Cortical Progenitors Function in the Generation of Cortical Cell Diversity
What is the mechanism by which a common pool of cortical progenitors within the germinal zone gives rise to a broad diversity of neurons over a prolonged
period of time? It has been postulated that the mode of cellular division utilized by the progenitor is central to this process. During early periods of neurogenesis, a majority of cortical progenitors undergo symmetric divisions that produce two progenitor cells, in order to expand the cortical progenitor pool. However, that a common progenitor produces distinct types of neurons over multiple cell divisions also implies that later there is a second process by which two daughter cells can assume distinct cell fates. Given that postmitotic daughter cells are produced sequentially, it suggests the existence of asymmetric divisions where one daughter cell reenters the cell cycle, while the other exits to differentiate into a mature neuron. That this can occur hints that during an asymmetric division the two daughter cells adopt fundamentally distinct intrinsic cell-fate programs, where asymmetric divisions unequally bestow cell-fate determinants into only one of the two daughter cells. Considerable efforts have been made to identify these ‘determinants.’ Drosophila neurogenesis has provided a perfect context to study this question, as neuroblasts in the central nervous system (CNS) as well as sensory organ precursor cells in the PNS undergo asymmetric divisions, depending on the precise orientation of their cleavage plane. Notably, the protein Numb, together with Prospero and the adaptor proteins Miranda and Pon, are asymmetrically localized to the basal surface of neuroblasts during metaphase. Therefore, when the orientation of the cleavage plane is horizontal to the apical surface, the daughter cell on the basal side inherits the majority of determinants, such as Numb protein. Thus, asymmetrical cell division requires two events, establishing the cell polarity by regulating the orientation of the mitotic spindle along the apical–basal axis, and targeting Numb and associated proteins to the basal side. Identification of asymmetric cell-fate determinants has come from analysis of the aberrations in the neural lineages of mutant flies, where one of the critical determinants, such as Numb or Prospero, is lost. The loss of these results in a randomized spindle orientation and a corresponding perturbed cell fate. Although the precise function of asymmetric localization of Numb is not clear, it is likely that Numb functions as an inhibitor of Notch signaling, which is required for maintaining neuroblasts in a progenitor state. In the cerebral cortex, it has been suggested that the plane of cell cleavage may also determine the fate of the resulting daughter cells. Examination of the ferret cortex has shown that around 15% of the mitosis in the ventricular zone has a horizontally oriented cleavage plane, where, following division, one daughter cell retains the apical contact and remains in the ventricular zone, while the basal daughter cell migrates away from the ventricular
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zone and becomes a postmitotic neuron. Interestingly, the mouse homolog of Drosophila Numb, m-Numb, is asymmetrically localized to the apical membrane of the dividing progenitors in the ventricular zone, suggesting a conserved function of Numb in determining progenitor outcome in mammals. However, a majority of the cell divisions exhibit a vertical or randomly oriented cleavage plane. Thus, the cleavage orientation is not clearly bifurcated into distinct division modes as is the case in flies. Hence, as the 15% of the horizontal cleavage cannot account for all neuronal production during peak neurogenesis period, the relationship between the cleavage plane and asymmetric division, and its contribution to developmental fate in the cerebral cortex, need to be clarified. What, then, is the function of Numb and Notch proteins in the cerebral cortex? In mammals, the interpretation of loss-of-function of either of the genes is not straightforward, as there are multiple homologs of both genes that appear to functionally compensate for each other. In mouse two numb homologs exist, mouse numb (m-numb) and numblike (nbl). Mice that lack m-numb have highly perturbed development and die prior to E11.5. Although the nbl –/– mutant alone does not exhibit an apparent neural developmental phenotype, this is likely a result of functional compensation between these two genes, as double mutants demonstrate a more severe phenotype and die around E8.5. To circumvent this early lethality, a conditional allele of m-numb was generated to test the requirement of this gene during the neurogenic period. The outcome of these experiments suggested that Numb has pleiotropic roles during the corticogenesis period, as distinct phenotypes were observed, depending on when the gene was removed. Conditionally ablating m-numb and nbl with Crerecombinase driven under the Nestin promoter, which specifically removes genes starting around E8.5, resulted in an early depletion of the cortical progenitor pool. Similarly, compound removal of m-numb and nbl during later corticogenesis using a late Cre driver, D6-Cre (which becomes active only after E10.5), also suggested that both genes are required to maintain the progenitor cells in an undifferentiated state. However, removal of m-numb/nbl gene function using an Emx1-Cre (which becomes active at E9.5 and results in complete recombination within the cerebral cortex by E12.5) resulted in hyperproliferation of the cortical progenitors as well as inhibition of neuronal differentiation, suggesting that in this context Numb acts to direct progenitors to a neuronal fate. Taken together it is apparent that further work is needed to assess why the function of Numb is diametrically opposite, depending on the context of its removal.
More clear-cut is the function of Notch signaling during neurogenesis. Loss of Notch signaling results in the precocious differentiation of neural progenitors, while overexpressing a constitutively active form of Notch (caNotch) results in inhibition of neurogenesis and promotion of a radial glia cell fate. It is notable that the transient ectopic activation of Notch signaling early in neurogenesis during the period when deep-layer neurons are produced does not disrupt the normal temporal progression of neurogenesis. Upon removal of the ectopic Notch activation, upper-layer corticogenesis resumes normally. Hence even though the transient activation of Notch prevents progenitors from differentiating, on removal of Notch signaling the normal temporal progression of neurogenesis resumes. This implies that the primary function of Notch in cortical development is to maintain the progenitors in an undifferentiated state, but it does so without disrupting the developmental clock that controls the competence of progenitors. In the future, better understanding of the precise roles of Numb and Notch in cortical development, as well as their genetic and biochemical interactions, should prove informative. In addition, whether asymmetrical cell division determines cell fate or merely controls the length of corticogenesis as a means of expanding the number of neural progenitors needs to be clarified. Does the Segregation of Progenitors to the Ventricular Zone and Subventricular Zone Represent a Bifurcation of Cell Fates?
The types of neurons produced in the cerebral cortex are clearly restricted by temporal cues within the ventricular zone; however, the mechanisms that determine layer neuron specification may not solely depend on temporal changes in progenitor competence. Recent studies have suggested that the restriction in neuronal class production can occur by a spatial segregation of cortical progenitors. In this regard, the emergence of the secondary germinal zone in the neocortex, the subventricular zone (SVZ), may play an important role in determining the identity of upper-layer neurons in the cerebral cortex. The SVZ is a transient zone that is juxtapositioned between the ventricular zone and the intermediate zone. It was traditionally defined by those progenitors that do not undergo interkinetic nuclear migration as they transit through the cell cycle. The SVZ is also distinguishable from the ventricular zone by its lack of pseudostratified epithelial morphology. It has been postulated that since SVZ develops later than the VZ, SVZ progenitors predominantly give rise to the glial cells at the end of corticogenesis. However, since the peak neurogenesis period of upper-layer (layer II–IV) neurons overlaps with the
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expansion of SVZ progenitors, the alternative view is that the spatial segregation between SVZ and VZ may directly restrict the fate of distinct layer neurons. This hypothesis is further supported by the strong correlation in genes expressed in both SVZ and upper-layer neurons. Two genes, Svet1 and Cux2, specifically label mitotic cells in the SVZ. Notably, the level of expression in the SVZ population declines as the progenitors in this pool are depleted. In turn, Svet1 and Cux2 are expressed in postmitotic neurons that comprise neurons in layers II–IV. Beyond this simple correlation in gene expression, mutants that have abnormal development in SVZ also show abnormalities in the differentiation of upper layers of cortex. In Pax6 mutants, the expression of Svet1 and Cux2 is downregulated in SVZ progenitors, and the upper-layer neurons fail to be properly established. Furthermore, expression of both Svet1 and Cux2 is inverted in reeler mutants, suggesting that these genes define the intrinsic properties of upper-layer neurons rather than their laminar position or migratory patterns. It is notable that progenitors change their mode of cell division as they transit from the VZ to the SVZ. During corticogenesis, a majority of VZ progenitors undergo asymmetric divisions, in which the two daughter cells adopt distinct fates to remain a progenitor or become a postmitotic neuron (Figure 1, top panel). However, SVZ progenitors are more likely to follow a symmetric pattern of division, where two daughter cells either become a pair of postmitotic neurons or together remain as progenitors (Figure 1, bottom panel). It is worth mentioning that not all the progenitors undergo mitosis in the SVZ, even during the peak neurogenesis of upper-layer neurons, and it remains possible that within the VZ and the SVZ there exists greater heterogeneity of progenitors than has yet to be appreciated. In this regard it is notable that in the primate neocortex studies of neurogenesis have subdivided the SVZ into inner and outer zones, the latter of which has been described to be another major source of cortical neurons. The emergence of SVZ during evolution may be the consequence of an evolutional strategy of expanding the diversity or numbers of cortical neurons that can be produced at a given time. Hence during evolution, in addition to the size and complexity of the architecture of the cerebral cortex, the cellular diversity may also have increased. Intrinsic Factors of Cortical Cell-Fate Specification
Fundamental to cortical neurogenesis is the prolonged production of different siblings from a common pool of progenitors within the VZ and SVZ. This
almost certainly depends on intrinsic cues to direct distinct neuronal fates. This process allows the progenitor pool to expand and simultaneously produce different cell types at precise times during neural development. What, then, is the identity of temporal determinants that alter the fate of the neuron types produced? As previously mentioned, cortical progenitors appear to utilize a ratcheting mechanism by which the neuronal types produced over time become progressively restricted. Moreover, later-born progenitors retain a limited ability to differentiate into neuron types prior to their own birthdate. In rodents, the molecular identity of this progressive restriction is best understood in the switch from generating the earliest-born CR cells to deep-layer projection neurons. This process is regulated by the transcriptional repressor Foxg1. In the cerebral cortex, Foxg1 is expressed in the majority of the cortical projection neurons, with the exception of layer I CR neurons. Moreover, in the absence of Foxg1, cortical progenitors fail to generate later-born neurons and instead continue to produce the earliest-born CR neurons. In order to determine the cell-autonomous function of Foxg1 in cell-fate specification, Foxg1 was conditionally inactivated in deep-layer progenitors. Interestingly, removal of Foxg1 at E13 (the birthdate of deep-layer neurons) results in the resumption of CR cell production in the cerebral cortex. By labeling the progenitors that were born subsequent to the removal of Foxg1 with BrdU, it was confirmed that the cells that were normally destined to become deep-layer neurons instead adopted an earliest-cell fate. This work has recently been replicated in vitro, where the fate of individual neuroblasts can be directly followed. These studies suggest the early cell-fate specification in the mammalian cerebral cortex is determined by an active mechanism whereby a transcriptional repressor prevents the later-born neurons from adopting an earlier fate. How is the cortical cell fate established during the subsequent corticogenesis period? It has been reported that a zinc-finger-containing transcription factor, Fezl, which is specifically expressed in corticospinal motor neurons in layer V, directs both the specification and identity of these cell types. Loss of Fezl results in both the loss of corticospinal neurons and the failure of subcortical projections. Furthermore, overexpression of Fezl near the end of layer V genesis can partially override the transition of cortical progenitors to produce neurons that migrate to layer IV, which instead send a projection through the internal capsule. It remains to be explored whether the competence window of cortical progenitors responding to either Foxg1 or Fezl is limited. Beyond this, it seems clear that a great deal
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more remains to be learned concerning the intrinsic mechanisms by which temporal competence is controlled within the cerebral cortex. A Cascade of Intrinsic Determinants Controls the Production of Neurons in Drosophila
The progressive restriction of progenitors appears to be central for the specification of distinct cell types in a variety of tissues, such as in the retina and hematopoietic systems. This mechanism appears to be evolutionarily conserved, as similar rules appear to underlie the specification of cells in the nerve cord of invertebrates. In particular, work in Drosophila has begun to elucidate the molecular determinants controlling the precisely orchestrated production of different neuronal subtypes during development. In flies, the cell-lineage relationships of distinct neuron types have been well characterized and transcription factors that are specifically expressed in neuroblasts at distinct times have been shown to be required for the acquisition of specific neuronal identities. For instance, the well-characterized NB7–1 lineage consists of 30þ neurons, including 5 Eveþ neurons (U1–U5) and their siblings, and subsequently 20þ interneurons. Analogous to the mammalian cortical neurons, the earliest-born U1 neurons reside in the deepest position and project a longest distance to their target muscle, whereas the latest-born U5 neurons reside superficially and send their processes only proximally. The neuroblasts that produce the 5 Eveþ distinct types express Hunchback (Hb), Hb/Kruppel (Kr), Kr, Pdm, and Pdm/Castor (Cas). It has been demonstrated by removing the function or overexpressing these genes that both Hb and Kr are necessary and sufficient to determine the first-born and second-born cell fates, respectively. Furthermore, overexpression of Hb induces Kr, and similarly expression of Kr induces Pdm, showing that sequential expression of these transcription cascades is tightly regulated to produce these distinct neuron types in an invariant order. It has also been demonstrated that the sequential expression of fate-determining genes is largely cell intrinsic, as the expression occurs in isolated neuroblasts in culture and even can partially progress in the absence of cell cycle progression (Kr-Pdm-Cas). Although corresponding mammalian homologs of Hb (Ikaros family genes), Pdm (SCIP/ Oct-6), and Cas (Casz1) exist, the contribution of these factors to temporal lineage progression is not equivalent (SCIP) or has not been addressed (Ikaros, Casz1). It is also noteworthy that in Drosophila, the temporal progression in neuroblast identity is dependent on transcriptional factors that positively
promote neuroblast identity. This is in contrast with the model in the mammalian cerebral cortex, where at least during the earliest corticogenesis, cell fate is regulated by a repressive mechanism by which a transcriptional repressor, Foxg1, prevents deep-layer neurons from adopting an earliest-born CR cell fate. The extent to which future analysis will reveal further similarities between mammalian cortical neurogenesis and that seen in the Drosophila nerve cord is presently unclear. Regardless of the precise details, it seems likely that the study of Drosophila neuroblasts will prove informative in helping us understand how cortical progenitors generate the diversity seen in the mature cortex. Furthermore, it seems inevitable that considerable progress will be forthcoming concerning the specific transcription cascades involved in generating cerebral cortex neuron diversity, as well as with regard to the mechanisms that ensure symmetric and asymmetric cell divisions that allow the production of the appropriate numbers of neurons at the appropriate times. See also: Cerebral Cortex: Inhibitory Cells; Cerebral Cortex; Memory Consolidation: Cerebral Cortex; Neocortex: Origins; Neurogenesis in the Intact Adult Brain; Neurogenesis and Neural Precursors, Progenitors, and Stem Cells in the Adult Brain; Radial Glial Cells: Brain Functions.
Further Reading Chenn A and McConnell SK (1995) Cleavage orientation and the asymmetric inheritance of Notch1 immunoreactivity in mammalian neurogenesis. Cell 82: 631–641. Desai AR and McConnell SK (2000) Progressive restriction in fate potential by neural progenitors during cerebral cortical development. Development 127: 2863–2872. Fishell G and Kriegstein AR (2003) Neurons from radial glia: The consequences of asymmetric inheritance. Current Opinion in Neurobiology 13: 34–41. Hanashima C, Li SC, Shen L, et al. (2004) Foxg1 suppresses early cortical cell fate. Science 303: 56–59. Lu B, Jan L, and Jan YN (2000) Control of cell divisions in the nervous system: Symmetry and asymmetry. Annual Review of Neuroscience 23: 531–556. McConnell SK and Kaznowski CE (1991) Cell cycle dependence of laminar determination in developing neocortex. Science 254: 282–285. Mizutani K and Saito T (2005) Progenitors resume generating neurons after temporary inhibition of neurogenesis by Notch activation in the mammalian cerebral cortex. Development 132: 1295–1304. Molyneaux BJ, Arlotta P, Hirata T, et al. (2005) Fez1 is required for the birth and specification of corticospinal motor neurons. Neuron 47: 817–831. Noctor SC, Flint AC, Weissman TA, et al. (2001) Neurons derived from radial glial cells establish radial units in neocortex. Nature 409: 714–720.
Cerebral Cortex: Symmetric vs. Asymmetric Cell Division 791 Noctor SC, Martinez-Cerdeno V, Ivic L, et al. (2004) Cortical neurons arise in symmetric and asymmetric division zones and migrate through specific phases. Nature Neuroscience 7: 136–144. Pearson BJ and Doe CQ (2004) Specification of temporal identity in the developing nervous system. Annual Review of Cell and Developmental Biology 20: 619–647. Reid CB, Tavazoie SF, and Walsh CA (1997) Clonal dispersion and evidence for asymmetric cell division in ferret cortex. Development 124: 2441–2450.
Tarabykin V, Stoykova A, Usman N, et al. (2001) Cortical upper layer neurons derive from the subventricular zone as indicated by Svet1 gene expression. Development 128: 1983–1993. Zhong W, Feder JN, Jiang MM, et al. (1996) Asymmetric localization of a mammalian numb homolog during mouse cortical neurogenesis. Neuron 17: 43–53. Zimmer C, Tiveron MC, Bodmer R, et al. (2004) Dynamics of Cux2 expression suggests that an early pool of SVZ precursors is fated to become upper cortical layer neurons. Cerebral Cortex 14(12): 1408–1420.