Role of Nrp1 in controlling cortical inter-hemispheric circuits.

Axons of the corpus callosum (CC) mediate the interhemispheric communication required for complex perception in mammals. In the somatosensory (SS) cortex, the CC exchanges inputs processed by the primary (S1) and secondary (S2) areas, which receive tactile and pain stimuli. During early postnatal life, a multistep process involving axonal navigation, growth, and refinement, leads to precise CC connectivity. This process is often affected in neurodevelopmental disorders such as autism and epilepsy. We herein show that in mice, expression of the axonal signaling receptor Neuropilin 1 (Nrp1) in SS layer (L) 2/3 is temporary and follows patterns that determine CC connectivity. At postnatal day 4, Nrp1 expression is absent in the SS cortex while abundant in the motor area, creating a sharp border. During the following 3 weeks, Nrp1 is transiently upregulated in subpopulations of SS L2/3 neurons, earlier and more abundantly in S2 than in S1. In vivo knock-down and overexpression experiments demonstrate that transient expression of Nrp1 does not affect the initial development of callosal projections in S1 but is required for subsequent S2 innervation. Moreover, knocking-down Nrp1 reduces the number of S2L2/3 callosal neurons due to excessive postnatal refinement. Thus, an exquisite temporal and spatial regulation of Nrp1 expression determines SS interhemispheric maps.

Developmental Phase Transitions in Spatial Organization of Spontaneous Activity in Postnatal Barrel Cortex Layer 4.

Spatially-organized spontaneous activity is a characteristic feature of developing mammalian sensory systems. However, the transitions of spontaneous-activity spatial organization during development and related mechanisms remain largely unknown. We reported previously that layer 4 (L4) glutamatergic neurons in the mouse barrel cortex exhibit spontaneous activity with a patchwork-type pattern at postnatal day (P)5, which is during barrel formation. In the current work, we revealed that spontaneous activity in mouse barrel-cortex L4 glutamatergic neurons exhibits at least three phases during the first two weeks of postnatal development. Phase I activity has a patchwork-type pattern and is observed not only at P5, but also P1, before barrel formation. Phase II is found at P9, by which time barrel formation is completed, and exhibits broadly synchronized activity across barrel borders. Phase III emerges around P11 when L4-neuron activity is desynchronized. The Phase I activity, but not Phase II or III activity, is blocked by thalamic inhibition, demonstrating that the Phase I to II transition is associated with loss of thalamic dependency. Dominant-negative (DN)-Rac1 expression in L4 neurons hampers the Phase II to III transition. It also suppresses developmental increases in spine density and excitatory synapses of L4 neurons in the second postnatal week, suggesting that Rac1-mediated synapse maturation could underlie the Phase II to III transition. Our findings revealed the presence of distinct mechanisms for Phase I to II and Phase II to III transition. They also highlighted the role of a small GTPase in the developmental desynchronization of cortical spontaneous activity.SIGNIFICANCE STATEMENT Developing neocortex exhibits spatially-organized spontaneous activity, which plays a critical role in cortical circuit development. The features of spontaneous-activity spatial organization and the mechanisms underlying its changes during development remain largely unknown. In the present study, using two-photon in vivo imaging, we revealed three phases (Phases I, II, and III) of spontaneous activity in barrel-cortex layer 4 (L4) glutamatergic neurons during the first two postnatal weeks. We also demonstrated the presence of distinct mechanisms underlying phase transitions. Phase I to II shift arose from the switch in the L4-neuron driving source, and Phase II to III transition relied on L4-neuron Rac1 activity. These results provide new insights into the principles of developmental transitions of neocortical spontaneous-activity spatial patterns.

COUP-TFI/Nr2f1 Orchestrates Intrinsic Neuronal Activity during Development of the Somatosensory Cortex.

The formation of functional cortical maps in the cerebral cortex results from a timely regulated interaction between intrinsic genetic mechanisms and electrical activity. To understand how transcriptional regulation influences network activity and neuronal excitability within the neocortex, we used mice deficient for Nr2f1 (also known as COUP-TFI), a key determinant of primary somatosensory (S1) area specification during development. We found that the cortical loss of Nr2f1 impacts on spontaneous network activity and synchronization of S1 cortex at perinatal stages. In addition, we observed alterations in the intrinsic excitability and morphological features of layer V pyramidal neurons. Accordingly, we identified distinct voltage-gated ion channels regulated by Nr2f1 that might directly influence intrinsic bioelectrical properties during critical time windows of S1 cortex specification. Altogether, our data suggest a tight link between Nr2f1 and neuronal excitability in the developmental sequence that ultimately sculpts the emergence of cortical network activity within the immature neocortex.

Cortical distribution of GABAergic interneurons is determined by migration time and brain size.

Cortical interneurons (CINs) originate in the ganglionic eminences (GEs) and migrate tangentially to the cortex guided by different attractive and repulsive cues. Once inside the cortex, the cellular and molecular mechanisms determining the migration of CINs along the rostrocaudal axis are less well understood. Here, we investigated the cortical distribution of CINs originating in the medial and caudal GEs at different time points. Using molecular and genetic labeling, we showed that, in the mouse, early- and late-born CINs (E12 versus E15) are differentially distributed along the rostrocaudal axis. Specifically, late-born CINs are preferentially enriched in cortical areas closer to their respective sites of origin in the medial or caudal GE. Surprisingly, our in vitro experiments failed to show a preferential migration pattern along the rostrocaudal axis for medial- or caudal-born CINs. Moreover, in utero transplantation experiments suggested that the rostrocaudal dispersion of CINs depends on the developmental stage of the host brain and is limited by the migration time and the increasing size of the developing brain. These data suggest that the embryonic expansion of the cortex contributes to the rostrocaudal distribution of CINs.

Long-lived zebrafish Rohon-Beard cells.

During normal development of the nervous system, extensive neuronal proliferation as well as death occurs. The extent of development death varies considerably between neuronal populations from little to almost 100%. Early born somatosensory neurons, known as Rohon-Beard cells, have served as an example of neurons that disappear during early developmental stages, presumably as their function is taken over by later developing dorsal root ganglion neurons. However, recent studies have raised questions about the extent to which zebrafish Rohon-Beard cells die during embryogenesis. While Rohon-Beard cells have distinguishing morphological features during embryonic stages development, they subsequently undergo substantial changes in their shape, size and position that hinder their unambiguous identification at later stages. To overcome this obstacle, we identify Rohon-Beard cells at one day, and using a combination of mosaic and stable transgenic labeling and repeated observation, follow them for 13-16 days post fertilization. We find that about 40% survive to late larval stages. Our studies also reveal that Rohon-Beard cells display an unusual repertoire of cell death properties. At one day, about 25% Rohon-Beard cells expose phosphatidyl serine at the surface membrane, but less than one Rohon-Beard cell/embryo expresses activated-caspase-3. Further, the temporal delay between detection of cell death markers and loss of the soma ranges from

Dorsal-to-Ventral Cortical Expansion Is Physically Primed by Ventral Streaming of Early Embryonic Preplate Neurons.

Despite recent studies elucidating the molecular mechanisms underlying cortical patterning and map formation, very little is known about how the embryonic pallium expands ventrally to form the future cortex and the nature of the underlying force-generating events. We find that neurons born at embryonic day 10 (E10) in the mouse dorsal pallium ventrally stream until E13, thereby superficially spreading the preplate, and then constitute the subplate from E14. From E11 to E12, the preplate neurons migrate, exerting pulling and pushing forces at the process and the soma, respectively. At E13, they are morphologically heterogeneous, with ∼40% possessing corticofugal axons, which are found to be in tension. Ablation of these E10-born neurons attenuates both deflection of radial glial fibers (by E13) and extension of the cortical plate (by E14), which should occur ventrally, and subsequently shrinks the postnatal neocortical map dorsally. Thus, the preplate stream physically primes neocortical expansion and arealization.

In utero Exposure to Anesthetics Alters Neuronal Migration Pattern in Developing Cerebral Cortex and Causes Postnatal Behavioral Deficits in Rats.

During fetal development, cerebral cortical neurons are generated in the proliferative zone along the ventricles and then migrate to their final positions. To examine the impact of in utero exposure to anesthetics on neuronal migration, we injected pregnant rats with bromodeoxyuridine to label fetal neurons generated at embryonic Day (E) 17 and then randomized these rats to 9 different groups receiving 3 different means of anesthesia (oxygen/control, propofol, isoflurane) for 3 exposure durations (20, 50, 120 min). Histological analysis of brains from 54 pups revealed that significant number of neurons in anesthetized animals failed to acquire their correct cortical position and remained dispersed within inappropriate cortical layers and/or adjacent white matter. Behavioral testing of 86 littermates pointed to abnormalities that correspond to the aberrations in the brain areas that are specifically developing during the E17. In the second set of experiments, fetal brains exposed to isoflurane at E16 had diminished expression of the reelin and glutamic acid decarboxylase 67, proteins critical for neuronal migration. Together, these results call for cautious use of anesthetics during the neuronal migration period in pregnancy and more comprehensive investigation of neurodevelopmental consequences for the fetus and possible consequences later in life.

Prenatal activity from thalamic neurons governs the emergence of functional cortical maps in mice.

The mammalian brain's somatosensory cortex is a topographic map of the body's sensory experience. In mice, cortical barrels reflect whisker input. We asked whether these cortical structures require sensory input to develop or are driven by intrinsic activity. Thalamocortical columns, connecting the thalamus to the cortex, emerge before sensory input and concur with calcium waves in the embryonic thalamus. We show that the columnar organization of the thalamocortical somatotopic map exists in the mouse embryo before sensory input, thus linking spontaneous embryonic thalamic activity to somatosensory map formation. Without thalamic calcium waves, cortical circuits become hyperexcitable, columnar and barrel organization does not emerge, and the somatosensory map lacks anatomical and functional structure. Thus, a self-organized protomap in the embryonic thalamus drives the functional assembly of murine thalamocortical sensory circuits.

Ethanol Exposure In Utero Disrupts Radial Migration and Pyramidal Cell Development in the Somatosensory Cortex.

Deficits in sensory processing in Fetal Alcohol Spectrum Disorders (FASD) implicate dysfunction in the somatosensory cortex. However, the effects of prenatal ethanol exposure on the development of this region await elucidation. Here, we used an established mouse model of FASD with binge-type ethanol exposure from embryonic day 13.5-16.5 to investigate the effects of prenatal ethanol exposure on pyramidal neurons in the somatosensory cortex. Specifically, we focused on the radial migration of primordial pyramidal neurons during embryonic corticogenesis and their morphology and function during active synaptogenesis in early postnatal development. We found that prenatal ethanol exposure resulted in aberrant radial migration, particularly affecting the populations of postmitotic pyramidal neurons. In addition, there was an enduring effect of prenatal ethanol exposure on glutamate-mediated synaptic transmission in layer V/VI pyramidal neurons. This persisted beyond a transient decrease in pyramidal neuron dendritic complexity that was evident only during early postnatal development. Adolescent mice exposed prenatally to ethanol also displayed decreased tactile sensitivity, as revealed by a modified adhesive tape removal assay. Our findings demonstrate the persistent effects of binge-type in utero ethanol exposure on pyramidal neuron form and function and ultimately sensory processing, the latter being reminiscent of that seen in individuals with FASD.

Alteration of the nucleus basalis of Meynert afferents to vibrissae-related sensory cortex in de-whiskered adolescent congenital hypothyroid rats.

INTRODUCTION: Thyroid hypofunction during early development results in anatomical alterations in the cerebellum, cerebrum, hippocampus and other brain structures. The plastic organization of the nucleus basalis of Meynert (nBM) projections to the whiskers-related somatosensory (wS1) cortex in adolescent pups with maternal thyroid hypofunction and sensory deprivation was assessed through retrograde WGA-HRP labeling. METHODS: Congenital hypothyroidism induced by adding PTU (25 ppm) to the drinking water from embryonic day 16 to postnatal day (PND) 60. Pregnant rats were divided to intact and congenital hypothyroid groups. In each group, the total whiskers of pups (4 of 8) were trimmed continuously from PND 0 to PND 60. RESULTS: Following separately WGA-HRP injections into wS1, retrogradely labeled neurons were observed in nBM. The number of labeled neurons in nBM were higher in the congenital hypothyroid and whisker deprived groups compared to their controls (P < 0.05). CONCLUSION: Based on our results both congenital hypothyroidism and sensory deprivation may disturb normal development of cortical circuits in of nBM afferents to the wS1 cortex.

mGluR5 Tunes NGF/TrkA Signaling to Orient Spiny Stellate Neuron Dendrites Toward Thalamocortical Axons During Whisker-Barrel Map Formation.

Neurons receive and integrate synaptic inputs at their dendrites, thus dendritic patterning shapes neural connectivity and behavior. Aberrant dendritogenesis is present in neurodevelopmental disorders such as Down's syndrome and autism. Abnormal glutamatergic signaling has been observed in these diseases, as has dysfunction of the metabotropic glutamate receptor 5 (mGluR5). Deleting mGluR5 in cortical glutamatergic neurons disrupted their coordinated dendritic outgrowth toward thalamocortical axons and perturbed somatosensory circuits. Here we show that mGluR5 loss-of-function disrupts dendritogenesis of cortical neurons by increasing mRNA levels of nerve growth factor (NGF) and fibroblast growth factor 10 (FGF10), in part through calcium-permeable AMPA receptors (CP-AMPARs), as the whisker-barrel map is forming. Postnatal NGF and FGF10 expression in cortical layer IV spiny stellate neurons differentially impacted dendritic patterns. Remarkably, NGF-expressing neurons exhibited dendritic patterns resembling mGluR5 knockout neurons: increased total dendritic length/complexity and reduced polarity. Furthermore, suppressing the kinase activity of TrkA, a major NGF receptor, prevents aberrant dendritic patterning in barrel cortex of mGluR5 knockout neurons. These results reveal novel roles for NGF-TrkA signaling and CP-AMPARs for proper dendritic development of cortical neurons. This is the first in vivo demonstration that cortical neuronal NGF expression modulates dendritic patterning during postnatal brain development.

Early patterns of activity in the developing cortex: Focus on the sensorimotor system.

Early development of somatotopic cortical maps occurs during the fetal period in humans and during the postnatal period in rodents. During this period, the sensorimotor cortex expresses transient patterns of correlated neuronal activity including delta waves, gamma- and spindle-burst oscillations. These early activity patterns are largely driven by the thalamus and triggered, in a topographic manner, by sensory feedback resulting from spontaneous movements. Early cortical activities are instrumental for competitive interactions between sensory inputs for the cortical territories, they prevent cortical neurons from apoptosis and their alteration may lead to disturbances in cortical network development in a number of neurodevelopmental diseases.

FGF-FGFR Mediates the Activity-Dependent Dendritogenesis of Layer IV Neurons during Barrel Formation.

Fibroblast growth factors (FGFs) and FGF receptors (FGFRs) are known for their potent effects on cell proliferation/differentiation and cortical patterning in the developing brain. However, little is known regarding the roles of FGFs/FGFRs in cortical circuit formation. Here we show that Fgfr1/2/3 and Fgf7/9/10/22 mRNAs are expressed in the developing primary somatosensory (S1) barrel cortex. Barrel cortex layer IV spiny stellate cells (bSCs) are the primary recipients of ascending sensory information via thalamocortical axons (TCAs). Detail quantification revealed distinctive phases for bSC dendritogenesis: orienting dendrites toward TCAs, adding de novo dendritic segments, and elongating dendritic length, while maintaining dendritic patterns. Deleting Fgfr1/2/3 in bSCs had minimal impact on dendritic polarity but transiently increased the number of dendritic segments. However, 6 d later, FGFR1/2/3 loss of function reduced dendritic branch numbers. These data suggest that FGFs/FGFRs have a role in stabilizing dendritic patterning. Depolarization of cultured mouse cortical neurons upregulated the levels of several Fgf/Fgfr mRNAs within 2 h. In vivo, within 6 h of systemic kainic acid administration at postnatal day 6, mRNA levels of Fgf9, Fgf10, Fgfr2c, and Fgfr3b in S1 cortices were enhanced, and this was accompanied by exuberant dendritogenesis of bSCs by 24 h. Deleting Fgfr1/2/3 abolished kainic acid-induced bSC dendritic overgrowth. Finally, FGF9/10 gain of function also resulted in extensive dendritogenesis. Together, our data suggest that FGFs/FGFRs can be regulated by glutamate transmission to modulate/stabilize bSC dendritic complexity. Both male and female mice were used for our study.SIGNIFICANCE STATEMENT Glutamatergic transmission plays critical roles in cortical circuit formation. Its dysregulation has been proposed as a core factor in the etiology of many neurological diseases. We found that excessive glutamate transmission upregulated mRNA expression of Fgfrs and their ligands Fgfs Deleting Fgfr1/2/3 not only impaired bSC dendritogenesis but also abolished glutamate transmission-induced dendritic overgrowth. Overexpressing FGF9 or FGF10 in cortical glutamatergic neurons results in excessive dendritic outgrowth within 24 h, resembling the changes induced by excessive glutamate transmission. Our findings provide strong evidence for the physiological role of fibroblast growth factors (FGFs) and FGF receptors (FGFRs) in establishing and maintaining cortical circuits. Perturbing the expression levels of FGFs/FGFRs by excessive glutamatergic neurotransmission could lead to abnormal neuronal circuits, which may contribute to neurological and psychiatric disease.

K-Cl Cotransporter 2-mediated Cl- Extrusion Determines Developmental Stage-dependent Impact of Propofol Anesthesia on Dendritic Spines.

BACKGROUND: General anesthetics potentiating γ-aminobutyric acid (GABA)-mediated signaling are known to induce a persistent decrement in excitatory synapse number in the cerebral cortex when applied during early postnatal development, while an opposite action is produced at later stages. Here, the authors test the hypothesis that the effect of general anesthetics on synaptogenesis depends upon the efficacy of GABA receptor type A (GABAA)-mediated inhibition controlled by the developmental up-regulation of the potassium-chloride (K-Cl) cotransporter 2 (KCC2). METHODS: In utero electroporation of KCC2 was used to prematurely increase the efficacy of (GABAA)-mediated inhibition in layer 2/3 pyramidal neurons in the immature rat somatosensory cortex. Parallel experiments with expression of the inward-rectifier potassium channel Kir2.1 were done to reduce intrinsic neuronal excitability. The effects of these genetic manipulations (n = 3 to 4 animals per experimental group) were evaluated using iontophoretic injection of Lucifer Yellow (n = 8 to 12 cells per animal). The total number of spines analyzed per group ranged between 907 and 3,371. RESULTS: The authors found a robust effect of the developmental up-regulation of KCC2-mediated Cl transport on the age-dependent action of propofol on dendritic spines. Premature expression of KCC2, unlike expression of a transport-inactive KCC2 variant, prevented a propofol-induced decrease in spine density. In line with a reduction in neuronal excitability, the above result was qualitatively replicated by overexpression of Kir2.1. CONCLUSIONS: The KCC2-dependent developmental increase in the efficacy of GABAA-mediated inhibition is a major determinant of the age-dependent actions of propofol on dendritic spinogenesis.

Microglia contact induces synapse formation in developing somatosensory cortex.

Microglia are the immune cells of the central nervous system that play important roles in brain pathologies. Microglia also help shape neuronal circuits during development, via phagocytosing weak synapses and regulating neurogenesis. Using in vivo multiphoton imaging of layer 2/3 pyramidal neurons in the developing somatosensory cortex, we demonstrate here that microglial contact with dendrites directly induces filopodia formation. This filopodia formation occurs only around postnatal day 8-10, a period of intense synaptogenesis and when microglia have an activated phenotype. Filopodia formation is preceded by contact-induced Ca(2+) transients and actin accumulation. Inhibition of microglia by genetic ablation decreases subsequent spine density, functional excitatory synapses and reduces the relative connectivity from layer 4 neurons. Our data provide the direct demonstration of microglial-induced spine formation and provide further insights into immune system regulation of neuronal circuit development, with potential implications for developmental disorders of immune and brain dysfunction.

Cited2 Regulates Neocortical Layer II/III Generation and Somatosensory Callosal Projection Neuron Development and Connectivity.

UNLABELLED: The neocortex contains hundreds to thousands of distinct subtypes of precisely connected neurons, allowing it to perform remarkably complex tasks of high-level cognition. Callosal projection neurons (CPN) connect the cerebral hemispheres via the corpus callosum, integrating cortical information and playing key roles in associative cognition. CPN are a strikingly diverse set of neuronal subpopulations, and development of this diversity requires precise control by a complex, interactive set of molecular effectors. We have found that the transcriptional coregulator Cited2 regulates and refines two stages of CPN development. Cited2 is expressed broadly by progenitors in the embryonic day 15.5 subventricular zone, during the peak of superficial layer CPN birth, with a progressive postmitotic refinement in expression, becoming restricted to CPN of the somatosensory cortex postnatally. We generated progenitor-stage and postmitotic forebrain-specific Cited2 conditional knock-out mice, using the Emx1-Cre and NEX-Cre mouse lines, respectively. We demonstrate that Cited2 functions in progenitors, but is not necessary postmitotically, to regulate both (1) broad generation of layer II/III CPN and (2) acquisition of precise area-specific molecular identity and axonal/dendritic connectivity of somatosensory CPN. This novel CPN subtype-specific and area-specific control from progenitor action of Cited2 adds yet another layer of complexity to the multistage developmental regulation of neocortical development. SIGNIFICANCE STATEMENT: This study identifies Cited2 as a novel subtype-specific and area-specific control over development of distinct subpopulations within the broad population of callosal projection neurons (CPN), whose axons connect the two cerebral hemispheres via the corpus callosum (CC). Currently, how the remarkable diversity of CPN subtypes is specified, and how they differentiate to form highly precise and specific circuits, are largely unknown. We found that Cited2 functions within subventricular zone progenitors to both broadly regulate generation of superficial layer CPN throughout the neocortex, and to refine precise area-specific development and connectivity of somatosensory CPN. Gaining insight into molecular development and heterogeneity of CPN will advance understanding of both diverse functions of CPN and of the remarkable range of neurodevelopmental deficits correlated with CPN/CC development.

Area-specific development of distinct projection neuron subclasses is regulated by postnatal epigenetic modifications.

During cortical development, the identity of major classes of long-distance projection neurons is established by the expression of molecular determinants, which become gradually restricted and mutually exclusive. However, the mechanisms by which projection neurons acquire their final properties during postnatal stages are still poorly understood. In this study, we show that the number of neurons co-expressing Ctip2 and Satb2, respectively involved in the early specification of subcerebral and callosal projection neurons, progressively increases after birth in the somatosensory cortex. Ctip2/Satb2 postnatal co-localization defines two distinct neuronal subclasses projecting either to the contralateral cortex or to the brainstem suggesting that Ctip2/Satb2 co-expression may refine their properties rather than determine their identity. Gain- and loss-of-function approaches reveal that the transcriptional adaptor Lmo4 drives this maturation program through modulation of epigenetic mechanisms in a time- and area-specific manner, thereby indicating that a previously unknown genetic program postnatally promotes the acquisition of final subtype-specific features.

Developmental expression of Kir4.1 in astrocytes and oligodendrocytes of rat somatosensory cortex and hippocampus.

Kir4.1 is the principal K(+) channel expressed in glial cells. It has been shown that it plays a fundamental role in K(+)-spatial buffering, an astrocyte-specific process where excess extracellular concentration of K(+) ions, generated by synaptic activity, is spatially redistributed to distant sites via astrocytic syncytia. Experimental and clinical evidence suggested that abnormality of Kir4.1 function in the brain is involved in different neurological diseases such as epilepsy, dysmyelination, and Huntington's disease. Although it has been shown that Kir4.1 is expressed predominantly in astrocytes in certain areas of the rat brain and its transcript is present in the rat forebrain as early as embryonic day E14, no information is available concerning the temporal sequence of Kir4.1 protein appearance during embryonic and post-natal development. Aim of this work was to study the expression pattern of Kir4.1 channel in rat somatosensory cortex and hippocampus during development and to examine its cellular localization with the glial and oligodendroglial markers S100-ß, GFAP, and Olig-2. Kir4.1 protein was detected since E20 and a gradual increase of Kir4.1 expression occurred between early postnatal period and adulthood. We showed a gradual shift in Kir4.1 subcellular localization from the soma of astrocytes to distal glial processes. Double immunofluorescence experiments confirmed the cellular localization of Kir4.1 in glial cells. Our data provide the first overview of Kir4.1 developmental expression both in the cortex and hippocampus and support the glial role of Kir4.1 in K(+) spatial buffering.

Quantitative analysis of somatosensory cortex development in eutherians, with a comparison with metatherians and monotremes.

Extant eutherians exhibit a wide range of adult brain sizes and degree of cortical gyrification. Quantitative analysis of parietal isocortical sections held in museum collections was used to compare the pace of somatosensory cortex development relative to body size and pallial thickness among diverse eutherian embryos, foetuses, and neonates. Analysis indicated that, for most eutherians, cortical plate aggregation begins at about 6-18 mm greatest length or about 120-320 µm pallial thickness. Expansion of the proliferative compartment occurs at a similar pace in most eutherians, but exceptionally rapidly in hominoids. Involution of the pallial proliferative zones occurs over a wide range of body sizes (42 mm to over 500 mm greatest length) or when the cerebral cortex reaches a thickness of 1.2-9.8 mm depending on the eutherian group. Many of these values overlap with those for metatherians. The findings suggest that there is less evolutionary flexibility in the timing of cortical plate aggregation than in the rate of expansion of the pallial proliferative compartment and the duration of proliferative zone activity.

Satb2 Regulates the Differentiation of Both Callosal and Subcerebral Projection Neurons in the Developing Cerebral Cortex.

The chromatin-remodeling protein Satb2 plays a role in the generation of distinct subtypes of neocortical pyramidal neurons. Previous studies have shown that Satb2 is required for normal development of callosal projection neurons (CPNs), which fail to extend axons callosally in the absence of Satb2 and instead project subcortically. Here we conditionally delete Satb2 from the developing neocortex and find that neurons in the upper layers adopt some electrophysiological properties characteristic of deep layer neurons, but projections from the superficial layers do not contribute to the aberrant subcortical projections seen in Satb2 mutants. Instead, axons from deep layer CPNs descend subcortically in the absence of Satb2. These data demonstrate distinct developmental roles of Satb2 in regulating the fates of upper and deep layer neurons. Unexpectedly, Satb2 mutant brains also display changes in gene expression by subcerebral projection neurons (SCPNs), accompanied by a failure of corticospinal tract (CST) formation. Altering the timing of Satb2 ablation reveals that SCPNs require an early expression of Satb2 for differentiation and extension of the CST, suggesting that early transient expression of Satb2 in these cells plays an essential role in development. Collectively these data show that Satb2 is required by both CPNs and SCPNs for proper differentiation and axon pathfinding.