Development of the human cerebral cortex: Boulder Committee revisited

Ependymal cells: biology and pathology

The literature was reviewed to summarize the current understanding of the role of ciliated ependymal cells in the mammalian brain. Previous reviews were summarized. Publications from the past 10 years highlight interactions between ependymal cells and the subventricular zone and the possible role of restricted ependymal populations in neurogenesis. Ependymal cells provide trophic support and possibly metabolic support for progenitor cells. Channel proteins such as aquaporins may be important for determining water fluxes at the ventricle wall. The junctional and anchoring proteins are now fairly well understood, as are proteins related to cilia function. Defects in ependymal adhesion and cilia function can cause hydrocephalus through several different mechanisms, one possibility being loss of patency of the cerebral aqueduct. Ependymal cells are susceptible to infection by a wide range of common viruses; while they may act as a line of first defense, they eventually succumb to repeated attacks in long-lived organisms. Ciliated ependymal cells are almost certainly important during brain development. However, the widespread absence of ependymal cells from the adult human lateral ventricles suggests that they may have only regionally restricted value in the mature brain of large size.

The discovery of the subpial granular layer in the human cerebral cortex

The subpial granular layer (SGL) is a transient accumulation of tangentially migrating small granular neurons in the marginal zone of the developing fetal neocortex. It has recently attracted attention as a possible additional source of future cortical interneurons, or even as a putative precursor pool for generation of Cajal-Retzius cells. The discovery of the SGL is generally attributed to Otto Ranke and it is usually claimed that the SGL is specific for human brain. The aim of this review is: (1) to demonstrate that the first to observe SGL in the human cerebral cortex was not Otto Ranke in 1910, but Franz Boll in 1874; (2) to provide an English translation of Ranke’s original description of the SGL and thus demonstrate that he described the SGL in both human and animal brain; and (3) to provide a concise review of current studies concerning the developmental fate and possible functions of the transient fetal SGL.

Fate and manipulations of endogenous neural stem cells following brain ischemia

Stem cells have been proposed as a new form of cell-based therapy in a variety of disorders, including acute and degenerative brain diseases. Endogenous neural stem cells (eNSCs) have been identified in the central nervous system where they reside largely in the subventricular zone and in the subgranular zone of the hippocampus. eNSCs are capable of self-renewal and differentiation into functional glia and neurons throughout life. However, spontaneous brain regeneration does not suffice to induce significant behavioral improvement in acute or chronic brain injury. Nevertheless, eNSCs responses can be considerably increased by tweaking the pathways governing cell proliferation, migration and differentiation. Contemporary evidence now suggests that such perturbations may lead to better functional outcome after brain injury.

Long-term outcome of preterm infants and the role of neuroimaging

Preterm birth has been defined as one of the major public health problems of this decade, preterm neonates being at high risk for neurodevelopmental disabilities. As preterm survival rates increase, the next great imperative for perinatal medicine is to understand and prevent the serious adverse neurodevelopmental outcomes of preterm birth. The challenge for neonatologists and neurologists alike is identifying early markers of outcome in the prematurely born. This article reviews current trends in prevalence, mortality, and morbidity, and the present status of outcome data for cognitive and neurosensory neurodevelopmental dysfunctions in preterm infants. New neuroimaging modalities and analysis tools are contributing to the understanding of neurologic sequelae of preterm birth by providing microstructural evidence of injury sustained by the preterm brain.

The encephalopathy of prematurity--brain injury and impaired brain development inextricably intertwined

The field of neonatal neurology, and specifically its focus on the premature infant, had its inception in neuropathologic studies. Since then, the development of advanced imaging techniques has guided our developing understanding of the etiology and nature of neonatal brain injury. This review promotes the concept that neonatal brain injury has serious and diverse effects on subsequent brain development, and that these effects likely are more important than simple tissue loss in determining neurologic outcome. Brain injury in the premature infant is best illustrative of this concept. This "encephalopathy of prematurity" is reviewed in the context of the remarkable array of developmental events actively proceeding during the last 16-20 weeks of human gestation. Recent insights into the brain abnormalities in survivors of preterm birth obtained by both advanced magnetic resonance imaging and neuropathologic techniques suggest that this encephalopathy is a complex amalgam of destructive and developmental disturbances. The interrelations between destructive and developmental mechanisms in the genesis of the encephalopathy are emphasized. In the future, advances in neonatal neurology will likely reiterate the dependence of this field on neuropathologic studies, including new cellular and molecular approaches in developmental neurobiology.

Primate-specific origins and migration of cortical GABAergic neurons

Gamma-aminobutyric-acidergic (GABAergic) cells form a very heterogeneous population of neurons that play a crucial role in the coordination and integration of cortical functions. Their number and diversity increase through mammalian brain evolution. Does evolution use the same or different developmental rules to provide the increased population of cortical GABAergic neurons? In rodents, these neurons are not generated in the pallial proliferative zones as glutamatergic principal neurons. They are produced almost exclusively by the subpallial proliferative zones, the ganglionic eminence (GE) and migrate tangentially to reach their target cortical layers. The GE is organized in molecularly different subdomains that produce different subpopulations of cortical GABAergic neurons. In humans and non-human primates, in addition to the GE, cortical GABAergic neurons are also abundantly generated by the proliferative zones of the dorsal telencephalon. Neurogenesis in ventral and dorsal telencephalon occurs with distinct temporal profiles. These dorsal and ventral lineages give rise to different populations of GABAergic neurons. Early-generated GABAergic neurons originate from the GE and mostly migrate to the marginal zone and the subplate. Later-generated GABAergic neurons, originating from both proliferative sites, populate the cortical plate. Interestingly, the pool of GABAergic progenitors in dorsal telencephalon produces mainly calretinin neurons, a population known to be significantly increased and to display specific features in primates. We conclude that the development of cortical GABAergic neurons have exclusive features in primates that need to be considered in order to understand pathological mechanisms leading to some neurological and psychiatric diseases.

Zinc finger protein 191 (ZNF191/Zfp191) is necessary to maintain neural cells as cycling progenitors

The identification of the factors that allow better monitoring of stem cell renewal and differentiation is of paramount importance for the implementation of new regenerative therapies, especially with regard to the nervous and hematopoietic systems. In this article, we present new information on the function of zinc finger protein 191 (ZNF/Zfp191), a factor isolated in hematopoietic cell lines, within progenitors of the central nervous system (CNS). ZNF/Zfp191 has been found to be principally expressed in progenitors of the developing CNS of humans and mice. Such an overlap of the expression patterns in addition to the high homology of the protein in mammals suggested that ZNF/Zfp191 exerts a conserved function within such progenitors. Indeed, ZNF191 knockdown in human neural progenitors inhibits proliferation and leads to the exit of the cell cycle. Conversely, ZNF191 misexpression maintains progenitors in cycle and exerts negative control on the Notch pathway, which prevents them from differentiating. The present data, together with the fact that the inactivation of Zfp191 leads to embryonic lethality, confirm ZNF191 as an essential factor acting for the promotion of the cell cycle and thus maintenance in the progenitor stage. On the bases of expression data, such a function can be extended to progenitor cells of other tissues such as the hematopoietic system, which emphasizes the important issue of further understanding the molecular events controlled by ZNF/Zfp191.

Tubulin-related cortical dysgeneses: microtubule dysfunction underlying neuronal migration defects

The fine tuning of proliferation and neurogenesis, neuronal migration and differentiation and connectivity underlies the proper development of the cerebral cortex. Mutations in genes involved in these processes are responsible for neurodevelopmental disorders, such as cortical dysgeneses, which are usually associated with severe mental retardation and epilepsy. Over the past few years, the importance of cytoskeleton components in cellular processes crucial for cortical development has emerged from a body of functional data. This was reinforced by the association of mutations in the LIS1 and DCX genes, which both encode proteins involved in microtubule (MT) homeostasis, with cerebral cortex developmental disorders. The recent discovery of patients with lissencephaly and bilateral asymmetrical polymicrogyria (PMG) carrying mutations in the alpha- and beta-tubulin-encoding genes TUBA1A and TUBB2B further supports this view, and also raises interesting questions about the specific roles played by certain tubulin isotypes during the development of the cortex.

Subplate cells: amplifiers of neuronal activity in the developing cerebral cortex

Due to their unique structural and functional properties, subplate cells are ideally suited to function as important amplifying units within the developing neocortical circuit. Subplate neurons have extensive dendritic and axonal ramifications and relatively mature functional properties, i.e. their action potential firing can exceed frequencies of 40 Hz. At earliest stages of corticogenesis subplate cells receive functional synaptic inputs from the thalamus and from other cortical and non-cortical sources. Glutamatergic and depolarizing GABAergic inputs arise from cortical neurons and neuromodulatory inputs arise from the basal forebrain and other sources. Activation of postsynaptic metabotropic receptors, i.e. muscarinic receptors, elicits in subplate neurons oscillatory burst discharges which are transmitted via electrical and chemical synapses to neighbouring subplate cells and to immature neurons in the cortical plate. The tonic non-synaptic release of GABA from GABAergic subplate cells facilitates the generation of burst discharges. These cellular bursts are amplified by prominent gap junction coupling in the subplate and cortical plate, thereby eliciting 10–20 Hz oscillations in a local columnar network. Thus, we propose that neuronal networks are organized at earliest stages in a gap junction coupled columnar syncytium. We postulate that the subplate does not only serve as a transient relay station for afferent inputs, but rather as an active element amplifying the afferent and intracortical activity.

Cross-species analyses of the cortical GABAergic and subplate neural populations

Cortical GABAergic (gamma-aminobutyric acidergic) neurons include a recently identified subset whose projections extend over relatively long distances in adult rodents and primates. A number of these inhibitory projection neurons are located in and above the conventionally identified white matter, suggesting their persistence from, or a correspondence with, the developmental subplate. GABAergic and subplate neurons share some unique properties unlike those of the more prevalent pyramidal neurons. To better understand the GABAergic and subplate populations, we constructed a database of neural developmental events common to the three species most frequently used in experimental studies: rat, mouse, and macaque, using data from the online database www.translatingtime.net as well as GABAergic and subplate developmental data from the empirical literature. We used a general linear model to test for similarities and differences, a valid approach because the sequence of most neurodevelopmental events is remarkably conserved across mammalian species. Similarities between the two rodent populations are striking, permitting us to identify developmental dates for GABAergic and subplate neural events in rats that were previously identified only in mice, as well as the timing in mouse development for events previously identified in rats. Primate comparative data are also compelling, although slight variability in statistical error measurement indicates differences in primate GABAergic and subplate events when compared to rodents. Although human extrapolations are challenging because fewer empirical data points are available, and because human data display more variability, we also produce estimates of dates for GABAergic and subplate neural events that have not yet been, or cannot be, determined empirically in humans.

Evolution of the neocortex: a perspective from developmental biology

The enlargement and species-specific elaboration of the cerebral neocortex during evolution holds the secret to the mental abilities of humans; however, the genetic origin and cellular mechanisms that generated the distinct evolutionary advancements are not well understood. This article describes how novelties that make us human may have been introduced during evolution, based on findings in the embryonic cerebral cortex in different mammalian species. The data on the differences in gene expression, new molecular pathways and novel cellular interactions that have led to these evolutionary advances may also provide insight into the pathogenesis and therapies for human-specific neuropsychiatric disorders.

Imaging biomarkers of outcome in the developing preterm brain

The neurodevelopmental disabilities of those who were born prematurely have been well described, yet the underlying alterations in brain development that lead to these changes remain poorly understood. Processes that are vulnerable to injury in the developing brain include maturation of oligodendrocyte precursors and genetically programmed changes in cortical connectivity; recent data have indicated that diffuse injury of the white matter accompanied by neuronal and axonal disruption is common in prematurely born infants. Recent advances in MRI include diffusion tensor imaging and sophisticated image analysis tools, such as functional connectivity, voxel-based morphometry, and mathematical morphology-based cortical folding strategies. These advanced techniques have shown that white matter structure is dependent on gestational age and have started to provide important information about the dynamic interactions between development, injury, and functional recovery in the preterm brain. Identification of early biomarkers for outcome could enable physicians and scientists to develop targeted pharmacological and behavioural therapies to restore functional connectivity.

Understanding the mechanisms of callosal development through the use of transgenic mouse models

The cerebral cortex is the area of the brain where higher-order cognitive processing occurs. The 2 hemispheres of the cerebral cortex communicate through one of the largest fiber tracts in the brain, the corpus callosum. Malformation of the corpus callosum in human beings occurs in 1 in 4000 live births, and those afflicted experience an extensive range of neurologic disorders, from relatively mild to severe cognitive deficits. Understanding the molecular and cellular processes involved in these disorders would therefore assist in the development of prognostic tools and therapies. During the past 3 decades, mouse models have been used extensively to determine which molecules play a role in the complex regulation of corpus callosum development. This review provides an update on these studies, as well as highlights the value of using mouse models with the goal of developing therapies for human acallosal syndromes.

The molecular and genetic mechanisms of neocortex development

This article reviews key recent findings in the field of human cortical development. This development is divided into three major time-dependent phases: neural proliferation of inhibitory and excitatory neurons in spatially distinct regions, migration through multiple cellular boundaries, and maturation through morphologic changes that result in the elaboration of dendrites and axons and that establish the multitude of cellular contacts that underlie neuronal processing. Many of the neurocognitive disorders treated in the clinic can trace their origin to a disorder in one or more of these key steps. Along with this update, work is highlighted that offers a glimpse at the future of therapy for developmental brain disorders that can result from disorders of these cellular events.

Electrical excitability of early neurons in the human cerebral cortex during the second trimester of gestation

Information about development of the human cerebral cortex (proliferation, migration, and differentiation of neurons) is largely based on postmortem histology. Physiological properties of developing human cortical neurons are difficult to access experimentally and therefore remain largely unexplored. Animal studies have shown that information about the arousal of electrical activity in individual cells within fundamental cortical zones (subventricular zone [SVZ], intermediate zone, subplate [SP], and cortical plate [CP]) is necessary for understanding normal brain development. Here we ask where, in what cortical zone, and when, in what gestational week (gw), human neurons acquire the ability to generate nerve impulses (action potentials [APs]). We performed electrical recordings from individual cells in acute brain slices harvested postmortem from the human fetal cerebral cortex (16-22 gw). Tetrodotoxin-sensitive Na(+) current occurs more frequently among CP cells and with significantly greater peak amplitudes than in SVZ. As early as 16 gw, a relatively small population of CP neurons (27%) was able to generate sodium APs upon direct current injection. Neurons located in the SP exhibited the highest level of cellular differentiation, as judged by their ability to fire repetitive APs. At 19 gw, a fraction of human CP and SP neurons possess beta IV spectrin-positive axon initial segments populated with voltage-gated sodium channels (PanNav). These results yield the first physiological characterization of developing human fetal cortical neurons with preserved morphologies in intact surrounding brain tissue.

Insights from in vitro fetal magnetic resonance imaging of cerebral development

The development of the cerebral cortex, white matter microstructure, and the basal ganglia can be well characterized using structural magnetic resonance imaging (MRI). In this review, we analyzed structural in vitro MRI studies of transient cellular cerebral zones that are sites of neurogenetic events (proliferation, migration, cell aggregation, growth of axonal pathways, myelinization, and synaptogenesis). During early fetal life, from 9-13 postconceptional weeks, a thick, densely packed cellular ventricular/subventricular zone and ganglionic eminence indicate intensive proliferation of neuroepithelial stem cells. During the mid and late fetal phase, other cellular zones also became discernable: (1) the intermediate zone as a migratory and axonal growth zone; (2) the subplate zone as a synaptic, extracellular matrix-rich "waiting" compartment; and (3) the cell-dense cortical plate with postmigratory neurons. The preterm phase is characterized by the growth of cortical, thalamic, and striatal pathways; formation of white matter segments; and stratification within the subplate. Thalamocortical fibers cause lamination in the cortical plate, which leads to the formation of a substrate of sensory input. Preterm cerebral immaturity is characterized by considerable extracellular space at sites of axonal growth and a delineable subplate. The intensity of axonal growth, together with a high, gradient-dependent requirement for axonal guidance, forms a substrate for selective vulnerability of specific segments of cerebral white matter in the preterm brain. In summary, the combination of in vitro MRI, histologic analysis, and in vivo MRI is a promising new approach for studying the etiology and treatment of developmental disorders.

Mathematical modeling supports substantial mouse neural progenitor cell death

Background

Existing quantitative models of mouse cerebral cortical development are not fully constrained by experimental data.

Results

Here, we use simple difference equations to model neural progenitor cell fate decisions, incorporating intermediate progenitor cells and initially low rates of neural progenitor cell death. Also, we conduct a sensitivity analysis to investigate possible uncertainty in the fraction of cells that divide, differentiate, and die at each cell cycle.

Conclusion

We demonstrate that uniformly low-level neural progenitor cell death, as concluded in previous models, is incompatible with normal mouse cortical development. Levels of neural progenitor cell death up to and exceeding 50% are compatible with normal cortical development and may operate to prevent forebrain overgrowth as observed following cell death attenuation, as occurs in caspase 3-null mutant mice.

ZHX2 Interacts with Ephrin-B and regulates neural progenitor maintenance in the developing cerebral cortex

Neural progenitor cells in the ventricular zone of the developing mammalian cerebral cortex give rise to specialized cortical cell types via consecutive rounds of proliferation and differentiation, but the mechanisms by which progenitor cell self-renewal and differentiation are regulated during cortical development are not well understood. Here, we show that zinc-finger and homeodomain protein 2 (ZHX2) is specifically expressed in neural progenitor cells during cortical neurogenesis. ZHX2 binds to the cytoplasmic domain of ephrin-B1, which is expressed in cortical neural progenitors and plays a role in neural progenitor cell maintenance. ZHX2 acts as a transcriptional repressor in cell, and its repressor activity is enhanced by coexpression of an ephrin-B1 intracellular domain. Blocking ZHX2 function in cultured neural progenitor cells or in the embryonic cortex causes neuronal differentiation, whereas overexpression of ZHX2 and an ephrin-B1 intracellular domain disrupts the normal differentiation of cortical neural progenitor cells. This study identifies ZHX2 as a novel regulator of neural progenitor cell maintenance and suggests a potential nuclear mechanism of the ephrin-B function in the cortex.

Developmental origins of mosaic brain evolution: Morphometric analysis of the developing zebra finch brain

In adult zebra finches (Taeniopygia guttata), the telencephalon occupies 64% of the entire brain. This fraction is similar to what is seen in parrots, but many other birds possess a significantly smaller telencephalon. The aim of the present study was to determine the developmental time course and cellular basis of telencephalic enlargement in zebra finches, and then to compare these findings with what is known about telencephalic enlargement in other birds. To this end we estimated the volumes of all major brain regions from serial sections in embryonic and post-hatching zebra finches. We also labeled proliferating cells with antibodies against proliferating cell nuclear antigen and phosphorylated histone H3. An important finding to emerge from this work is that the telencephalon of zebra finches at hatching contains a thick proliferative subventricular zone (SVZ) that extends from the subpallium into the dorsal pallium. The data also show that the onset and offset of telencephalic neurogenesis are both delayed in zebra finches relative to quail (Galliformes). This delay in neurogenesis, in conjunction with the expanded SVZ, probably accounts for most of the telencephalic enlargement in passerines such as the zebra finch. In addition, passerines enlarged their telencephalon by decreasing the proportional size of their midbrain tectum. Because the presumptive tectum is proportionally smaller in zebra finches than quail before neurogenesis begins, this difference in tectum size cannot be due to evolutionary alterations in neurogenesis timing. Collectively these findings indicate that several different developmental mechanisms underlie the evolution of a large telencephalon in passerines.

Fibroblast growth factor signaling in development of the cerebral cortex

Despite substantial and exciting recent progress in our understanding of developmental processes in the cerebral cortex, there is still much to be learned about the molecular and cellular mechanisms that account for formation of the cortical structures, and in turn, how the regulation of these mechanisms is linked to cortical functions and behaviors in animals and humans. Fibroblast growth factors (FGFs) are a classic family of growth factors that are important in neural development and whose structures and signaling have been well-studied molecularly and biochemically. Recent advances have revealed their diverse but specific functions in patterning and neurogenesis during cortical development, as evidenced by multiple experimental approaches using in vivo models. Importantly, changes in FGF signaling during development have been shown to influence structure and function of the cerebral cortex as well as animal behavior, and have been implicated in disorders of nervous system function and intellectual development in humans. For example, disturbance of FGF pathways during development has been implicated in the pathogenesis of autism spectrum disorders. Experimental models with altered cortical structure due to perturbations of FGF signaling present a unique opportunity whereby molecular and cellular mechanisms that underlie cortical function and animal behavior can be directly studied and linked to each other.

Exocyst is involved in polarized cell migration and cerebral cortical development

Neuronal migration is essential for proper development of the cerebral cortex. As a first step, a postmitotic cell extends its leading process, presumably by adding new membrane at the growing tip, which would enable directed locomotion. The goal of the present study was to determine if biosynthetic exocytic pathway is polarized in migrating cells and whether polarized exocytosis promotes directed cell migration. A promising candidate for controlling the spatial sites of vesicle tethering and fusion at the plasma membrane is a protein complex called the exocyst. We found that cell migration in a wound assay, as well as cortical neuronal migration during embryonic development was impaired when the exocyst was disturbed. By combining TIRF microscopy and a stochastic model of exocytosis, we found that vesicle exocytosis is preferentially distributed close to the leading edge of polarized cells, that the exocytic process is organized into hotspots, and that the polarized delivery of vesicles and their clustering in hotspots depend on the intact exocyst complex. The exocyst complex seems to achieve this spatial regulation by determining the sites at the membrane where secretory vesicles tether. Thus, our study supports the notion that polarized membrane traffic regulated by the exocyst is an essential component of cell migration and that its deficit may lead to cortical abnormalities involving cortical neuronal malpositioning.

Functional and evolutionary insights into human brain development through global transcriptome analysis

Our understanding of the evolution, formation, and pathological disruption of human brain circuits is impeded by a lack of comprehensive data on the developing brain transcriptome. A whole-genome, exon-level expression analysis of 13 regions from left and right sides of the mid-fetal human brain revealed that 76% of genes are expressed, and 44% of these are differentially regulated. These data reveal a large number of specific gene expression and alternative splicing patterns, as well as coexpression networks, associated with distinct regions and neurodevelopmental processes. Of particular relevance to cognitive specializations, we have characterized the transcriptional landscapes of prefrontal cortex and perisylvian speech and language areas, which exhibit a population-level global expression symmetry. We show that differentially expressed genes are more frequently associated with human-specific evolution of putative cis-regulatory elements. These data provide a wealth of biological insights into the complex transcriptional and molecular underpinnings of human brain development and evolution.

The somatostatin 2A receptor is enriched in migrating neurons during rat and human brain development and stimulates migration and axonal outgrowth

The neuropeptide somatostatin has been suggested to play an important role during neuronal development in addition to its established modulatory impact on neuroendocrine, motor and cognitive functions in adults. Although six somatostatin G protein-coupled receptors have been discovered, little is known about their distribution and function in the developing mammalian brain. In this study, we have first characterized the developmental expression of the somatostatin receptor sst2A, the subtype found most prominently in the adult rat and human nervous system. In the rat, the sst2A receptor expression appears as early as E12 and is restricted to post-mitotic neuronal populations leaving the ventricular zone. From E12 on, migrating neuronal populations immunopositive for the receptor were observed in numerous developing regions including the cerebral cortex, hippocampus and ganglionic eminences. Intense but transient immunoreactive signals were detected in the deep part of the external granular layer of the cerebellum, the rostral migratory stream and in tyrosine hydroxylase- and serotonin- positive neurons and axons. Activation of the sst2A receptor in vitro in rat cerebellar microexplants and primary hippocampal neurons revealed stimulatory effects on neuronal migration and axonal growth, respectively. In the human cortex, receptor immunoreactivity was located in the preplate at early development stages (8 gestational weeks) and was enriched to the outer part of the germinal zone at later stages. In the cerebellum, the deep part of the external granular layer was strongly immunoreactive at 19 gestational weeks, similar to the finding in rodents. In addition, migrating granule cells in the internal granular layer were also receptor-positive. Together, theses results strongly suggest that the somatostatin sst2A receptor participates in the development and maturation of specific neuronal populations during rat and human brain ontogenesis.

Efficient CPP-mediated Cre protein delivery to developing and adult CNS tissues

Background

Understanding and manipulating gene function in physiological conditions is a major objective for both fundamental and applied research. In contrast to other experimental settings, which use either purely genetic or gene delivery (viral or non-viral) strategies, we report here a strategy based on direct protein delivery to central nervous system (CNS) tissues. We fused Cre recombinase with cell-penetrating peptides and analyzed the intracellular biological activity of the resulting chimerical proteins when delivered into cells endowed with Cre-mediated reporter gene expression.

Results

We show that active Cre enzymatic conjugates are readily internalized and exert their enzymatic activity in the nucleus of adherent cultured cells. We then evaluated this strategy in organotypic cultures of neural tissue explants derived from reporter mice carrying reporter "floxed" alleles. The efficacy of two protocols was compared on explants, either by direct addition of an overlying drop of protein conjugate or by implantation of conjugate-coated beads. In both cases, delivery of Cre recombinase resulted in genomic recombination that, with the bead protocol, was restricted to discrete areas of embryonic and adult neural tissues. Furthermore, delivery to adult brain tissue resulted in the transduction of mature postmitotic populations of neurons.

Conclusion

We provide tools for the spatially restricted genetic modification of cells in explant culture. This strategy allows to study lineage, migration, differentiation and death of neural cells. As a proof-of-concept applied to CNS tissue, direct delivery of Cre recombinase enabled the selective elimination of an interneuron subpopulation of the spinal cord, thereby providing a model to study early events of neurodegenerative processes. Thus our work opens new perspectives for both fundamental and applied cell targeting protocols using proteic cargoes which need to retain full bioactivity upon internalisation, as illustrated here with the oligomeric Cre recombinase.

New horizons for the subplate zone and its pioneering neurons

Transitional neuronal layers are a hallmark of the prenatal and neonatal brain yet their contribution to the development of higher functions is not clear. Evidence accumulated over the last 3 decades shows that early connectivity and functional activity start in a transitional layer called the subplate zone (SPZ). The SPZ is host to a heterogeneous population of neurons and its evolutionary complexity peaked in the human brain. In this issue of Cerebral Cortex, three reports (Hoerder-Suabedissen et al., 2008; McKellar and Shatz, 2008; Moore et al., 2008) present new data and evidence in three species (mouse, rat, human) as to the function of the SPZ, to the heterogeneity of its cellular composition, and to the genetic basis of its development.

Focal laser-lesions activate an endogenous population of neural stem/progenitor cells in the adult visual cortex

CNS lesions stimulate adult neurogenic niches. Endogenous neural stem/progenitor cells represent a potential resource for CNS regeneration. Here, we investigate the response to unilateral focal laser-lesions applied to the visual cortex of juvenile rats. Within 3 days post-lesion, an ipsilateral increase of actively cycling cells was observed in cortical layer one and in the callosal white matter within the lesion penumbra. The cells expressed the neural stem/progenitor cell marker Nestin and the 473HD-epitope. Tissue prepared from the lesion area by micro-dissection generated self-renewing, multipotent neurospheres, while cells from the contralateral visual cortex did not. The newly formed neural stem/progenitor cells in the lesion zone might support neurogenesis, as suggested by the expression of Pax6 and Doublecortin, a marker of newborn neurons. We propose that focal laser-lesions may induce the emergence of stem/progenitor cells with neurogenic potential. This could underlie the beneficial effects of laser application in neurosurgery.

Brain injury in premature infants: a complex amalgam of destructive and developmental disturbances

Brain injury in premature infants is of enormous public health importance because of the large number of such infants who survive with serious neurodevelopmental disability, including major cognitive deficits and motor disability. This type of brain injury is generally thought to consist primarily of periventricular leukomalacia (PVL), a distinctive form of cerebral white matter injury. Important new work shows that PVL is frequently accompanied by neuronal/axonal disease, affecting the cerebral white matter, thalamus, basal ganglia, cerebral cortex, brain stem, and cerebellum. This constellation of PVL and neuronal/axonal disease is sufficiently distinctive to be termed "encephalopathy of prematurity". The thesis of this Review is that the encephalopathy of prematurity is a complex amalgam of primary destructive disease and secondary maturational and trophic disturbances. This Review integrates the fascinating confluence of new insights into both brain injury and brain development during the human premature period.

Modulation of GABAergic and glutamatergic transmission by ethanol in the developing neocortex: an in vitro test of the excessive inhibition hypothesis of fetal alcohol spectrum disorder

Exposure to ethanol during development triggers neuronal cell death and this is thought to play a central role in the pathophysiology of fetal alcohol spectrum disorder (FASD). Studies suggest that ethanol-induced neurodegeneration during the period of synaptogenesis results from widespread potentiation of GABA(A) receptors and inhibition of NMDA receptors throughout the brain, with neocortical layer II being particularly sensitive. Here, we tested whether ethanol modulates the function of these receptors during this developmental period using patch-clamp electrophysiological and Ca(2+) imaging techniques in acute slices from postnatal day 7-9 rats. We focused on pyramidal neurons in layer II of the parietal cortex (with layer III as a control). Ethanol (70mM) increased spontaneous action potential-dependent GABA release in layer II (but not layer III) neurons without affecting postsynaptic GABA(A) receptors. Protein and mRNA expression for both the Cl(-) importer, NKCC1, and the Cl(-) exporter, KCC2, were detected in layer II/III neurons. Perforated-patch experiments demonstrated that E(Cl)((-)) is shifted to the right of E(m); activation of GABA(A) receptors with muscimol depolarized E(m), decreased action potential firing, and minimally increased [Ca(2+)](i). However, the ethanol-induced increase of GABAergic transmission did not affect neuronal excitability. Ethanol had no effect on currents exogenously evoked by NMDA or AMPA receptor-mediated spontaneous excitatory postsynaptic currents. Acute application of ethanol in the absence of receptor antagonists minimally increased [Ca(2+)](i). These findings are inconsistent with the excessive inhibition model of ethanol-induced neurodegeneration, supporting the view that ethanol damages developing neurons via more complex mechanisms that vary among specific neuronal populations.

Characterization of a t(5;8)(q31;q21) translocation in a patient with mental retardation and congenital heart disease: implications for involvement of RUNX1T1 in human brain and heart development

The chromosome break points of the t(8;21)(q21.3;q22.12) translocation associated with acute myeloid leukemia disrupt the RUNX1 gene (also known as AML1) and the RUNX1T1 gene (also known as CBFA2T3, MTG8 and ETO) and generate a RUNX1-RUNX1T1 fusion protein. Molecular characterization of the translocation break points in a t(5;8)(q32;q21.3) patient with mild-to-moderate mental retardation and congenital heart disease revealed that one of the break points was within the RUNX1T1 gene. Analysis of RUNX1T1 expression in human embryonic and fetal tissues suggests a role of RUNX1T1 in brain and heart development and support the notion that disruption of the RUNX1T1 gene is associated with the patient's phenotype.

Modes and mishaps of neuronal migration in the mammalian brain

The ability of neurons to migrate to their appropriate positions in the developing brain is critical to brain architecture and function. Recent research has elucidated different modes of neuronal migration and the involvement of a host of signaling factors in orchestrating the migration, as well as vulnerabilities of this process to environmental and genetic factors. Here we discuss the role of cytoskeleton, motor proteins, and mechanisms of nuclear translocation in radial and tangential migration of neurons. We will also discuss how these and other events essential for normal migration of neurons can be disrupted by genetic and environmental factors that contribute to neurological disease in humans.

Intermediate neuronal progenitors (basal progenitors) produce pyramidal-projection neurons for all layers of cerebral cortex

The developing cerebral cortex contains apical and basal types of neurogenic progenitor cells. Here, we investigated the cellular properties and neurogenic output of basal progenitors, also called intermediate neuronal progenitors (INPs). We found that basal mitoses expressing transcription factor Tbr2 (an INP marker) were present throughout corticogenesis, from embryonic day 10.5 through birth. Postnatally, Tbr2(+) progenitors were present in the dentate gyrus, subventricular zone (SVZ), and posterior periventricle (pPV). Two morphological subtypes of INPs were distinguished in the embryonic cortex, "short radial" in the ventricular zone (VZ) and multipolar in the SVZ, probably corresponding to molecularly defined INP subtypes. Unexpectedly, many short radial INPs appeared to contact the apical (ventricular) surface and some divided there. Time-lapse video microscopy suggested that apical INP divisions produced daughter INPs. Analysis of neurogenic divisions (Tis21-green fluorescent protein [GFP](+)) indicated that INPs may produce the majority of projection neurons for preplate, deep, and superficial layers. Conversely, proliferative INP divisions (Tis21-GFP(-)) increased from early to middle corticogenesis, concomitant with SVZ growth. Our findings support the hypothesis that regulated amplification of INPs may be an important factor controlling the balance of neurogenesis among different cortical layers.

Assessment and surgical outcomes for mild type I and severe type II cortical dysplasia: a critical review and the UCLA experience

Recent findings on the clinical, electroencephalography (EEG), neuroimaging, and surgical outcomes are reviewed comparing patients with Palmini type I (mild) and type II (severe) cortical dysplasia. Resources include peer-reviewed studies on surgically treated patients and a subanalysis of the 2004 International League Against Epilepsy (ILAE) Survey of Pediatric Epilepsy Surgery. These sources were supplemented with data from University of California, Los Angeles (UCLA). Cortical dysplasia is the most frequent histopathologic substrate in children, and the second most common etiology in adult epilepsy surgery patients. Cortical dysplasia patients present with seizures at an earlier age than other surgically treated etiologies, and 33-50% have nonlocalized scalp EEG and normal magnetic resonance imaging (MRI) scans. 2-((18)F)Fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) is positive in 75-90% of cases. After complete resection, 80% of patients are seizure free compared with 20% with incomplete resections. Compared with type I, patients with type II cortical dysplasia present at younger ages, have higher seizure frequencies, and are extratemporal. Type I dysplasia is found more often in adult patients in the temporal lobe and is often MRI negative. These findings identify characteristics of patients with mild and severe cortical dysplasia that define surgically treated epilepsy syndromes. The authors discuss future challenges to identifying and treating medically refractory epilepsy patients with cortical dysplasia.

Differences of migratory behavior between direct progeny of apical progenitors and basal progenitors in the developing cerebral cortex

Cerebral cortical neurons are known to be produced from both apical progenitors in the ventricular zone (VZ) and basal (intermediate) progenitors in the subventricular zone (SVZ). On the other hand, we have shown that many SVZ cells assume multipolar morphology and show a characteristic movement termed "multipolar migration." The relationship between multipolar cells and basal progenitors in the SVZ has yet to be investigated. Herein, we followed postmitotic cells generated in the VZ and found that they stayed for more than 10 h in the VZ after becoming postmitotic and then accumulated in the lower part of the SVZ (multipolar cell accumulation zone: MAZ) as multipolar cells (slowly exiting population: SEP), whereas basal progenitors rapidly migrated into the SVZ or intermediate zone (IZ) (rapidly exiting population: REP) with somal translocation morphology. Although REP reached the SVZ/IZ earlier than the SEP, REP stayed within in the SVZ/IZ, whereas SEP moved steadily and entered the CP prior to the REP. We also observed SEP to eventually differentiate into pyramidal neurons in layers II/III. This study provides in vivo evidence of differences in migration modes between postmitotic cells generated from apical progenitors and basal progenitors.

Etiopathogenesis of Parkinson disease: a new beginning?

Parkinson disease (PD) probably represents a syndrome of different disorders and origins converging into a relatively uniform neurodegenerative process. Although clinical-pathological studies have suggested that the presymptomatic phase of PD may be relatively short, perhaps less than a decade, the authors postulate that the pathogenic mechanisms may begin much earlier, possibly even in the prenatal period. Thus, some patients with PD may be born with a fewer than normal number of dopaminergic (and nondopaminergic) neurons as a result of genetic or other abnormalities sustained during the prenatal or perinatal period; as a result of normal age-related neuronal attrition, they eventually reach the critical threshold (60% or more) of neuronal loss needed for onset of PD to become clinically manifest. The authors review the emerging evidence that genetic disruption of normal development, coupled with subsequent environmental factors (the so called multiple-hit hypothesis), plays an important role in the etiopathogenesis of PD.

Tbr2 directs conversion of radial glia into basal precursors and guides neuronal amplification by indirect neurogenesis in the developing neocortex

T-brain gene-2 (Tbr2) is specifically expressed in the intermediate (basal) progenitor cells (IPCs) of the developing cerebral cortex; however, its function in this biological context has so far been overlooked due to the early lethality of Tbr2 mutant embryos. Conditional ablation of Tbr2 in the developing forebrain resulted in the loss of IPCs and their differentiated progeny in mutant cortex. Intriguingly, early loss of IPCs led to a decrease in cortical surface expansion and thickness with a neuronal reduction observed in all cortical layers. These findings suggest that IPC progeny contribute to the correct morphogenesis of each cortical layer. Our observations were confirmed by tracing Tbr2+ IPC cell fate using Tbr2::GFP transgenic mice. Finally, we demonstrated that misexpression of Tbr2 is sufficient to induce IPC identity in ventricular radial glial cells (RGCs). Together, these findings identify Tbr2 as a critical factor for the specification of IPCs during corticogenesis.

Neuroimaging studies of normal brain development and their relevance for understanding childhood neuropsychiatric disorders

OBJECTIVE

To review the maturational events that occur during prenatal and postnatal brain development and to present neuroimaging findings from studies of healthy individuals that identify the trajectories of normal brain development.

METHOD

Histological and postmortem findings of early brain development are presented, followed by a discussion of anatomical, diffusion tensor, proton spectroscopy, and functional imaging findings from studies of healthy individuals, with special emphasis on longitudinal data.

RESULTS

Early brain development occurs through a sequence of major events, beginning with the formation of the neural tube and ending with myelination. Brain development at a macroscopic level typically proceeds first in sensorimotor areas, spreading subsequently and progressively into dorsal and parietal, superior temporal, and dorsolateral prefrontal cortices throughout later childhood and adolescence. These patterns of anatomical development parallel increasing activity in frontal cortices that subserves the development of higher-order cognitive functions during late childhood and adolescence. Disturbances in these developmental patterns seem to be involved centrally in the pathogenesis of various childhood psychiatric disorders including childhood-onset schizophrenia, attention-deficit/hyperactivity disorder, developmental dyslexia, Tourette's syndrome, and bipolar disorder.

CONCLUSIONS

Advances in imaging techniques have enhanced our understanding of normal developmental trajectories in the brain, which may improve insight into the abnormal patterns of development in various childhood psychiatric disorders.

Making bigger brains-the evolution of neural-progenitor-cell division

Relative brain size differs markedly between species. This variation might ultimately result from differences in the cell biology of neural progenitors, which might underlie their different proliferative potential. On the basis of the cell-biological properties of neural progenitors of animals of varying brain size and complexity (namely, Drosophila melanogaster, rodents and primates), we hypothesize that the evolution of four related cell-biological features has contributed to increases in neuron number. Three of these features-the pseudostratification of the progenitor layer, the loss of (Inscuteable-mediated) mitotic-spindle rotation and the evolution of proteins (such as Aspm) that maintain the precision of symmetric progenitor division-affect the mode of cell division in the apically dividing progenitors of the ventricular zone. The fourth feature, however, concerns the evolution of the basally dividing progenitors of the subventricular zone. In rodents, these basal (or intermediate) progenitors lack cell polarity, whereas in primates a subpopulation of radial, presumably polarized, progenitors has evolved (outer-subventricular-zone progenitors). These cells undergo basal mitoses and are thought to retain epithelial characteristics. We propose the epithelial-progenitor hypothesis, which argues that evolutionary changes that promote the maintenance of epithelial features in neural progenitors, including outer-subventricular-zone progenitors, have been instrumental in the expansion of the cerebral cortex in primates.

The Tsc1-Tsc2 complex influences neuronal polarity by modulating TORC1 activity and SAD levels

Neuronal function depends on the specification of neuronal processes as axons or dendrites. In this issue of Genes & Development Choi and colleagues (2485-2495) show that without Tuberous Sclerosis Complex 1 (Tsc1) or Tsc2, molecules linked to the autosomal dominant disease tuberous sclerosis, an increase in the activity of the translational regulator Target of Rapamycin 1 (TORC1) causes neurons to have multiple axons and the translation of SAD kinase increases as well. Thus, in addition to the kinase LKB1, the Tsc1-Tsc2 complex, acting through TORC1, also modulates SAD to regulate axon formation.

The role of ATP signaling in the migration of intermediate neuronal progenitors to the neocortical subventricular zone

Most neurons of the cerebral cortex are generated in the germinal zones near the embryonic cerebral ventricle and migrate radially to the overlying cortical plate. Initially, all dividing cells are attached to the surface of the embryonic ventricle (ventricular zone) until a subset of dividing cells (basal or intermediate neuronal progenitors, INPs), recognized by their immunoreactivity to Tbr2, detach from the ventricular surface and migrate a short distance to establish a secondary proliferative compartment (the subventricular zone). The mechanism that regulates migration of the Tbr2(+) INPs from the ventricular to the subventricular zones is unknown. Here, we show that INPs, unlike the postmitotic neurons that tend to lose the ATP response, continue to express the purinergic P2Y1 receptor. Furthermore, blocking ATP signaling by the P2Y1 blockers, MRS2176, suramin, and apyrase, reduces Ca(2+) transients and retards INP migration to the subventricular zone. In addition, genetic knockdown of the P2Y1 receptor by in vivo application of short hairpin RNA selectively impairs the migration of INPs to the subventricular zone. Together, these results suggest that intercellular ATP signaling is essential for the migration of INPs and the proper formation of the subventricular zone. Interference of ATP signaling or abnormal Ca(2+) fluctuations in INPs may play a significant role in variety of genetic or acquired cortical malformations.