Relation Between Putative Transmitter Phenotypes and Connectivity of Subplate Neurons During Cerebral Cortical Development

Early retinal deprivation crossmodally alters nascent subplate circuits and activity in the auditory cortex during the precritical period

Sensory perturbation in one modality results in the adaptive reorganization of neural pathways within the spared modalities, a phenomenon known as "crossmodal plasticity," which has been examined during or after the classic "critical period." Because peripheral perturbations can alter the auditory cortex (ACX) activity and functional connectivity of the ACX subplate neurons (SPNs) even before the critical period, called the precritical period, we investigated if retinal deprivation at birth crossmodally alters the ACX activity and SPN circuits during the precritical period. We deprived newborn mice of visual inputs after birth by performing bilateral enucleation. We performed in vivo widefield imaging in the ACX of awake pups during the first two postnatal weeks to investigate cortical activity. We found that enucleation alters spontaneous and sound-evoked activities in the ACX in an age-dependent manner. Next, we performed whole-cell patch clamp recording combined with laser scanning photostimulation in ACX slices to investigate circuit changes in SPNs. We found that enucleation alters the intracortical inhibitory circuits impinging on SPNs, shifting the excitation-inhibition balance toward excitation and this shift persists after ear opening. Together, our results indicate that crossmodal functional changes exist in the developing sensory cortices at early ages before the onset of the classic critical period.

Changing subplate circuits: Early activity dependent circuit plasticity

Early neural activity in the developing sensory system comprises spontaneous bursts of patterned activity, which is fundamental for sculpting and refinement of immature cortical connections. The crude early connections that are initially refined by spontaneous activity, are further elaborated by sensory-driven activity from the periphery such that orderly and mature connections are established for the proper functioning of the cortices. Subplate neurons (SPNs) are one of the first-born mature neurons that are transiently present during early development, the period of heightened activity-dependent plasticity. SPNs are well integrated within the developing sensory cortices. Their structural and functional properties such as relative mature intrinsic membrane properties, heightened connectivity via chemical and electrical synapses, robust activation by neuromodulatory inputs-place them in an ideal position to serve as crucial elements in monitoring and regulating spontaneous endogenous network activity. Moreover, SPNs are the earliest substrates to receive early sensory-driven activity from the periphery and are involved in its modulation, amplification, and transmission before the maturation of the direct adult-like thalamocortical connectivity. Consequently, SPNs are vulnerable to sensory manipulations in the periphery. A broad range of early sensory deprivations alters SPN circuit organization and functions that might be associated with long term neurodevelopmental and psychiatric disorders. Here we provide a comprehensive overview of SPN function in activity-dependent development during early life and integrate recent findings on the impact of early sensory deprivation on SPNs that could eventually lead to neurodevelopmental disorders.

Chromatin remodeler Arid1a regulates subplate neuron identity and wiring of cortical connectivity

Loss-of-function mutations in chromatin remodeler gene ARID1A are a cause of Coffin-Siris syndrome, a developmental disorder characterized by dysgenesis of corpus callosum. Here, we characterize Arid1a function during cortical development and find unexpectedly selective roles for Arid1a in subplate neurons (SPNs). SPNs, strategically positioned at the interface of cortical gray and white matter, orchestrate multiple developmental processes indispensable for neural circuit wiring. We find that pancortical deletion of Arid1a leads to extensive mistargeting of intracortical axons and agenesis of corpus callosum. Sparse Arid1a deletion, however, does not autonomously misroute callosal axons, implicating noncell-autonomous Arid1a functions in axon guidance. Supporting this possibility, the ascending axons of thalamocortical neurons, which are not autonomously affected by cortical Arid1a deletion, are also disrupted in their pathfinding into cortex and innervation of whisker barrels. Coincident with these miswiring phenotypes, which are reminiscent of subplate ablation, we unbiasedly find a selective loss of SPN gene expression following Arid1a deletion. In addition, multiple characteristics of SPNs crucial to their wiring functions, including subplate organization, subplate axon-thalamocortical axon cofasciculation ("handshake"), and extracellular matrix, are severely disrupted. To empirically test Arid1a sufficiency in subplate, we generate a cortical plate deletion of Arid1a that spares SPNs. In this model, subplate Arid1a expression is sufficient for subplate organization, subplate axon-thalamocortical axon cofasciculation, and subplate extracellular matrix. Consistent with these wiring functions, subplate Arid1a sufficiently enables normal callosum formation, thalamocortical axon targeting, and whisker barrel development. Thus, Arid1a is a multifunctional regulator of subplate-dependent guidance mechanisms essential to cortical circuit wiring.

The Ferret as a Model System for Neocortex Development and Evolution

The neocortex is the largest part of the cerebral cortex and a key structure involved in human behavior and cognition. Comparison of neocortex development across mammals reveals that the proliferative capacity of neural stem and progenitor cells and the length of the neurogenic period are essential for regulating neocortex size and complexity, which in turn are thought to be instrumental for the increased cognitive abilities in humans. The domesticated ferret, Mustela putorius furo, is an important animal model in neurodevelopment for its complex postnatal cortical folding, its long period of forebrain development and its accessibility to genetic manipulation in vivo. Here, we discuss the molecular, cellular, and histological features that make this small gyrencephalic carnivore a suitable animal model to study the physiological and pathological mechanisms for the development of an expanded neocortex. We particularly focus on the mechanisms of neural stem cell proliferation, neuronal differentiation, cortical folding, visual system development, and neurodevelopmental pathologies. We further discuss the technological advances that have enabled the genetic manipulation of the ferret in vivo. Finally, we compare the features of neocortex development in the ferret with those of other model organisms.

The enigmatic fetal subplate compartment forms an early tangential cortical nexus and provides the framework for construction of cortical connectivity

The most prominent transient compartment of the primate fetal cortex is the deep, cell-sparse, synapse-containing subplate compartment (SPC). The developmental role of the SPC and its extraordinary size in humans remain enigmatic. This paper evaluates evidence on the development and connectivity of the SPC and discusses its role in the pathogenesis of neurodevelopmental disorders. A synthesis of data shows that the subplate becomes a prominent compartment by its expansion from the deep cortical plate (CP), appearing well-delineated on MR scans and forming a tangential nexus across the hemisphere, consisting of an extracellular matrix, randomly distributed postmigratory neurons, multiple branches of thalamic and long corticocortical axons. The SPC generates early spontaneous non-synaptic and synaptic activity and mediates cortical response upon thalamic stimulation. The subplate nexus provides large-scale interareal connectivity possibly underlying fMR resting-state activity, before corticocortical pathways are established. In late fetal phase, when synapses appear within the CP, transient the SPC coexists with permanent circuitry. The histogenetic role of the SPC is to provide interactive milieu and capacity for guidance, sorting, "waiting" and target selection of thalamocortical and corticocortical pathways. The new evolutionary role of the SPC and its remnant white matter neurons is linked to the increasing number of associative pathways in the human neocortex. These roles attributed to the SPC are regulated using a spatiotemporal gene expression during critical periods, when pathogenic factors may disturb vulnerable circuitry of the SPC, causing neurodevelopmental cognitive circuitry disorders.

Subplate Neurons as an Organizer of Mammalian Neocortical Development

Subplate neurons (SpNs) are one of the earliest born and matured neurons in the developing cerebral cortex and play an important role in the early development of the neocortex. It has been known that SpNs have an essential role in thalamocortical axon (TCA) pathfinding and the establishment of the first neural circuit from the thalamus towards cortical layer IV. In addition to this function, it has recently been revealed in mouse corticogenesis that SpNs play an important role in the regulation of radial neuronal migration during the mid-embryonic stage. Moreover, accumulating studies throw light on the possible roles of SpNs in adult brain functions and also their involvement in psychiatric or other neurological disorders. As SpNs are unique to mammals, they may have contributed to the evolution of the mammalian neocortex by efficiently organizing cortical formation during the limited embryonic period of corticogenesis. By increasing our knowledge of the functions of SpNs, we will clarify how SpNs act as an organizer of mammalian neocortical formation.

High Angular Resolution Diffusion MRI Reveals Conserved and Deviant Programs in the Paths that Guide Human Cortical Circuitry

Diffusion magnetic resonance (MR) tractography represents a novel opportunity to investigate conserved and deviant developmental programs between humans and other species such as mice. To that end, we acquired high angular resolution diffusion MR scans of mice [embryonic day (E) 10.5 to postnatal week 4] and human brains [gestational week (GW) 17-30] at successive stages of fetal development to investigate potential evolutionary changes in radial organization and emerging pathways between humans and mice. We compare radial glial development as well as commissural development (e.g., corpus callosum), primarily because our findings can be integrated with previous work. We also compare corpus callosal growth trajectories across primates (i.e., humans and rhesus macaques) and rodents (i.e., mice). One major finding is that the developing cortex of humans is predominated by pathways likely associated with a radial glial organization at GW 17-20, which is not as evident in age-matched mice (E 16.5, 17.5). Another finding is that, early in development, the corpus callosum follows a similar developmental timetable in primates (i.e., macaques and humans) as in mice. However, the corpus callosum grows for an extended period of time in primates compared with rodents. Taken together, these findings highlight deviant developmental programs underlying the emergence of cortical pathways in the human brain.

Kcnab1 Is Expressed in Subplate Neurons With Unilateral Long-Range Inter-Areal Projections

Subplate (SP) neurons are among the earliest-born neurons in the cerebral cortex and heterogeneous in terms of gene expression. SP neurons consist mainly of projection neurons, which begin to extend their axons to specific target areas very early during development. However, the relationships between axon projection and gene expression patterns of the SP neurons, and their remnant layer 6b (L6b) neurons, are largely unknown. In this study, we analyzed the corticocortical projections of L6b/SP neurons in the mouse cortex and searched for a marker gene expressed in L6b/SP neurons that have ipsilateral inter-areal projections. Retrograde tracing experiments demonstrated that L6b/SP neurons in the primary somatosensory cortex (S1) projected to the primary motor cortex (M1) within the same cortical hemisphere at postnatal day (PD) 2 but did not show any callosal projection. This unilateral projection pattern persisted into adulthood. Our microarray analysis identified the gene encoding a β subunit of voltage-gated potassium channel (Kcnab1) as being expressed in L6b/SP. Double labeling with retrograde tracing and in situ hybridization demonstrated that Kcnab1 was expressed in the unilaterally-projecting neurons in L6b/SP. Embryonic expression was specifically detected in the SP as early as embryonic day (E) 14.5, shortly after the emergence of SP. Double immunostaining experiments revealed different degrees of co-expression of the protein product Kvβ1 with L6b/SP markers Ctgf (88%), Cplx3 (79%), and Nurr1 (58%), suggesting molecular subdivision of unilaterally-projecting L6b/SP neurons. In addition to expression in L6b/SP, scattered expression of Kcnab1 was observed during postnatal stages without layer specificity. Among splicing variants with three alternative first exons, the variant 1.1 explained all the cortical expression mentioned in this study. Together, our data suggest that L6b/SP neurons have corticocortical projections and Kcnab1 expression defines a subpopulation of L6b/SP neurons with a unilateral inter-areal projection.

The Integrative Function of Silent Synapses on Subplate Neurons in Cortical Development and Dysfunction

The thalamocortical circuit is of central importance in relaying information to the cortex. In development, subplate neurons (SPNs) form an integral part of the thalamocortical pathway. These early born cortical neurons are the first neurons to receive thalamic inputs and excite neurons in the cortical plate. This feed-forward circuit topology of SPNs supports the role of SPNs in shaping the formation and plasticity of thalamocortical connections. Recently it has been shown that SPNs also receive inputs from the developing cortical plate and project to the thalamus. The cortical inputs to SPNs in early ages are mediated by N-methyl-D-aspartate (NMDA)-receptor only containing synapses while at later ages α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-receptors are present. Thus, SPNs perform a changing integrative function over development. NMDA-receptor only synapses are crucially influenced by the resting potential and thus insults to the developing brain that causes depolarizations, e.g., hypoxia, can influence the integrative function of SPNs. Since such insults in humans cause symptoms of neurodevelopmental disorders, NMDA-receptor only synapses on SPNs might provide a crucial link between early injuries and later circuit dysfunction. We thus here review subplate associated circuits, their changing functions, and discuss possible roles in development and disease.

The Superior Function of the Subplate in Early Neocortical Development

During early development the structure and function of the cerebral cortex is critically organized by subplate neurons (SPNs), a mostly transient population of glutamatergic and GABAergic neurons located below the cortical plate. At the molecular and morphological level SPNs represent a rather diverse population of cells expressing a variety of genetic markers and revealing different axonal-dendritic morphologies. Electrophysiologically SPNs are characterized by their rather mature intrinsic membrane properties and firing patterns. They are connected via electrical and chemical synapses to local and remote neurons, e.g., thalamic relay neurons forming the first thalamocortical input to the cerebral cortex. Therefore SPNs are robustly activated at pre- and perinatal stages by the sensory periphery. Although SPNs play pivotal roles in early neocortical activity, development and plasticity, they mostly disappear by programmed cell death during further maturation. On the one hand, SPNs may be selectively vulnerable to hypoxia-ischemia contributing to brain damage, on the other hand there is some evidence that enhanced survival rates or alterations in SPN distribution may contribute to the etiology of neurological or psychiatric disorders. This review aims to give a comprehensive and up-to-date overview on the many functions of SPNs during early physiological and pathophysiological development of the cerebral cortex.

Early human brain development: Starring the subplate

This review summarizes early human brain development on the basis of neuroanatomical data and functional connectomics. It indicates that the most significant changes in the brain occur during the second half of gestation and the first three months post-term, in particular in the cortical subplate and cerebellum. As the transient subplate pairs a high rate of intricate developmental changes and interactions with clear functional activity, two phases of development are distinguished: a) the transient cortical subplate phase, ending at 3 months post-term when the permanent circuitries in the primary motor, somatosensory and visual cortices have replaced the subplate; and subsequently, b) the phase in which the permanent circuitries dominate. In the association areas the subplate dissolves in the remainder of the first postnatal year. During both phases developmental changes are paralleled by continuous reconfigurations in network activity. The reviewed literature also suggests that disruption of subplate development may play a pivotal role in developmental disorders, such as cerebral palsy, autism spectrum disorders, attention deficit hyperactivity disorder and schizophrenia.

Subset of Cortical Layer 6b Neurons Selectively Innervates Higher Order Thalamic Nuclei in Mice

The thalamus receives input from 3 distinct cortical layers, but input from only 2 of these has been well characterized. We therefore investigated whether the third input, derived from layer 6b, is more similar to the projections from layer 6a or layer 5. We studied the projections of a restricted population of deep layer 6 cells ("layer 6b cells") taking advantage of the transgenic mouse Tg(Drd1a-cre)FK164Gsat/Mmucd (Drd1a-Cre), that selectively expresses Cre-recombinase in a subpopulation of layer 6b neurons across the entire cortical mantle. At P8, 18% of layer 6b neurons are labeled with Drd1a-Cre::tdTomato in somatosensory cortex (SS), and some co-express known layer 6b markers. Using Cre-dependent viral tracing, we identified topographical projections to higher order thalamic nuclei. VGluT1+ synapses formed by labeled layer 6b projections were found in posterior thalamic nucleus (Po) but not in the (pre)thalamic reticular nucleus (TRN). The lack of TRN collaterals was confirmed with single-cell tracing from SS. Transmission electron microscopy comparison of terminal varicosities from layer 5 and layer 6b axons in Po showed that L6b varicosities are markedly smaller and simpler than the majority from L5. Our results suggest that L6b projections to the thalamus are distinct from both L5 and L6a projections.

Neocortical Layer 6B as a Remnant of the Subplate - A Morphological Comparison

The fate of the subplate (SP) is still a matter of debate. The SP and layer 6 (which is ontogenetically the oldest and innermost neocortical lamina) develop coincidentally. Yet, the function of sublamina 6B is largely unknown. It has been suggested that it consists partly of neurons from the transient SP, however, experimental evidence for this hypothesis is still missing. To obtain first insights into the neuronal complement of layer 6B in the somatosensory rat barrel cortex, we used biocytin stainings of SP neurons (aged 0-4 postnatal days, PND) and layer 6B neurons (PND 11-35) obtained during in vitro whole-cell patch-clamp recordings. Neurons were reconstructed for a quantitative characterization of their axonal and dendritic morphology. An unsupervised cluster analysis revealed that the SP and layer 6B consist of heterogeneous but comparable neuronal cell populations. Both contain 5 distinct spine-bearing cell types whose relative fractions change with increasing age. Pyramidal cells were more prominent in layer 6B, whereas non-pyramidal neurons were less frequent. Because of the high morphological similarity of SP and layer 6B neurons, we suggest that layer 6B consists of persistent non-pyramidal neurons from the SP and cortical L6B pyramidal neurons.

Postnatal development of GABAergic interneurons in the neocortical subplate of mice

The subplate (SP) plays important roles in developmental and functional events in the neocortex, such as thalamocortical and corticofugal projection, cortical oscillation generation and corticocortical connectivity. Although accumulated evidence indicates that SP interneurons are crucial for SP function, the molecular composition of SP interneurons as well as their developmental profile and distribution remain largely unclear. In this study, we systematically investigated dynamic development of SP thickness and chemical marker expression in SP interneurons in distinct cortical regions during the first postnatal month. We found that, although the relative area of the SP in the cerebral cortex significantly declined with postnatal development, the absolute thickness did not change markedly. We also found that somatostatin (SOM), the ionotropic serotonin receptor 3A (5HT3AR), and parvalbumin (PV) reliably identify three distinct non-overlapping subpopulations of SP interneurons. The SOM group, which represents ~30% of total SP interneurons, expresses neuronal nitric oxide synthase (nNOS) and calbindin (CB) and colocalizes entirely with neuropeptide Y (NPY). The 5HT3AR group, which accounts for ~60% of the total interneuronal population, expresses calretinin (CR) and GABA-A receptor subunit delta (GABAARδ). The PV group accounts for ~10% of total SP interneurons and coexpressed GABAARδ. Moreover, distinct interneuron subtypes show characteristic temporal and spatial distribution in the SP. nNOS(+) interneurons in the SP increase from the anterior motor cortex to posterior visual cortex, while CR(+) and CB(+) interneurons the opposite. Interestedly, the majority of GABAARδ(+) neurons in SP are non-GABAergic neurons in contrast to other cortical layers. These findings clarify and extend our understanding of SP interneurons in the developing cerebral cortex and will underpin further study of SP function.

The relevance of human fetal subplate zone for developmental neuropathology of neuronal migration disorders and cortical dysplasia

The human fetal cerebral cortex develops through a series of partially overlapping histogenetic events which occur in transient cellular compartments, such as the subplate zone. The subplate serves as waiting compartment for cortical afferent fibers, the major site of early synaptogenesis and neuronal differentiation and the hub of the transient fetal cortical circuitry. Thus, the subplate has an important but hitherto neglected role in the human fetal cortical connectome. The subplate is also an important compartment for radial and tangential migration of future cortical neurons. We review the diversity of subplate neuronal phenotypes and their involvement in cortical circuitry and discuss the complexity of late neuronal migration through the subplate as well as its potential relevance for pathogenesis of migration disorders and cortical dysplasia. While migratory neurons may become misplaced within the subplate, they can easily survive by being involved in early subplate circuitry; this can enhance their subsequent survival even if they have immature or abnormal physiological activity and misrouted connections and thus survive into adulthood. Thus, better understanding of subplate developmental history and various subsets of its neurons may help to elucidate certain types of neuronal disorders, including those accompanied by epilepsy.

Cerebral cortical development in rodents and primates

Rodents and primates both show considerable variation in the overall size, the radial and tangential dimensions, folding and subdivisions into distinct areas of their cerebral cortex. Our current understanding of brain development is based on a handful of model systems. A detailed comparative analysis of the cellular and molecular mechanisms that regulate neural progenitor production, cell migration, and circuit assembly can provide much needed insights into the working of neocortical evolution. From the limited comparative data currently available, it is apparent that the emergence and variation of the neuronal progenitor cells have led to the production of increased neuronal populations and the evolution of the cortex. Further diversification and compartmentalization of the germinal zone together with changing proportions of radial glia in the ventricular zone and various intermediate progenitors in the subventricular zone may have been the driving force behind increased cell numbers in larger brains both in rodents and primates. Radial and tangential migratory patterns are both present in rodents and primates, but in different proportions. There are apparent differences between mouse and human in the generation and elaboration of the interneuronal subtypes and also in gene expression patterns associated with the appearance of distinct cortical areas. The increased cortical dimensions and the formation of a more elaborate cortical architecture in primates require a larger and more compartmentalized transient subplate zone during development. More comparative analysis in rodent and primate species with large, small, and smooth and folded brains is needed to reveal the biological significance of the alterations in these cortical developmental programs.

Hypothesis on the dual origin of the Mammalian subplate

The development of the mammalian neocortex relies heavily on subplate. The proportion of this cell population varies considerably in different mammalian species. Subplate is almost undetectable in marsupials, forms a thin, but distinct layer in mouse and rat, a larger layer in carnivores and big-brained mammals as pig, and a highly developed embryonic structure in human and non-human primates. The evolutionary origin of subplate neurons is the subject of current debate. Some hypothesize that subplate represents the ancestral cortex of sauropsids, while others consider it to be an increasingly complex phylogenetic novelty of the mammalian neocortex. Here we review recent work on expression of several genes that were originally identified in rodent as highly and differentially expressed in subplate. We relate these observations to cellular morphology, birthdating, and hodology in the dorsal cortex/dorsal pallium of several amniote species. Based on this reviewed evidence we argue for a third hypothesis according to which subplate contains both ancestral and newly derived cell populations. We propose that the mammalian subplate originally derived from a phylogenetically ancient structure in the dorsal pallium of stem amniotes, but subsequently expanded with additional cell populations in the synapsid lineage to support an increasingly complex cortical plate development. Further understanding of the detailed molecular taxonomy, somatodendritic morphology, and connectivity of subplate in a comparative context should contribute to the identification of the ancestral and newly evolved populations of subplate neurons.

Comparative aspects of subplate zone studied with gene expression in sauropsids and mammals

There is currently a debate about the evolutionary origin of the earliest generated cortical preplate neurons and their derivatives (subplate and marginal zone). We examined the subplate with murine markers including nuclear receptor related 1 (Nurr1), monooxygenase Dbh-like 1 (Moxd1), transmembrane protein 163 (Tmem163), and connective tissue growth factor (Ctgf) in developing and adult turtle, chick, opossum, mouse, and rat. Whereas some of these are expressed in dorsal pallium in all species studied (Nurr1, Ctgf, and Tmem163), we observed that the closely related mouse and rat differed in the expression patterns of several others (Dopa decarboxylase, Moxd1, and thyrotropin-releasing hormone). The expression of Ctgf, Moxd1, and Nurr1 in the oppossum suggests a more dispersed subplate population in this marsupial compared with mice and rats. In embryonic and adult chick brains, our selected subplate markers are primarily expressed in the hyperpallium and in the turtle in the main cell dense layer of the dorsal cortex. These observations suggest that some neurons that express these selected markers were present in the common ancestor of sauropsids and mammals.

The subplate and early cortical circuits

The developing mammalian cerebral cortex contains a distinct class of cells, subplate neurons (SPns), that play an important role during early development. SPns are the first neurons to be generated in the cerebral cortex, they reside in the cortical white matter, and they are the first to mature physiologically. SPns receive thalamic and neuromodulatory inputs and project into the developing cortical plate, mostly to layer 4. Thus SPns form one of the first functional cortical circuits and are required to relay early oscillatory activity into the developing cortical plate. Pathophysiological impairment or removal of SPns profoundly affects functional cortical development. SPn removal in visual cortex prevents the maturation of thalamocortical synapses, the maturation of inhibition in layer 4, the development of orientation selective responses and the formation of ocular dominance columns. SPn removal also alters ocular dominance plasticity during the critical period. Therefore, SPns are a key regulator of cortical development and plasticity. SPns are vulnerable to injury during prenatal stages and might provide a crucial link between brain injury in development and later cognitive malfunction.

Subplate in the developing cortex of mouse and human

The subplate is a largely transient zone containing precocious neurons involved in several key steps of cortical development. The majority of subplate neurons form a compact layer in mouse, but are dispersed throughout a much larger zone in the human. In rodent, subplate neurons are among the earliest born neocortical cells, whereas in primate, neurons are added to the subplate throughout cortical neurogenesis. Magnetic resonance imaging and histochemical studies show that the human subplate grows in size until the end of the second trimester. Previous microarray experiments in mice have shown several genes that are specifically expressed in the subplate layer of the rodent dorsal cortex. Here we examined the human subplate for some of these markers. In the human dorsal cortex, connective tissue growth factor-positive neurons can be seen in the ventricular zone at 15-22 postconceptional weeks (PCW) (most at 17 PCW) and are present in the subplate at 22 PCW. The nuclear receptor-related 1 protein is mostly expressed in the subplate in the dorsal cortex, but also in lower layer 6 in the lateral and perirhinal cortex, and can be detected from 12 PCW. Our results suggest that connective tissue growth factor- and nuclear receptor-related 1-positive cells are two distinct cell populations of the human subplate. Furthermore, our microarray analysis in rodent suggested that subplate neurons produce plasma proteins. Here we demonstrate that the human subplate also expresses α2zinc-binding globulin and Alpha-2-Heremans-Schmid glycoprotein/human fetuin. In addition, the established subplate neuron marker neuropeptide Y is expressed superficially, whereas potassium/chloride co-transporter (KCC2)-positive neurons are localized in the deep subplate at 16 PCW. These observations imply that the human subplate shares gene expression patterns with rodent, but is more compartmentalized into superficial and deep sublayers. This increased complexity of the human subplate may contribute to differential vulnerability in response to hypoxia/ischaemia across the depth of the cortex. Combining knowledge of cell-type specific subplate gene expression with modern imaging methods will enable a better understanding of neuropathologies involving the subplate.

Developmental history of the subplate zone, subplate neurons and interstitial white matter neurons: relevance for schizophrenia

The subplate zone is a transient cytoarchitectonic compartment of the fetal telencephalic wall and contains a population of subplate neurons which are the main neurons of the fetal neocortex and play a key role in normal development of cerebral cortical structure and connectivity. While the subplate zone disappears during the perinatal and early postnatal period, numerous subplate neurons survive and remain embedded in the superficial (gyral) white matter of adolescent and adult brain as so-called interstitial neurons. In both fetal and adult brain, subplate/interstitial neurons belong to two major classes of cortical cells: (a) projection (glutamatergic) neurons and (b) local circuit (GABAergic) interneurons. As interstitial neurons remain strategically positioned at the cortical/white matter interface through which various cortical afferent systems enter the deep cortical layers, they probably serve as auxiliary interneurons involved in differential "gating" of cortical input systems. It is widely accepted that prenatal lesions which alter the number of surviving subplate neurons (i.e., the number of interstitial neurons) and/or the nature of their involvement in cortical circuitry represent an important causal factor in pathogenesis of at least some types of schizophrenia--e.g., in the subgroup of patients with cognitive impairment and deficits of frontal lobe functions. The abnormal functioning of cortical circuitry in schizophrenia becomes manifest during the adolescence, when there is an increased demand for proper functioning of the prefrontal cortex. In this review, we describe developmental history of subplate zone, subplate neurons and surviving interstitial neurons, as well as presumed consequences of the increased number of GABAergic interstitial neurons in the prefrontal cortex. We propose that the increased number of GABAergic interstitial neurons leads to the increased inhibition of prefrontal cortical neurons. This inhibitory action of GABAergic interstitial neurons is facilitated by their strategic position at the cortical/white matter interface where limbic and modulatory afferent pathways enter the prefrontal cortex. Thus, enlarged population of inhibitory interstitial neurons (even if they represent a minor fraction of total neuron number, as in the cerebral cortex itself) may alter the differential "gating" of limbic and modulatory inputs (as well as other cortical and subcortical inputs) and cause a functional disconnectivity between the prefrontal and limbic cortex in the adolescent brain. In conclusion, fetal subplate neurons and surviving postnatal interstitial neurons are important modulators of cortical functions in both normal and schizophrenic cerebral cortex.

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.

Dynamic integration of subplate neurons into the cortical barrel field circuitry during postnatal development in the Golli-tau-eGFP (GTE) mouse

In the Golli-tau-eGFP (GTE) transgenic mouse the reporter gene expression is largely confined to the layer of subplate neurons (SPn), providing an opportunity to study their intracortical and extracortical projections. In this study, we examined the thalamic afferents and layer IV neuron patterning in relation to the SPn neurites in the developing barrel cortex in GTE mouse at ages embryonic day 17 (E17) to postnatal day 14 (P14). Serotonin transporter immunohistochemistry or cytochrome oxydase histochemistry was used to reveal thalamic afferent patterning. Bizbenzimide staining identified the developing cytoarchitecture in coronal and tangential sections of GTE brains. Enhanced green fluorescent protein (GFP)-labelled neurites and thalamic afferents were both initially diffusely present in layer IV but by P4-P6 both assumed the characteristic periphery-related pattern and became restricted to the barrel hollows. This pattern gradually changed and by P10 the GFP-labelled neurites largely accumulated at the layer IV-V boundary within the barrel septa whereas thalamic afferents remained in the hollows. To investigate whether this reorganisation is dependent on sensory input, the whiskers of row 'a' or 'c' were removed at P0 or P5 and the organisation of GFP-labelled neurites in the barrel cortex was examined at P6 or P10. In the contralateral region corresponding to row 'a' or 'c' the lack of hollow to septa rearrangement of the GFP-labelled neurites was observed after P0 row removal at P10 but not at P6. Our findings suggest a dynamic, sensory periphery-dependent integration of SPn neurites into the primary somatosensory cortex during the period of barrel formation.

Glycine receptors mediate excitation of subplate neurons in neonatal rat cerebral cortex

The development of the cerebral cortex depends on genetic factors and early electrical activity patterns that form immature neuronal networks. Subplate neurons (SPn) are involved in the construction of thalamocortical innervation, generation of oscillatory network activity, and in the proper formation of the cortical columnar architecture. Because glycine receptors play an important role during early corticogenesis, we analyzed the functional consequences of glycine receptor activation in visually identified SPn in neocortical slices from postnatal day 0 (P0) to P4 rats using whole cell and perforated patch-clamp recordings. In all SPn the glycinergic agonists glycine, beta-alanine, and taurine induced dose-dependent inward currents with the affinity for glycine being higher than that for beta-alanine and taurine. Glycine-induced responses were blocked by the glycinergic antagonist strychnine, but were unaffected by either the GABAergic antagonist gabazine, the N-methyl-d-aspartate-receptor antagonist d-2-amino-5-phosphonopentanoic acid, or picrotoxin and cyanotriphenylborate, antagonists of alpha-homomeric and alpha1-subunit-containing glycine receptors, respectively. Under perforated-patch conditions, glycine induced membrane depolarizations that were sufficient to trigger action potentials (APs) in most cells. Furthermore, glycine and taurine decreased the injection currents as well as the synaptic stimulation strength required to elicit APs, indicating that glycine receptors have a consistent excitatory effect on SPn. Inhibition of taurine transport and application of hypoosmolar solutions induced strychnine-sensitive inward currents, suggesting that taurine can act as a possible endogenous agonist on SPn. In summary, these results demonstrate that SPn express glycine receptors that mediate robust excitatory membrane responses during early postnatal development.

Subplate neurons and the patterning of thalamocortial connections

The patterning of the cerebral cortex into functionally distinct domains relies on the formation of appropriate connections between the thalamus and the cortex during development. To identify the mechanisms that underlie cortical target selection by thalamic axons, we have examined the role of cellular interactions in the formation of connections between the lateral geniculate nucleus (LGN) and the visual cortex during development of the cat visual system. The morphology of LGN axons as they grow towards the visual cortex suggests that interactions within the subplate zone may be important in the development of geniculocortical connections. The requirement for subplate neurons in this process was examined by ablating subplate neurons underlying the visual cortex at various developmental stages. When subplate cells are deleted between E38 and E42, prior to target innervation by LGN axons, these axons fail to select the visual cortex as their correct target and instead grow past it, staying restricted to the white matter. Deletion of subplate cells at later stages, between P2 and P7, does not affect target selection, but instead it prevents the segregation of LGN axons into ocular dominance columns within layer IV of the cortex. The effects of subplate neuron ablation suggest that interactions between thalamic axons and subplate cells are of critical importance in the specification of thalamocortical connections during development.

Visualization of corticofugal projections during early cortical development in a tau-GFP-transgenic mouse

The first postmitotic neurons in the developing neocortex establish the preplate layer. These early-born neurons have a significant influence on the circuitry of the developing cortex. However, the exact timing and trajectory of their projections, between cortical hemispheres and intra- and extra-cortical regions, remain unresolved. Here, we describe the creation of a transgenic mouse using a 1.3 kb golli promoter element of the myelin basic protein gene to target expression of a tau-green fluorescent protein (GFP) fusion protein in the cell bodies and processes of pioneer cortical neurons. During embryonic and early neonatal development, the timing and patterning of process extension from these neurons was examined. Analysis of tau-GFP fluorescent fibers revealed that progression of early labeled projections was interrupted unexpectedly by transient pauses at the corticostriatal and telencephalic-diencephalic boundaries before invading the thalamus just prior to birth. After birth the pioneering projections differentially invaded the thalamus, excluding some nuclei, e.g. medial and lateral geniculate, until postnatal days 10-14. Early labeled projections were also found to cross to the contralateral hemisphere as well as to the superior colliculus. These results indicate that early corticothalamic projections appear to pause before invading specific subcortical regions during development, that there is developmental regulation of innervation of individual thalamic nuclei, and that these early-generated neurons also establish early projections to commissural and subcortical targets.

GABAergic neurons in the lateral superior olive of the hamster are distinguished by differential expression of gad isoforms during development

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter that is synthesized by two isoforms of glutamic acid decarboxylase (GAD), GAD65 and GAD67. Using in situ hybridization and immunocytochemical techniques in hamsters, we investigated the postnatal development of GAD isoforms within the lateral superior olive (LSO) where GABAergic neurons form part of a descending efferent projection to the cochlea. In the neonatal hamster LSO, GAD67 immunoreactivity, GAD67 transcript labeling, and intense GABA immunostaining are at low levels. However, robust GAD65 mRNA expression is found throughout the LSO during the early postnatal period. The neonatal GABAergic expression patterns are in stark contrast to the adult where the LSO has robust GAD67 mRNA expression and weak GAD65 mRNA expression. Cells exhibiting intense GABA immunolabeling were also found in the same LSO locations as robust GAD67 mRNA expression and intense GAD67 immunoreactivity. Additionally, GAD67-positive cells in the LSO were retrogradely labeled from the cochlea confirming that these cells are a part of the lateral olivocochlear system. The late onset of GAD67 expression and intense GABA immunoreactivity in LSO neurons are consistent with the relatively late maturation of the lateral olivocochlear neurons inferred from previous studies. During development, these data lead us to conclude that the GABAergic portion of the lateral olivocochlear system is distinguished by preferential GAD67 expression, intense GABA immunoreactivity, and relatively late postnatal onset.

A unique subpopulation of Tbr1-expressing deep layer neurons in the developing cerebral cortex

Cells of the subplate (SP) and deep cortical plate (CP) are among the pioneer neurons of the developing cerebral cortex, an important group of early-born cells that impact cortical organization and function. Similarities between pioneer neurons in different cortical positions and heterogeneities in pioneer cells in the same cortical location, however, have made it difficult to appreciate the characteristics and functions of particular sets of these cells. Here, we provide a tool to illuminate a unique subset of SP and deep CP neurons: expression of a Tbrain-1 (Tbr1)-driven transgene. Transgene-expressing cells were consistently positive for neuronal but not glial markers, were born early in corticogenesis, representing just a subset of SP and deep CP neurons, were morphologically complex during the formation of the cortex, and were maintained into maturity. This analysis reveals a novel group of pioneer neurons and demonstrates unrecognized diversity within this cortical population. In the future, this information will help to uncover the roles of discrete pioneer populations in cortical development.

Ancestry of the mammalian preplate and its derivatives: evolutionary relicts or embryonic adaptations?

Mammalian cortical development is preceded by the elaboration of a transient preplate, which is split into a superficial marginal zone and a deep subplate after the arrival of the cortical plate. There has been some controversy in the evolutionary interpretation of this transient structure, as some propose it to represent the ancestral cortex or pallium of non-mammals, while others consider it to be a phylogenetic novelty. The preplate and its derivatives contain components derived by both tangential and radial migration. Tangentially migrating elements include pioneer neurons and interneurons, both of subpallial origin, and Cajal-Retzius cells, mostly of pallial origin. Pioneer neurons were probably present in the ancestors of mammals, but may have changed their original superficial position to one below the developing cortex, thus attracting thalamic afferents in the subcortical white matter, and making them penetrate the cortex radially. In mammals, Cajal-Retzius cells appear to have increased both in number and on their level of reelin expression, perhaps in the context of controlling the final stages of migration in a radially expanding neoocortex. Radial-migrating cells are partly represented by the pyramidal-like cells of the subplate. These neurons resemble the excitatory elements of the adult reptilian cortex, but is not clear whether they are their true homologues. One possibility is that these cells appeared by virtue of a heterochronic process in which the earliest radial elements of the cortical plate began to be produced at progressively earlier developmental stages. Thus, we conclude that the mammalian preplate and its derivatives contain both ancestral and derived elements, all of which have been modified in the course of mammalian evolution to support an increasingly complex cortical plate development.

Systemic prenatal insults disrupt telencephalon development: implications for potential interventions

Infants born prematurely are prone to chronic neurologic deficits including cerebral palsy, epilepsy, cognitive delay, behavioral problems, and neurosensory impairments. In affected children, imaging and neuropathological findings demonstrate significant damage to white matter. The extent of cortical damage has been less obvious. Advances in the understanding of telencephalon development provide insights into how systemic intrauterine insults affect the developing white matter, subplate, and cortex, and lead to multiple neurologic impairments. In addition to white matter oligodendrocytes and axons, other elements at risk for perinatal brain injury include subplate neurons, GABAergic neurons migrating through white matter and subplate, and afferents of maturing neurotransmitter systems. Common insults including hypoxia-ischemia and infection often affect the developing brain differently than the mature brain, and insults precipitate a cascade of damage to multiple neural lineages. Insights from development can identify potential targets for therapies to repair the damaged neonatal brain before it has matured.

Perinatal subplate neuron injury: implications for cortical development and plasticity

Perinatal brain injury may result in widespread deficits in visual, motor and cognitive systems suggesting disrupted brain development. Neurosensory and cognitive impairment are observed at increasing frequency with decreasing gestational ages, suggesting a unique vulnerability of the developing brain. The peak of human subplate neuron development coincides with the gestational ages of highest vulnerability to perinatal brain injury in the premature infant. At the same time, human thalamocortical connections are forming and being refined by activity-dependent mechanisms during critical periods. Subplate neurons are the first cortical neurons to mature and are selectively vulnerable to early hypoxic-ischemic brain injury in animal models. Timing of subplate neuron death determines the resulting defect in thalamocortical development: very early excitotoxic subplate neuron death results in failure of thalamocortical innervation, while later subplate neuron death interferes with the refinement of thalamocortical connections into mature circuits. We suggest that subplate neuron injury may be a central component of perinatal brain injury resulting in specific neurodevelopmental consequences.

A unique subpopulation of Tbr1-expressing deep layer neurons in the developing cerebral cortex

Cells of the subplate (SP) and deep cortical plate (CP) are among the pioneer neurons of the developing cerebral cortex, an important group of early-born cells that impact cortical organization and function. Similarities between pioneer neurons in different cortical positions and heterogeneities in pioneer cells in the same cortical location, however, have made it difficult to appreciate the characteristics and functions of particular sets of these cells. Here, we provide a tool to illuminate a unique subset of SP and deep CP neurons: expression of a Tbrain-1 (Tbr1)-driven transgene. Transgene-expressing cells were consistently positive for neuronal but not glial markers, were born early in corticogenesis, representing just a subset of SP and deep CP neurons, were morphologically complex during the formation of the cortex, and were maintained into maturity. This analysis reveals a novel group of pioneer neurons and demonstrates unrecognized diversity within this cortical population. In the future, this information will help to uncover the roles of discrete pioneer populations in cortical development.

Abnormal cerebral structure is present at term in premature infants

BACKGROUND

Long-term studies of the outcome of very prematurely born infants have clearly documented that the majority of such infants have significant motor, cognitive, and behavioral deficits. However, there is a limited understanding of the nature of the cerebral abnormality underlying these adverse neurologic outcomes.

AIM

The overall aim of this study was to define quantitatively the alterations in cerebral tissue volumes at term equivalent in a large longitudinal cohort study of very low birth weight premature infants in comparison to term-born infants by using advanced volumetric 3-dimensional magnetic resonance imaging (MRI) techniques. We also aimed to define any relationship of such perinatal lesions as white matter (WM) injury or other potentially adverse factors to the quantitative structural alterations. Additionally, we wished to identify the relationship of the structural alterations to short-term neurodevelopmental outcome.

METHODS

From November 1998 to December 2000, 119 consecutive premature infants admitted to the neonatal intensive care units at Christchurch Women's Hospital (Christchurch, New Zealand) and the Royal Women's Hospital (Melbourne, Australia) were recruited (88% of eligible) after informed parental consent to undergo an MRI scan at term equivalent. Twenty-one term-born infants across both sites were recruited also. Postacquisition advanced 3-dimensional tissue segmentation with 3-dimensional reconstruction was undertaken to estimate volumes of cerebral tissues: gray matter (GM; cortical and deep nuclear structures), WM (myelinated and unmyelinated), and cerebrospinal fluid (CSF).

RESULTS

In comparison to the term-born infants, the premature infants at term demonstrated prominent reductions in cerebral cortical GM volume (premature infants [mean +/- SD]: 178 +/- 41 mL; term infants: 227 +/- 26 mL) and in deep nuclear GM volume (premature infants: 10.8 +/- 4.1 mL; term infants: 13.8 +/- 5.2 mL) and an increase in CSF volume (premature infants: 45.6 +/- 22.1 mL; term infants: 28.9 +/- 16 mL). The major predictors of altered cerebral volumes were gestational age at birth and the presence of cerebral WM injury. Infants with significantly reduced cortical GM and deep nuclear GM volumes and increased CSF volume volumes exhibited moderate to severe neurodevelopmental disability at 1 year of age.

CONCLUSIONS

This MRI study of prematurely born infants further defines the nature of quantitative cerebral structural abnormalities present as early as term equivalent. The abnormalities particularly involve cerebral neuronal regions including both cortex and deep nuclear structures. The pattern of cerebral alterations is related most significantly to the degree of immaturity at birth and to concomitant WM injury. The alterations are followed by abnormal short-term neurodevelopmental outcome.

Neurons in the corpus callosum of the cat during postnatal development

The corpus callosum (CC) is a major telencephalic commissure containing mainly cortico-cortical axons and glial cells. We have identified neurons in the CC of the cat and quantified their number at different postnatal ages. An antibody against microtubule-associated protein 2 was used as a marker of neurons. Immunocytochemical double-labelling with neuron-specific enolase or gamma-aminobutyric acid antibodies in the absence of glial fibrillary acidic protein positivity confirmed the neuronal phenotype of these cells. CC neurons were also stained with anti-calbindin and anti-calretinin antibodies, typical for interneurons, and with an anti-neurofilament antibody, which in neocortex detects pyramidal neurons. Together, these findings suggest that the CC contains a mixed population of neuronal types. The quantification was corrected for double counting of adjacent sections and volume changes during CC development. Our data show that CC neurons are numerous early postnatally, and their number decreases with age. At birth, about 570 neurons are found within the CC boundaries and their number drops to about 200 in the adult. The distribution of the neurons within the CC also changes in development. Initially, many neurons are found throughout the CC, while at later ages they become restricted to the boundaries of the CC, and in the adult to the rostrum of the CC close to the septum pellucidum or to the indusium griseum. Although origin and function of transient CC neurons in development and in adulthood remain unknown, they are likely to be interstitial neurons. Some of them have well-developed and differentiated processes and resemble pyramidal cells or interneurons. An axon-guiding function during the early postnatal period can not be excluded.

Cellular physiology of the neonatal rat cerebral cortex

The early development of the cerebral cortex is characterized by neurogenesis, neuronal migration, cellular differentiation and programmed cell death. Cajal-Retzius cells, developing cortical plate neurons and subplate cells form a transient synaptic circuit which may serve as a template for the formation of cortical layers and columns. These three neuronal cell types show distinct electrophysiological properties and synaptic inputs. Endogenous or exogenous harmful disturbances during this developmental period may lead to the preservation of early cortical circuits, which may act as trigger zones for the initiation of pathophysiological activity.

Absence of selectivity in the loss of neurons from the developing cortical subplate of the rat

Neurons of the cortical subplate display evidence of cell death, although a significant population survives to the mature brain. The present study examined different populations of neurons to determine if the loss of cells was specific for a particular cell type. Immunocytochemical procedures for neurons expressing GluR2/3, GAD, or NPY, were used on tissue sections taken from animals at gestational day 18 to postnatal day 21. The rate of loss of labeled cells was similar for all groups of neurons. Thus, these data reveal no evidence that the loss of subplate neurons is specific to any major cell type.

Functional synaptic projections onto subplate neurons in neonatal rat somatosensory cortex

Subplate neurons (SPn) play an important role in the formation of thalamocortical connections during early development and show glutamatergic and GABAergic spontaneous synaptic activity. We characterized these synaptic inputs by performing whole-cell recordings from SPn in somatosensory cortical slices of postnatal day 0-3 rats. At -70 mV, electrical stimulation of the thalamocortical afferents elicited in 68% of the SPn a monosynaptic CNQX-sensitive postsynaptic current (PSC). These fast PSCs were mediated by AMPA receptors, because they were prolonged by cyclothiazide and blocked by GYKI 52466. On membrane depolarization, thalamocortical stimulation elicited in 50% of the cells an additional slow monosynaptic component mediated by NMDA receptors. Stimulation of the cortical plate evoked in 72% of SPn a monosynaptic AMPA receptor-mediated PSC with an additional NMDA component at depolarized membrane potentials and in 40% of the investigated cells polysynaptic responses, depending on GABA(A) and NMDA receptors. Stimulation of the subplate elicited in 67% of SPn a monosynaptic dual-component PSC mediated by AMPA and NMDA receptors activated at -70 mV and in 12% of SPn a monosynaptic single-component PSC mediated by AMPA receptors with an additional NMDA component activated at depolarized membrane potentials. A monosynaptic GABAergic response could be observed in 68% of SPn after stimulation of the subplate. In gramicidin-perforated patch recordings, bath application of GABA caused membrane depolarization to -40 mV and elicited action potentials. These results demonstrate that SPn receive distinct functional synaptic inputs arising from the thalamus, cortical plate, and subplate, indicating that SPn are capable of integrating and processing information from cortical and subcortical regions.

Do subplate neurons comprise a transient population of cells in developing neocortex of rats?

Studies were undertaken to determine whether neurons of the subplate layer represent a transient or stable population of cells in developing neocortex of rat. The first set of studies sought to determine the fraction of subplate neurons that is lost during early postnatal development. The optical dissector method was used to analyze fluorescently stained material in animals the age of postnatal day 0 (P0) to P40. These results demonstrate a reduction of slightly less than half of the total number of subplate neurons from P0 to P40. Counts of labeled cells in littermates at varied ages after [(3)H]thymidine or BRDU treatment on gestational day 14 (G14 - birthdate of occipital subplate neurons) or G18 (birthdate of layers III-IV neurons) demonstrate loss of approximately 50% of neurons in the subplate layer between P0 and P40, somewhat greater than the loss of neurons from cortical layers III-IV. The second set of studies investigated whether subplate neurons display cellular atrophy during postnatal development. Analysis of subplate neurons injected intracellularly with Lucifer yellow in fixed slice preparations indicates no reduction in soma size, number of dendrites, or extent of dendritic fields of subplate neurons taken from animals age P0 to P60. The third set of studies investigated whether functional markers of subplate neurons are reduced during postnatal development. Analysis of tissue stained histochemically for cytochrome oxidase or acetylcholinesterase, or stained immunocytochemically for GABA, somatostatin, or neuropeptide Y, demonstrate a remarkable loss of expression of staining patterns from late gestational ages to P20. These data demonstrate that, although subplate neurons seem not to be a transient population of cells in the usual sense of being eliminated by cell death or structural atrophy, the loss of histochemical and immunocytochemical markers indicates that they may be a functionally transient population of cells.

The lack of Emx2 causes impairment of Reelin signaling and defects of neuronal migration in the developing cerebral cortex

Neocorticogenesis in mice homozygous for an Emx2 null allele is the topic of this article. The development of both main components of neocortex, primordial plexiform layer derivatives and cortical plate, was analyzed, paying special attention to radial migration of neurons forming the cortical plate. The products of the Reelin gene, normally playing a key role in orchestrating radial migration of these neurons, display normal distribution at the beginning of the cortical neuronogenesis but are absent in the neocortical marginal zone of the mutant mice at the time when the cortical plate is laid down. As a consequence, the development of radial glia is impaired, and neurons making up the cortical plate display abnormal migration patterns. In addition, restricted defects along the rostrocaudal and the mediolateral axes are present in the subplate, suggesting an Emx2-specific role in priming the proper development of this layer.

Prenatal blockade of vasoactive intestinal peptide alters cell death and synaptic equipment in the murine neocortex

Vasoactive intestinal peptide (VIP) is a potent growth factor that stimulates murine neocortical astrocyte genesis during the period of ontogenesis corresponding to premature delivery in humans. In rodents, part of the VIP supplied to the fetal brain is maternal VIP that crosses the placenta. If these data also apply to human brain development, premature newborns may be partly VIP-deficient because of loss of the maternal supply, and this may adversely affect their brain development. The goal of the present study was to determine the effects of VIP blockade during mouse neocortical astrocyte genesis on neuritic survival and maturation. VIP blockade by a specific VIP antagonist on embryonic d 17 and 18 induced transient, postnatal depletion of astrocytes in the upper neocortical layers. Combined use of in situ DNA fragmentation analysis (terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling method, a marker of cell death); immunohistochemical detection of synaptophysin, microtubule-associated proteins, and neurofilaments; and quantification of mRNA for synaptophysin and N-methyl-D-aspartate R1 receptor subunit revealed that early VIP blockade significantly altered programmed neuritic death and impaired neuritic differentiation. VIP inhibition induced 1) exaggerated postnatal terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling of cortical neurons, 2) long-term overexpression of synaptophysin and N-methyl-D-aspartate R1 receptor subunit, and 3) long-term overexpression of microtubule-associated protein-5 and neurofilament 160 kD. Although the functional consequences of this deviant pattern of murine neocortical development remain to be determined, these data open up new avenues for investigating some of the cognitive deficits observed in human premature infants.

Connections between cells of the internal capsule, thalamus, and cerebral cortex in embryonic rat

The aim of our study is to understand the development of the earliest connections in the mammalian pallium by documenting the distribution of cells and fibres labelled from the dorsal and ventral thalamus, internal capsule, perirhinal, and dorsal cortex during the period between embryonic day (E) 14 and 17 by using carbocyanine dye tracing in fixed embryonic rat brains. Dye placed in the thalamus of E14 brains backlabels cells in the thalamic reticular nucleus and within the primitive internal capsule. Both anterograde and retrograde tracing confirmed that the first corticofugal projections reach the internal capsule by E14. At E15-E16, after the first cortical plate cells have migrated into the lateral cortex, some cells of the cortical plate and subplate and marginal zone, are backlabelled from the internal capsule, but still not from the dorsal thalamus, even with very long incubation periods. Crystal placement into the perirhinal cortex at E14-E15 labels numerous cells within the internal capsule, whereas no such cells are revealed from dorsal cerebral cortex until E17, suggesting that internal capsule cells establish early connections with the perirhinal and ventral but not dorsal cortex. We propose that the growth of axons from cortex to dorsal thalamus is delayed in two regions: first from E14-E15 at the lateral entrance of the internal capsule and then, from E16, closer to the thalamus, probably within the thalamic reticular nucleus. Subplate projections reach the proximity of the diencephalon at an early stage, but they might never enter the dorsal thalamus.

Establishment of patterned thalamocortical connections does not require nitric oxide synthase

Subplate neurons are early-generated neurons that project into the overlying neocortex and are required for the formation of ocular dominance columns. A subset of subplate neurons express nitric oxide synthase (NOS) and produce nitric oxide (NO), a neuronal messenger thought to be involved in adult hippocampal synaptic plasticity and also in the establishment of certain specific connections during visual system development. Here, we examine whether the NOS-containing subplate neurons are involved in ocular dominance column formation in the ferret visual system. Ocular dominance columns form in ferrets between postnatal day 35 (P35) and P60. NOS expression in the visual subplate is low at birth, increases to a maximum at the onset of ocular dominance column formation, and falls thereafter. Nevertheless, blockade of NOS with daily injections of nitroarginine from P14 to P56 fails to prevent the formation of ocular dominance columns, although NOS activity is reduced by >98%. To test further a requirement for NOS in the patterning of connections during CNS development, we examined the cortical barrels in the somatosensory system of mice carrying targeted disruptions of NOS that also received injections of nitroarginine; cortical barrels formed normally in these animals. In addition, barrel field plasticity induced by whisker ablation at birth was normal in nitroarginine-injected NOS knock-out mice. Thus, despite the dynamic regulation of NOS in subplate neurons, NO is unlikely to be essential for the patterning of thalamocortical connections either in visual or somatosensory systems.