The subplate and early cortical circuits

Development of the corticothalamic projections

In this review we discuss recent advances in the understanding of corticothalamic axon guidance; patterning of the early telencephalon, the sequence and choreography of the development of projections from subplate, layers 5 and 6. These cortical subpopulations display different axonal outgrowth kinetics and innervate distinct thalamic nuclei in a temporal pattern determined by cortical layer identity and subclass specificity. Guidance by molecular cues, structural cues, and activity-dependent mechanisms contribute to this development. There is a substantial rearrangement of the corticofugal connectivity outside the thalamus at the border of and within the reticular thalamic nucleus, a region that shares some of the characteristics of the cortical subplate during development. The early transient circuits are not well understood, nor the extent to which this developmental pattern may be driven by peripheral sensory activity. We hypothesize that transient circuits during embryonic and early postnatal development are critical in the matching of the cortical and thalamic representations and forming the cortical circuits in the mature brain.

Mechanisms controlling the guidance of thalamocortical axons through the embryonic forebrain

Thalamocortical axons must cross a complex cellular terrain through the developing forebrain, and this terrain has to be understood for us to learn how thalamocortical axons reach their destinations. Selective fasciculation, guidepost cells and various diencephalic and telencephalic gradients have been implicated in thalamocortical guidance. As our understanding of the relevant forebrain patterns has increased, so has our knowledge of the guidance mechanisms. Our aim here is to review recent observations of cellular and molecular mechanisms related to: the growth of thalamofugal projections to the ventral telencephalon, thalamic axon avoidance of the hypothalamus and extension into the telencephalon to form the internal capsule, the crossing of the pallial-subpallial boundary, and the growth towards the cerebral cortex. We shall review current theories for the explanation of the maintenance and alteration of topographic order in the thalamocortical projections to the cortex. It is now increasingly clear that several mechanisms are involved at different stages of thalamocortical development, and each contributes substantially to the eventual outcome. Revealing the molecular and cellular mechanisms can help to link specific genes to details of actual developmental mechanisms.

Pain assessment in human fetus and infants

In humans, painful stimuli can arrive to the brain at 20-22 weeks of gestation. Therefore several researchers have devoted their efforts to study fetal analgesia during prenatal surgery, and during painful procedures in premature babies. Aim of this paper is to gather from scientific literature the available data on the signals that the human fetus and newborns produce, and that can be interpreted as signals of pain. Several signs can be interpreted as signals of pain. We will describe them in the text. In infants, these signs can be combined to create specific and sensible pain assessment tools, called pain scales, used to rate the level of pain.

Impact of prenatal ischemia on behavior, cognitive abilities and neuroanatomy in adult rats with white matter damage

Early brain damage, such as white matter damage (WMD), resulting from perinatal hypoxia-ischemia in preterm and low birth weight infants represents a high risk factor for mortality and chronic disabilities, including sensory, motor, behavioral and cognitive disorders. In previous studies, we developed a model of WMD based on prenatal ischemia (PI), induced by unilateral ligation of uterine artery at E17 in pregnant rats. We have shown that PI reproduced some of the main deficits observed in preterm infants, such as white and gray matter damage, myelination deficits, locomotor, sensorimotor, and short-term memory impairments, as well as related musculoskeletal and neuroanatomical histopathologies [1-3]. Here, we determined the deleterious impact of PI on several behavioral and cognitive abilities in adult rats, as well as on the neuroanatomical substratum in various related brain areas. Adult PI rats exhibited spontaneous exploratory and motor hyperactivity, deficits in information encoding, and deficits in short- and long-term object memory tasks, but no impairments in spatial learning or working memory in watermaze tasks. These results were in accordance with white matter injury and damage in the medial and lateral entorhinal cortices, as detected by axonal degeneration, astrogliosis and neuronal density. Although there was astrogliosis and axonal degeneration in the fornix, hippocampus and cingulate cortex, neuronal density in the hippocampus and cingulate cortex was not affected by PI. Levels of spontaneous hyperactivity, deficits in object memory tasks, neuronal density in the medial and lateral entorhinal cortices, and astrogliosis in the fornix correlated with birth weight in PI rats. Thus, this rodent model of WMD based on PI appears to recapitulate the main neurobehavioral and neuroanatomical human deficits often observed in preterm children with a perinatal history of ischemia.

Is fetal pain a real evidence?

Due to the progress in fetal surgery, it is important to acquire data about fetal pain.

MATERIAL AND METHODS

We performed a Medline research from 1995, matching the following key words: "pain" and "fetus", with the following: "subplate", "thalamocortical", "myelination", "analgesia", "anesthesia", "brain", "behavioral states", "substance p". We focused on: (a) fetal development of nociceptive pathways; (b) fetal electrophysiological, endocrinological and behavioral reactions to stimuli and pain.

RESULTS

We retrieved 217 papers of which 157 were highly informative; some reported similar data or were only case-reports, and were not quoted. Most endocrinological, behavioral and electrophysiological studies of fetal pain are performed in the third trimester, and they seem to agree that the fetus in the 3rd trimester can experience pain. But the presence of fetal pain in the 2nd trimester is less evident. In favor of a 2nd trimester perception of pain is the early development of spino-thalamic pathways (approximately from the 20th week), and the connections of the thalamus with the subplate (approximately from the 23rd week). Against this possibility, some authors report the immaturity of the cortex with the consequent lack of awareness, and the almost continuous state of sleep of the fetus.

CONCLUSIONS

Most studies disclose the possibility of fetal pain in the third trimester of gestation. This evidence becomes weaker before this date, though we cannot exclude its increasing presence since the beginning of the second half of the gestation.

What can we get from 'barrels': the rodent barrel cortex as a model for studying the establishment of neural circuits

Sensory inputs triggered by external stimuli are projected into discrete arrays of neuronal modules in the primary sensory cortex. This whisker-to-barrel pathway has gained in popularity as a model system for studying the development of cortical circuits and sensory processing because its clear patterns facilitate the identification of genetically modified mice with whisker map deficits and make possible coordinated in vitro and in vivo electrophysiological studies. Numerous whisker map determinants have been identified in the past two decades. In this review, we summarize what have we learned from the detailed studies conducted in various mutant mice with cortical whisker map deficits. We will specifically focus on the anatomical and functional establishment of the somatosensory thalamocortical circuits.

Subplate neurons promote spindle bursts and thalamocortical patterning in the neonatal rat somatosensory cortex

Patterned neuronal activity such as spindle bursts in the neonatal cortex is likely to promote the maturation of cortical synapses and neuronal circuits. Previous work on cats has shown that removal of subplate neurons, a transient neuronal population in the immature cortex, prevents the functional maturation of thalamocortical and intracortical connectivity. Here we studied the effect of subplate removal in the neonatal rat primary somatosensory cortex (S1). Using intracortical EEG we show that after selective removal of subplate neurons in the limb region of S1, endogenous and sensory evoked spindle burst activity is largely abolished. Consistent with the reduced in vivo activity in the S1 limb region, we find by in vitro recordings that thalamocortical inputs to layer 4 neurons are weak. In addition, we find that removal of subplate neurons in the S1 barrel region prevents the development of the characteristic histological barrel-like appearance. Thus, subplate neurons are crucially involved in the generation of particular types of early network activity in the neonatal cortex, which are an important feature of cortical development. The altered EEG pattern following subplate damage could be applicable in the neurological assessment of human neonates.

Diversity of GABAergic interneurons in layer VIa and VIb of mouse barrel cortex

Neocortical layer VI modulates the thalamocortical transfer of information and has a significant impact on sensory processing. This function implicates local γ-aminobutyric acidergic (GABAergic) interneurons that have only been partly described at the present time. Here, we characterized 85 layer VI GABAergic interneurons in acute slices of mouse somatosensory barrel cortex, using whole-cell current-clamp recordings, single-cell reverse transcription-polymerase chain reaction, and biocytin labeling followed by Neurolucida reconstructions. Unsupervised clustering based on electrophysiological molecular and morphological properties disclosed 4 types of interneurons. The 2 major classes were fast-spiking cells transcribing parvalbumin (PV) (51%) and adapting interneurons transcribing somatostatin (SOM) (26%). The third population (18%) transcribed neuropeptide Y (NPY) and appeared very similar to neurogliaform cells. The last class (5%) was constituted by well-segregated GABAergic interneurons transcribing vasoactive intestinal peptide (VIP). Using transgenic mice expressing GFP under the control of the glutamic acid decarboxylase 67k (GAD67) promoter, we investigated the densities of GABAergic cells immunolabeled against PV, SOM, VIP, and NPY through the depth of layer VI. This analysis revealed that PV and NPY translating interneurons concentrate in the upper and lower parts of layer VI, respectively. This study provides an extensive characterization of the properties of layer VI interneurons.

Long-range temporal correlations in the EEG bursts of human preterm babies

The electrical activity in the very early human preterm brain, as recorded by scalp EEG, is mostly discontinuous and has bursts of high-frequency oscillatory activity nested within slow-wave depolarisations of high amplitude. The temporal organisation of the occurrence of these EEG bursts has not been previously investigated. We analysed the distribution of the EEG bursts in 11 very preterm (23-30 weeks gestational age) human babies through two estimates of the Hurst exponent. We found long-range temporal correlations (LRTCs) in the occurrence of these EEG bursts demonstrating that even in the very immature human brain, when the cerebral cortical structure is far from fully developed, there is non-trivial temporal structuring of electrical activity.

Developmental profiles of infant EEG: overlap with transient cortical circuits

OBJECTIVE

To quantify spectral power in frequency specific bands and commonly observed types of bursting activities in the EEG during early human development.

METHODS

An extensive archive of EEG data from human infants from 35 to 52 weeks postmenstrual age obtained in a prior multi-center study was analyzed using power spectrum analyses and a high frequency burst detection algorithm.

RESULTS

Low frequency power increased with age; however, high frequency power decreased from 35 to 45 weeks. This unexpected decrease was largely attributable to a rapid decline in the number of high frequency bursts.

CONCLUSIONS

The decline in high frequency bursting activity overlaps with a developmental shift in GABA's actions on neurons from depolarizing to hyperpolarizing and the dissolution of the gap junction circuitry of the cortical subplate.

SIGNIFICANCE

We postulate that quantitative characterization of features of the EEG unique to early development provide indices for tracking changes in specific neurophysiologic mechanisms that are critical for normal development of brain function.

Changing microcircuits in the subplate of the developing cortex

Subplate neurons (SPNs) are a population of neurons in the mammalian cerebral cortex that exist predominantly in the prenatal and early postnatal period. Loss of SPNs prevents the functional maturation of the cerebral cortex. SPNs receive subcortical input from the thalamus and relay this information to the developing cortical plate and thereby can influence cortical activity in a feedforward manner. Little is known about potential feedback projections from the cortical plate to SPNs. Thus, we investigated the spatial distribution of intracortical synaptic inputs to SPNs in vitro in mouse auditory cortex by photostimulation. We find that SPNs fell into two broad classes based on their distinct spatial patterns of synaptic inputs. The first class of SPNs receives inputs from only deep cortical layers, while the second class of SPNs receives inputs from deep as well as superficial layers including layer 4. We find that superficial cortical inputs to SPNs emerge in the second postnatal week and that SPNs that receive superficial cortical input are located more superficially than those that do not. Our data thus suggest that distinct circuits are present in the subplate and that, while SPNs participate in an early feedforward circuit, they are also involved in a feedback circuit at older ages. Together, our results show that SPNs are tightly integrated into the developing thalamocortical and intracortical circuit. The feedback projections from the cortical plate might enable SPNs to amplify thalamic inputs to SPNs.

Bisphenol A, an endocrine-disrupting chemical, and brain development

Bisphenol A (BPA) is an endocrine-disrupting chemical, widely used in various industries and the field of dentistry. The consequent increase in BPA exposure among humans has led us to some concerns regarding the potential deleterious effects on reproduction and brain development. The emphasis of this review is on the effects of prenatal and lactational exposure to low doses of BPA on brain development in mice. We demonstrated that prenatal exposure to BPA affected fetal murine neocortical development by accelerating neuronal differentiation/migration during the early embryonic stage, which was associated with up- and down-regulation of the genes critical for brain development, including the basic helix-loop-helix transcription factors. In the adult mice brains, both abnormal neocortical architecture and abnormal corticothalamic projections persisted in the group exposed to the BPA. Functionally, BPA exposure disturbed murine behavior, accompanied with a disrupted neurotransmitter system, including monoamines, in the postnatal development period and in adult mice. We also demonstrated that epigenetic alterations in promoter-associated CpG islands might underlie some of the effects on brain development after exposure to BPA.

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.

The Long and the Short of it: Gene and Environment Interactions During Early Cortical Development and Consequences for Long-Term Neurological Disease

Cortical development is a complex amalgamation of proliferation, migration, differentiation, and circuit formation. These processes follow defined timescales and are controlled by a combination of intrinsic and extrinsic factors. It is currently unclear how robust and flexible these processes are and whether the developing brain has the capacity to recover from disruptions. What is clear is that there are a number of cognitive disorders or conditions that are elicited as a result of disrupted cortical development, although it may take a long time for the full pathophysiology of the conditions to be realized clinically. The critical window for the manifestation of a neurodevelopmental disorder is prolonged, and there is the potential for a complex interplay between genes and environment. While there have been extended investigations into the genetic basis of a number of neurological and mental disorders, limited definitive associations have been discovered. Many environmental factors, including inflammation and stress, have been linked to neurodevelopmental disorders, and it may be that a better understanding of the interplay between genes and environment will speed progress in this field. In particular, the development of the brain needs to be considered in the context of the whole materno-fetal unit as the degree of the metabolic, endocrine, or inflammatory responses, for example, will greatly influence the environment in which the brain develops. This review will emphasize the importance of extending neurodevelopmental studies to the contribution of the placenta, vasculature, cerebrospinal fluid, and to maternal and fetal immune response. These combined investigations are more likely to reveal genetic and environmental factors that influence the different stages of neuronal development and potentially lead to the better understanding of the etiology of neurological and mental disorders such as autism, epilepsy, cerebral palsy, and schizophrenia.

Spatiotemporal properties of cortical haemodynamic response to auditory stimuli in sleeping infants revealed by multi-channel near-infrared spectroscopy

Multi-channel near-infrared spectroscopy (NIRS) has been used as a neuroimaging tool to study functional activation of the developing brain in infants. In this paper, we focus on spatiotemporal dynamics of cortical oxygenation changes during sensory processing in young infants. We use a 94-channel NIRS system to assess the activity of wide regions of the cortex in quietly sleeping three-month-old infants. Auditory stimuli composed of a random sequence of pure tones induced haemodynamic changes not only in the temporal auditory regions, but also in the occipital and frontal regions. Analyses of phase synchronization showed that mutual synchronizations of signal changes among the cortical regions were much stronger than the stimulus-induced synchronizations of signal changes. Furthermore, analyses of phase differences among cortical regions revealed phase advancement of the bilateral temporal auditory regions, and phase gradient in a posterior direction from the temporal auditory regions to the occipital regions and in an anterior direction within the frontal regions. We argue that multi-channel NIRS is capable of detecting the precise timing of cortical activation and its flow in the global network of the developing brain.

Pioneer GABA cells comprise a subpopulation of hub neurons in the developing hippocampus

Connectivity in the developing hippocampus displays a functional organization particularly effective in supporting network synchronization, as it includes superconnected hub neurons. We have previously shown that hub network function is supported by a subpopulation of GABA neurons. However, it is unclear whether hub cells are only transiently present or later develop into distinctive subclasses of interneurons. These questions are difficult to assess given the heterogeneity of the GABA neurons and the poor early expression of markers. To circumvent this conundrum, we used "genetic fate mapping" that allows for the selective labeling of GABA neurons based on their place and time of origin. We show that early-generated GABA cells form a subpopulation of hub neurons, characterized by an exceptionally widespread axonal arborization and the ability to single-handedly impact network dynamics when stimulated. Pioneer hub neurons remain into adulthood, when they acquire the classical markers of long-range projecting GABA neurons.

Evidence for structural and functional changes of subplate neurons in developing rat barrel cortex

In the developing sensory cortex, the subplate could serve as a transient relay station between the thalamus and cortical plate and assists the formation of thalamocortical projection. While the thalamus-layer IV connection is formed, the thalamic activation of subplate is diminished. In the present study, we aimed to explore the mechanism which may attribute to the decline of subplate activity. To resolve this issue, the developmental changes of subplate neurons (SPns) in rat somatosensory cortex were examined during the first two postnatal weeks which covers the stages prior and subsequent to the establishment of thalamocortical connection. During development, more SPns exhibited regular-spiking firing pattern and the membrane properties of SPns displayed a continual trend of maturation. In the meantime, the excitability of SPns decreased as revealed by increasing rheobase and rightwardly shifted frequency-current curves. On the other hand, increasing paired-pulse ratio and slowing MK-801 blocking rate were noted during development, implying the reduction of presynaptic transmitter release. Morphologically, the size of SPn soma increased with age while the shape became flat. The total length, branching nodes and segments of dendrites increased significantly during the first week. However, after peaking around day 10, these values decreased, implying a pruning process. Our findings here propose that the reduction of neuronal excitability, synaptic transmission and dendritic complexity may attribute to the decline of functional connectivity between thalamus and subplate and reduction of subplate activity while the thalamocortical pathway is established.

Neonatal fluoxetine exposure affects the action potential properties and dendritic development in cortical subplate neurons of rats

Selective serotonin reuptake inhibitor (SSRI)-type antidepressants might be given to depressive pregnant women and the developing fetuses are thus exposed to these drugs. Since serotonin plays important roles in the maturation of the nervous system, early SSRI exposure might influence the fetal brain development. To test this hypothesis, we treated the neonatal rat pups with fluoxetine (Flx) from the day of birth to postnatal day (P) 4, comparable to the third trimester of human gestation, and observed the physiological and morphological features of subplate neurons (SPns), a group of cells important for early cortical development and vulnerable to neonatal neural insults. Using whole-cell patch-clamp recording technique, we examined the passive membrane properties and characteristics of action potential (AP). In SPns of Flx-treated rats, the rheobase for generating an AP was increased and the width of APs was reduced, especially in the falling phase. In the morphological aspect, the dendritic remodeling of SPns including dendritic branching, elongation and pruning were affected by early Flx treatment. Together, our results demonstrate that the teratogenic effect of early SSRI exposure on the structure and function of developing SPns and these changes may lead to undesired brain activity and distorted behaviors later in life.

Developmental patterns of doublecortin expression and white matter neuron density in the postnatal primate prefrontal cortex and schizophrenia

Postnatal neurogenesis occurs in the subventricular zone and dentate gyrus, and evidence suggests that new neurons may be present in additional regions of the mature primate brain, including the prefrontal cortex (PFC). Addition of new neurons to the PFC implies local generation of neurons or migration from areas such as the subventricular zone. We examined the putative contribution of new, migrating neurons to postnatal cortical development by determining the density of neurons in white matter subjacent to the cortex and measuring expression of doublecortin (DCX), a microtubule-associated protein involved in neuronal migration, in humans and rhesus macaques. We found a striking decline in DCX expression (human and macaque) and density of white matter neurons (humans) during infancy, consistent with the arrival of new neurons in the early postnatal cortex. Considering the expansion of the brain during this time, the decline in white matter neuron density does not necessarily indicate reduced total numbers of white matter neurons in early postnatal life. Furthermore, numerous cells in the white matter and deep grey matter were positive for the migration-associated glycoprotein polysialiated-neuronal cell adhesion molecule and GAD65/67, suggesting that immature migrating neurons in the adult may be GABAergic. We also examined DCX mRNA in the PFC of adult schizophrenia patients (n = 37) and matched controls (n = 37) and did not find any difference in DCX mRNA expression. However, we report a negative correlation between DCX mRNA expression and white matter neuron density in adult schizophrenia patients, in contrast to a positive correlation in human development where DCX mRNA and white matter neuron density are higher earlier in life. Accumulation of neurons in the white matter in schizophrenia would be congruent with a negative correlation between DCX mRNA and white matter neuron density and support the hypothesis of a migration deficit in schizophrenia.

A transcriptomic atlas of mouse neocortical layers

In the mammalian cortex, neurons and glia form a patterned structure across six layers whose complex cytoarchitectonic arrangement is likely to contribute to cognition. We sequenced transcriptomes from layers 1-6b of different areas (primary and secondary) of the adult (postnatal day 56) mouse somatosensory cortex to understand the transcriptional levels and functional repertoires of coding and noncoding loci for cells constituting these layers. A total of 5,835 protein-coding genes and 66 noncoding RNA loci are differentially expressed ("patterned") across the layers, on the basis of a machine-learning model (naive Bayes) approach. Layers 2-6b are each associated with specific functional and disease annotations that provide insights into their biological roles. This new resource (http://genserv.anat.ox.ac.uk/layers) greatly extends currently available resources, such as the Allen Mouse Brain Atlas and microarray data sets, by providing quantitative expression levels, by being genome-wide, by including novel loci, and by identifying candidate alternatively spliced transcripts that are differentially expressed across layers.

Gene expression analysis of the embryonic subplate

The subplate layer of the cerebral cortex is comprised of a heterogeneous population of cells and contains some of the earliest-generated neurons. In the embryonic brain, subplate cells contribute to the guidance and areal targeting of thalamocortical axons. At later developmental stages, they are predominantly involved in the maturation and plasticity of the cortical circuitry and the establishment of functional modules. We aimed to further characterize the embryonic murine subplate population by establishing a gene expression profile at embryonic day (E) 15.5 using laser capture microdissection and microarrays. The microarray identified over 300 transcripts with higher expression in the subplate compared with the cortical plate at this stage. Using quantitative reverse transcription-polymerase chain reaction, in situ hybridization (ISH), and immunohistochemistry (IHC), we have confirmed specific expression in the E15.5 subplate for 13 selected genes, which have not been previously associated with this compartment (Abca8a, Cdh10, Cdh18, Csmd3, Gabra5, Kcnt2, Ogfrl1, Pls3, Rcan2, Sv2b, Slc8a2, Unc5c, and Zdhhc2). In the reeler mutant, the expression of the majority of these genes (9 of 13) was shifted in accordance with the altered position of subplate. These genes belong to several functional groups and likely contribute to synapse formation and axonal growth and guidance in subplate cells.

Neuroanatomical, sensorimotor and cognitive deficits in adult rats with white matter injury following prenatal ischemia

Perinatal brain injury including white matter damage (WMD) is highly related to sensory, motor or cognitive impairments in humans born prematurely. Our aim was to examine the neuroanatomical, functional and behavioral changes in adult rats that experienced prenatal ischemia (PI), thereby inducing WMD. PI was induced by unilateral uterine artery ligation at E17 in pregnant rats. We assessed performances in gait, cognitive abilities and topographical organization of maps, and neuronal and glial density in primary motor and somatosensory cortices, the hippocampus and prefrontal cortex, as well as axonal degeneration and astrogliosis in white matter tracts. We found WMD in corpus callosum and brainstem, and associated with the hippocampus and somatosensory cortex, but not the motor cortex after PI. PI rats exhibited mild locomotor impairments associated with minor signs of spasticity. Motor map organization and neuronal density were normal in PI rats, contrasting with major somatosensory map disorganization, reduced neuronal density, and a marked reduction of inhibitory interneurons. PI rats exhibited spontaneous hyperactivity in open-field test and short-term memory deficits associated with abnormal neuronal density in related brain areas. Thus, this model reproduces in adult PI rats the main deficits observed in infants with a perinatal history of hypoxia-ischemia and WMD.

A critical period for auditory thalamocortical connectivity

Neural circuits are shaped by experience during periods of heightened brain plasticity in early postnatal life. Exposure to acoustic features produces age-dependent changes through largely unresolved cellular mechanisms and sites of origin. We isolated the refinement of auditory thalamocortical connectivity by in vivo recordings and day-by-day voltage-sensitive dye imaging in an acute brain slice preparation. Passive tone-rearing modified response strength and topography in mouse primary auditory cortex (A1) during a brief, 3-d window, but did not alter tonotopic maps in the thalamus. Gene-targeted deletion of a forebrain-specific cell-adhesion molecule (Icam5) accelerated plasticity in this critical period. Consistent with its normal role of slowing spinogenesis, loss of Icam5 induced precocious stubby spine maturation on pyramidal cell dendrites in neocortical layer 4 (L4), identifying a primary locus of change for the tonotopic plasticity. The evolving postnatal connectivity between thalamus and cortex in the days following hearing onset may therefore determine a critical period for auditory processing.

How do barrels form in somatosensory cortex?

The somatosensory cortex of many rodents, lagomorphs, and marsupials contains distinct cytoarchitectonic features named "barrels" that reflect the pattern of large facial whiskers on the snout. Barrels are composed of clustered thalamocortical afferents relaying sensory information from one whisker surrounded by cell-dense walls or "barrels" in layer 4 of the cortex. In many ways, barrels are a simple and relatively accessible canonical cortical column, making them a common model system for the examination of cortical development and function. Despite their experimental accessibility and popularity, we still lack a basic understanding of how and why barrels form in the first place. In this review, we will examine what is known about mechanisms of barrel development, focusing specifically on the recent literature using the molecular-genetic power of mice as a model system for examining brain development.

Functional connectivity MRI in infants: exploration of the functional organization of the developing brain

Advanced neuroimaging techniques have been increasingly applied to the study of preterm and term infants in an effort to further define the functional cerebral architecture of the developing brain. Despite improved understanding of the complex relationship between structure and function obtained through these investigations, significant questions remain regarding the nature, location, and timing of the maturational changes which occur during early development. Functional connectivity magnetic resonance imaging (fcMRI) utilizes spontaneous, low frequency (< 0.1 Hz), coherent fluctuations in blood oxygen level dependent (BOLD) signal to identify networks of functional cerebral connections. Due to the intrinsic characteristics of its image acquisition and analysis, fcMRI offers a novel neuroimaging approach well suited to investigation of infants. Recently, this methodology has been successfully applied to examine neonatal populations, defining normative patterns of large-scale neural network development in the maturing brain. The resting-state networks (RSNs) identified in these studies reflect the evolving cerebral structural architecture, presumably driven by varied genetic and environmental influences. Principal features of these investigations and their role in characterization of the tenets of neural network development during this critical developmental period are highlighted in this review. Despite these successes, optimal methods for fcMRI data acquisition and analysis for this population have not yet been defined. Further, appropriate schemes for interpretation and translation of fcMRI results remain unknown, a matter of increasing importance as functional neuroimaging findings are progressively applied in the clinical arena. Notwithstanding these concerns, fcMRI provides insight into the earliest forms of cerebral connectivity and therefore holds great promise for future neurodevelopmental investigations.

Early folding patterns and asymmetries of the normal human brain detected from in utero MRI

Early cortical folding and the emergence of structural brain asymmetries have been previously analyzed by neuropathology as well as qualitative analysis of magnetic resonance imaging (MRI) of fetuses and preterm neonates. In this study, we present a dedicated image analysis framework and its application for the detection of folding patterns during the critical period for the formation of many primary sulci (20-28 gestational weeks). Using structural information from in utero MRI, we perform morphometric analysis of cortical plate surface development and modeling of early folding in the normal fetal brain. First, we identify regions of the fetal brain surface that undergo significant folding changes during this developmental period and provide precise temporal staging of these changes for each region of interest. Then, we highlight the emergence of interhemispheric structural asymmetries that may be related to future functional specialization of cortical areas. Our findings complement previous descriptions of early sulcogenesis based on neuropathology and qualitative evaluation of 2D in utero MRI by accurate spatial and temporal mapping of the emergence of individual sulci as well as structural brain asymmetries. The study provides the missing starting point for their developmental trajectories and extends our understanding of normal cortical folding.

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.

Dissecting functional connectivity of neuronal microcircuits: experimental and theoretical insights

Structure-function studies of neuronal networks have recently benefited from considerable progress in different areas of investigation. Advances in molecular genetics and imaging have allowed for the dissection of neuronal connectivity with unprecedented detail whereas in vivo recordings are providing much needed clues as to how sensory, motor and cognitive function is encoded in neuronal firing. However, bridging the gap between the cellular and behavioral levels will ultimately require an understanding of the functional organization of the underlying neuronal circuits. One way to unravel the complexity of neuronal networks is to understand how their connectivity emerges during brain maturation. In this review, we will describe how graph theory provides experimentalists with novel concepts that can be used to describe and interpret these developing connectivity schemes.

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.

Populations of subplate and interstitial neurons in fetal and adult human telencephalon

In the adult human telencephalon, subcortical (gyral) white matter contains a special population of interstitial neurons considered to be surviving descendants of fetal subplate neurons [Kostovic & Rakic (1980) Cytology and the time of origin of interstitial neurons in the white matter in infant and adult human and monkey telencephalon. J Neurocytol9, 219]. We designate this population of cells as superficial (gyral) interstitial neurons and describe their morphology and distribution in the postnatal and adult human cerebrum. Human fetal subplate neurons cannot be regarded as interstitial, because the subplate zone is an essential part of the fetal cortex, the major site of synaptogenesis and the 'waiting' compartment for growing cortical afferents, and contains both projection neurons and interneurons with distinct input-output connectivity. However, although the subplate zone is a transient fetal structure, many subplate neurons survive postnatally as superficial (gyral) interstitial neurons. The fetal white matter is represented by the intermediate zone and well-defined deep periventricular tracts of growing axons, such as the corpus callosum, anterior commissure, internal and external capsule, and the fountainhead of the corona radiata. These tracts gradually occupy the territory of transient fetal subventricular and ventricular zones.The human fetal white matter also contains distinct populations of deep fetal interstitial neurons, which, by virtue of their location, morphology, molecular phenotypes and advanced level of dendritic maturation, remain distinct from subplate neurons and neurons in adjacent structures (e.g. basal ganglia, basal forebrain). We describe the morphological, histochemical (nicotinamide-adenine dinucleotide phosphate-diaphorase) and immunocytochemical (neuron-specific nuclear protein, microtubule-associated protein-2, calbindin, calretinin, neuropeptide Y) features of both deep fetal interstitial neurons and deep (periventricular) interstitial neurons in the postnatal and adult deep cerebral white matter (i.e. corpus callosum, anterior commissure, internal and external capsule and the corona radiata/centrum semiovale). Although these deep interstitial neurons are poorly developed or absent in the brains of rodents, they represent a prominent feature of the significantly enlarged white matter of human and non-human primate brains.

The ongoing balance of cortical excitation and inhibition during early development

Two papers recently published in Nature propose that the balance between excitation and inhibition is important for the maturation of cortical function. Their conclusions however, are contradictory; one study suggests that balance is established before hearing onset, whereas the other proposes that balance is established after hearing onset. We carefully examined the data and found that the differences between the two groups are less dramatic than they first appear. Despite their methodological differences, both studies provide evidence that an ongoing balance between cortical excitation/inhibition accounts for the maturation and refinement of cortical function during early development.

Long-Range GABAergic Connections Distributed throughout the Neocortex and their Possible Function

Features and functions of long-range GABAergic projection neurons in the developing cerebral cortex have been reported previously, although until now their significance in the adult cerebral cortex has remained uncertain. The septo-hippocampal circuit is one exception – in this system, long-range mature GABAergic projection neurons have been well analyzed and their contribution to the generation of theta-oscillatory behavior in the hippocampus has been documented. To have a clue to the function of the GABAergic projection neurons in the neocortex, we view how the long-range GABAergic projections are integrated in the cortico-cortical, cortico-fugal, and afferent projections in the cerebral cortex. Then, we consider the possibility that the GABAergic projection neurons are involved in the generation, modification, and/or synchronization of oscillations in mature neocortical neuron activity. When markers that identify the GABAergic projection neurons are examined in anatomical and developmental studies, it is clear that neuronal NO synthetase (nNOS)-immunoreactivity can readily identify GABAergic projection neurons. GABAergic projection neurons account for 0.5% of the neocortical GABAergic neurons. To elucidate the role of the GABAergic projection neurons in the neocortex, it will be necessary to clarify the network constructed by nNOS-positive GABAergic projection neurons and their postsynaptic targets. Thus, our long-range goals will be to label and manipulate (including deleting) the GABAergic projection neurons using genetic tools driven by a nNOS promoter. We recognize that this may be a complex endeavor, as most excitatory neurons in the murine neocortex express nNOS transiently. Nevertheless, additional studies characterizing long-range GABAergic projection neurons will have great value to the overall understanding of mature cortical function.

Early history of subplate and interstitial neurons: from Theodor Meynert (1867) to the discovery of the subplate zone (1974)

In this historical review, we trace the early history of research on the fetal subplate zone, subplate neurons and interstitial neurons in the white matter of the adult nervous system. We arrive at several general conclusions. First, a century of research clearly testifies that interstitial neurons, subplate neurons and the subplate zone were first observed and variously described in the human brain - or, in more general terms, in large brains of gyrencephalic mammals, characterized by an abundant white matter and slow and protracted prenatal and postnatal development. Secondly, the subplate zone cannot be meaningfully defined using a single criterion - be it a specific population of cells, fibres or a specific molecular or genetic marker. The subplate zone is a highly dynamic architectonic compartment and its size and cellular composition do not remain constant during development. Thirdly, it is important to make a clear distinction between the subplate zone and the subplate (and interstitial) neurons. The transient existence of the subplate zone (as a specific architectonic compartment of the fetal telencephalic wall) should not be equated with the putative transient existence of subplate neurons. It is clear that in rodents, and to an even greater extent in humans and monkeys, a significant number of subplate cells survive and remain functional throughout life.

Renewed focus on the developing human neocortex

Many specifically human psychiatric and neurological conditions have developmental origins. Rodent models are extremely valuable for the investigation of brain development, but cannot provide insight into aspects that are specifically human. The human brain, and particularly the cerebral cortex, has some unique genetic, molecular, cellular and anatomical features, and these need to be further explored. Cortical expansion in human is not just quantitative; there are some novel types of neurons and cytoarchitectonic areas identified by their gene expression, connectivity and functions that do not exist in rodents. Recent research into human brain development has revealed more elaborated neurogenetic compartments, radial and tangential migration, transient cell layers in the subplate, and a greater diversity of early-generated neurons, including predecessor neurons. Recently there has been a renaissance of the study of human brain development because of these unique differences, made possible by the availability of new techniques. This review gives a flavour of the recent studies stemming from this renewed focus on the developing human brain.

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.

A conserved switch in sensory processing prepares developing neocortex for vision

Developing cortex generates endogenous activity that modulates the formation of functional units, but how this activity is altered to support mature function is poorly understood. Using recordings from the visual cortex of preterm human infants and neonatal rats, we report a "bursting" period of visual responsiveness during which the weak retinal output is amplified by endogenous network oscillations, enabling a primitive form of vision. This period ends shortly before delivery in humans and eye opening in rodents with an abrupt switch to the mature visual response. The switch is causally linked to the emergence of an activated state of continuous cortical activity dependent on the ascending neuromodulatory systems involved in arousal. This switch is sensory system specific but experience independent and also involves maturation of retinal processing. Thus, the early development of visual processing is governed by a conserved, intrinsic program that switches thalamocortical response properties in anticipation of patterned vision.

The development of the subplate and thalamocortical connections in the human foetal brain

The aim of this review is to present clinically relevant data on prenatal development of thalamocortical connections in the human brain. The analysis is based on extensive Zagreb Neuroembryological Collection, including more than 500 prenatal human brains stained with various classical neurohistological, as well as modern histochemical and immunohistochemical methods. The connection of thalamocortical axons during the 'waiting' period with transient cortical subplate zone and subsequent synaptic engagement in the cortical plate is the main connectivity event in the late foetus and preterm infant. This connectivity is the structural substrate for the endogeneous subplate and sensory-driven circuitry generating transient electrical phenomena and may represent a transient network in the developmental history of consciousness.

CONCLUSION

Findings presented in this review should be considered in the management of pain in preterm infants, in searching for the vulnerability of the subplate zone in diagnostic procedures using the in vivo MRI and in revealing the developmental origin of cognitive and mental disorders.

The applicability of spike time dependent plasticity to development

Spike time dependent plasticity (STDP) has been observed in both developing and adult animals. Theoretical studies suggest that it implicitly leads to both competition and homeostasis in addition to correlation-based plasticity, making it a good candidate to explain developmental refinement and plasticity in a number of systems. However, it has only been observed to play a clear role in development in a small number of cases. Because the fast time scales necessary to elicit STDP, it would likely be inefficient in governing synaptic modifications in the absence of fast correlations in neural activity. In contrast, later stages of development often depend on sensory inputs that can drive activity on much faster time scales, suggesting a role in STDP in many sensory systems after opening of the eyes and ear canals. Correlations on fast time scales can be also be present earlier in developing microcircuits, such as those produced by specific transient “teacher” circuits in the cerebral cortex. By reviewing examples of each case, we suggest that STDP is not a universal rule, but rather might be masked or phased in, depending on the information available to instruct refinement in different developing circuits. Thus, this review describes selected cases where STDP has been studied in developmental contexts, and uses these examples to suggest a more general framework for understanding where it could be playing a role in development.

Between molecules and experience: role of early patterns of coordinated activity for the development of cortical maps and sensory abilities

Sensory systems processing information from the environment rely on precisely formed and refined neuronal networks that build maps of sensory receptor epithelia at different subcortical and cortical levels. These sensory maps share similar principles of function and emerge according to developmental processes common in visual, somatosensory and auditory systems. Whereas molecular cues set the coarse organization of cortico-subcortical topography, its refinement is known to succeed under the influence of experience-dependent electrical activity during critical periods. However, coordinated patterns of activity synchronize the cortico-subcortical networks long before the meaningful impact of environmental inputs on sensory maps. Recent studies elucidated the cellular and network mechanisms underlying the generation of these early patterns of activity and highlighted their similarities across species. Moreover, the experience-independent activity appears to act as a functional template for the maturation of sensory networks and cortico-subcortical maps. A major goal for future research will be to analyze how this early activity interacts with the molecular cues and to determine whether it is permissive or rather supporting for the establishment of sensory topography.