Cortical progenitor expansion, self-renewal and neurogenesis-a polarized perspective

Neural stem and progenitor cells giving rise to neurons in developing mammalian neocortex fall into two principal classes with regard to location of mitosis-apical and basal, and into three principal classes in terms of cell polarity during mitosis-bipolar, monopolar, and nonpolar. Insight has been gained into how inheritance of polarized, apical and basal, cell constituents is related to symmetric versus asymmetric divisions of these progenitors, and how this inheritance is linked to their expansion, self-renewal, and neurogenesis. Retention and inheritance of the basal process emerge as key for self-renewal, notably for the monopolar progenitors of prospective gyrencephalic neocortex that undergo asymmetric mitoses at basal locations. The resulting expansion of the neocortex during evolution is proposed to be associated with an increased cone-shape of radial units.     

Comparative analysis of the subventricular zone in rat, ferret and macaque: evidence for an outer subventricular zone in rodents

The mammalian cerebral cortex arises from precursor cells that reside in a proliferative region surrounding the lateral ventricles of the developing brain. Recent work has shown that precursor cells in the subventricular zone (SVZ) provide a major contribution to prenatal cortical neurogenesis, and that the SVZ is significantly thicker in gyrencephalic mammals such as primates than it is in lissencephalic mammals including rodents. Identifying characteristics that are shared by or that distinguish cortical precursor cells across mammalian species will shed light on factors that regulate cortical neurogenesis and may point toward mechanisms that underlie the evolutionary expansion of the neocortex in gyrencephalic mammals. We immunostained sections of the developing cerebral cortex from lissencephalic rats, and from gyrencephalic ferrets and macaques to compare the distribution of precursor cell types in each species. We also performed time-lapse imaging of precursor cells in the developing rat neocortex. We show that the distribution of Pax6+ and Tbr2+ precursor cells is similar in lissencephalic rat and gyrencephalic ferret, and different in the gyrencephalic cortex of macaque. We show that mitotic Pax6+ translocating radial glial cells (tRG) are present in the cerebral cortex of each species during and after neurogenesis, demonstrating that the function of Pax6+ tRG cells is not restricted to neurogenesis. Furthermore, we show that Olig2 expression distinguishes two distinct subtypes of Pax6+ tRG cells. Finally we present a novel method for discriminating the inner and outer SVZ across mammalian species and show that the key cytoarchitectural features and cell types that define the outer SVZ in developing primates are present in the developing rat neocortex. Our data demonstrate that the developing rat cerebral cortex possesses an outer subventricular zone during late stages of cortical neurogenesis and that the developing rodent cortex shares important features with that of primates.     

Neural Precursor Cells from Adult Mouse Cerebral Cortex Differentiate into Both Neurons and Oligodendrocytes

Recent findings concerning adult neurogenesis in two selected structures of the mammalian brain, the olfactory bulb and dentate gyrus of the hippocampus, present the possibility that this mechanism of neurogenesis applies for all brain regions, including the cerebral neocortex. In this way, a small number of potential neural precursor cells may exist in the cerebral neocortex, but they do not normally differentiate into cortical neurons in vivo. It has, however, been reported recently that cycling cells isolated from non-neurogenic areas of adult rat cerebral cortex could generate neurons in vitro. In this study, we analyzed the lineage potential of cycling cells from the adult mouse neocortex. For the dissection of the cerebral cortex from the adult mouse, which is significantly smaller than that of the adult rat, we have modified the previous dissection protocol developed for the rat neocortex. As a result, cycling cells from adult mouse neocortex gave rise to neurons and oligodendrocytes, but not to astrocytes, whereas when the previous dissection method was used, cycling cells gave rise to neurons, oligodendrocytes and astrocytes. This discrepancy might stem from slight contamination of the dissected mouse neocortical tissue in the previous protocol used for the dissection of rat neocortex by cells from the surrounding subependymal zone, where typical adult neural stem cells exist. The results presented here will contribute to our understanding of the nature of cycling cells in the adult mammalian neocortex, and for which future stem cell research will provide new possibilities for cell replacement therapy to be used in the treatment of neurodegenerative conditions.     

Understanding primate brain evolution

We present a detailed reanalysis of the comparative brain data for primates, and develop a model using path analysis that seeks to present the coevolution of primate brain (neocortex) and sociality within a broader ecological and life-history framework. We show that body size, basal metabolic rate and life history act as constraints on brain evolution and through this influence the coevolution of neocortex size and group size. However, they do not determine either of these variables, which appear to be locked in a tight coevolutionary system. We show that, within primates, this relationship is specific to the neocortex. Nonetheless, there are important constraints on brain evolution; we use path analysis to show that, in order to evolve a large neocortex, a species must first evolve a large brain to support that neocortex and this in turn requires adjustments in diet (to provide the energy needed) and life history (to allow sufficient time both for brain growth and for 'software' programming). We review a wider literature demonstrating a tight coevolutionary relationship between brain size and sociality in a range of mammalian taxa, but emphasize that the social brain hypothesis is not about the relationship between brain/neocortex size and group size per se; rather, it is about social complexity and we adduce evidence to support this. Finally, we consider the wider issue of how mammalian (and primate) brains evolve in order to localize the social effects.     

Distribution and Morphological Features of Microglia in the Developing Cerebral Cortex of Gyrencephalic Mammals

Microglia have been attracting much attention because of their fundamental importance in both the mature brain and the developing brain. Though important roles of microglia in the developing cerebral cortex of mice have been uncovered, their distribution and roles in the developing cerebral cortex in gyrencephalic higher mammals have remained elusive. Here we examined the distribution and morphology of microglia in the developing cerebral cortex of gyrencephalic carnivore ferrets. We found that a number of microglia were accumulated in the germinal zones (GZs), especially in the outer subventricular zone (OSVZ), which is a GZ found in higher mammals. Furthermore, we uncovered that microglia extended their processes tangentially along inner fiber layer (IFL)-like fibers in the developing ferret cortex. The OSVZ and the IFL are the prominent features of the cerebral cortex of higher mammals. Our findings indicate that microglia may play important roles in the OSVZ and the IFL in the developing cerebral cortex of higher mammals.     

Slit2 activity in the migration of guidepost neurons shapes thalamic projections during development and evolution

How brain connectivity has evolved to integrate the mammalian-specific neocortex remains largely unknown. Here, we address how dorsal thalamic axons, which constitute the main input to the neocortex, are directed internally to their evolutionary novel target in mammals, though they follow an external path to other targets in reptiles and birds. Using comparative studies and functional experiments in chick, we show that local species-specific differences in the migration of previously identified "corridor" guidepost neurons control the opening of a mammalian thalamocortical route. Using in?vivo and ex vivo experiments in mice, we further demonstrate that the midline repellent Slit2 orients migration of corridor neurons and thereby switches thalamic axons from an external to a mammalian-specific internal path. Our study reveals that subtle differences in the migration of conserved intermediate target neurons trigger large-scale changes in thalamic connectivity, and opens perspectives on Slit functions and the evolution of brain wiring.     

New GABAergic interneurons in the adult neocortex and striatum are generated from different precursors

Ongoing neurogenesis in the adult mammalian dentate gyrus and olfactory bulb is generally accepted, but its existence in other adult brain regions is highly controversial. We labeled newly born cells in adult rats with the S-phase marker bromodeoxyuridine (BrdU) and used neuronal markers to characterize new cells at different time points after cell division. In the neocortex and striatum, we found BrdU-labeled cells that expressed each of the eight neuronal markers. Their size as well as staining for gamma-aminobutyric acid (GABA), glutamic acid decarboxylase 67, calretinin and/or calbindin, suggest that new neurons in both regions are GABAergic interneurons. BrdU and doublecortin-immunoreactive (BrdU+/DCX+) cells were seen within the striatum, suggesting migration of immature neurons from the subventricular zone. Surprisingly, no DCX+ cells were found within the neocortex. NG2 immunoreactivity in some new neocortical neurons suggested that they may instead be generated from the NG2+ precursors that reside within the cortex itself.     

Involvement of gap junctions in the development of the neocortex

Gap junctions play an important role during the development of the mammalian brain. In the neocortex, gap junctions are already expressed at very early stages of development and they seem to be involved in many processes like neurogenesis, migration and synapse formation. Gap junctions are found in all cell types including progenitor cells, glial cells and neurons. These direct cell-to-cell connections form clusters consisting of a distinct number of cells of a certain type. These clusters can be considered as communication compartments in which the information transfer is mediated electrically by ionic currents and/or chemically by, e.g., small second messenger molecules. Within the neocortex, four such communication compartments can be identified: (1) gap junction-coupled neuroblasts of the ventricular zone and gap junctions in migrating cells and radial glia, (2) gap junction-coupled glial cells (astrocytes and oligodendrocytes), (3) gap junction-coupled pyramidal cells (only during the first two postnatal weeks) and (4) gap junction-coupled inhibitory interneurons. These compartments can consist of sub-compartments and they may overlap to some degree. The compartments 1 and 3 disappear with ongoing develop, whereas compartments 2 and 4 persist in the mature neocortex. Gap junction-mediated coupling of glial cells seems to be important for stabilization of the extracellular ion homeostasis, uptake of neurotransmitters, migration of neurons and myelination of axons. Electrical synapses between inhibitory interneurons facilitate the synchronization of pyramidal cells. In this way, they contribute to the generation of oscillatory network activity correlated with higher cortical functions. The role of gap junctions present in neuroblasts of the ventricular zone as well as the role of gap junctions found in pyramidal cells during the early postnatal stages is less clear. It is assumed that they might help to form precursors of the functional columns observed in the mature neocortex. Although recent developments of new techniques led to the solution of many problems concerning gap junction-coupling between neurons and glial cells in the neocortex, there are many open questions which need to be answered before we can achieve a comprehensive understanding of the role of gap junctions in the development of the neocortex.     

How to keep proliferative neural stem/progenitor cells

It has long been argued that cell cycle regulators such as cyclins, cyclin-dependent kinases and their inhibitors affect the fate of neuronal progenitor cells. Recently, we identified that cyclin D2, which localizes at the basal tip of the radial glial cell (i.e., the neural progenitor in the developing neocortex), functions to give differential cell fates to its daughter cells just after cell division. This basally biased localization is due to transportation of cyclin D2 mRNA via its unique cis-regulatory sequence and local translation into cyclin D2 protein at the basal endfoot. During division of the neural progenitor cells, cyclin D2 protein is inherited by the daughter cell that retain the basal process, resulting in asymmetric distribution of cyclin D2 protein between the two daughter cells. Cyclin D2 is similarly localized in the human fetal cortical primordium, suggesting a common mechanism for the maintenance of neural progenitors and a possible scenario in evolution of primate brains. Here we introduce our recent findings and discuss how cyclin D2 functions in mammalian brain development and evolution.     

How to keep proliferative neural stem/progenitor cells: A critical role of asymmetric inheritance of cyclin D2

It has long been argued that cell cycle regulators such as cyclins, cyclin-dependent kinases and their inhibitors affect the fate of neuronal progenitor cells. Recently, we identified that cyclin D2, which localizes at the basal tip of the radial glial cell (i.e., the neural progenitor in the developing neocortex), functions to give differential cell fates to its daughter cells just after cell division. This basally biased localization is due to transportation of cyclin D2 mRNA via its unique cis-regulatory sequence and local translation into cyclin D2 protein at the basal endfoot. During division of the neural progenitor cells, cyclin D2 protein is inherited by the daughter cell that retain the basal process, resulting in asymmetric distribution of cyclin D2 protein between the two daughter cells. Cyclin D2 is similarly localized in the human fetal cortical primordium, suggesting a common mechanism for the maintenance of neural progenitors and a possible scenario in evolution of primate brains. Here we introduce our recent findings and discuss how cyclin D2 functions in mammalian brain development and evolution.     

Evidence for convergent evolution of a neocortex‐like structure in a late Permian therapsid

The special sensory, motor, and cognitive capabilities of mammals mainly depend upon the neocortex, which is the six‐layered cover of the mammalian forebrain. The origin of the neocortex is still controversial and the current view is that larger brains with neocortex first evolved in late Triassic Mammaliaformes. Here, we report the earliest evidence of a structure analogous to the mammalian neocortex in a forerunner of mammals, the fossorial anomodont Kawingasaurus fossilis from the late Permian of Tanzania. The endocranial cavity of Kawingasaurus is almost completely ossified, which allowed a less hypothetical virtual reconstruction of the brain endocast to be generated. A parietal foramen is absent. A small pit between the cerebral hemispheres is interpreted as a pineal body. The inflated cerebral hemispheres are demarcated from each other by a median sulcus and by a possible rhinal fissure from the rest of the endocast. The encephalization quotient estimated by using the method of Eisenberg is 0.52, which is 2–3 times larger than in other nonmammalian synapsids. Another remarkable feature are the extremely ramified infraorbital canals in the snout. The shape of the brain endocast, the extremely ramified maxillary canals as well as the small frontally placed eyes suggest that special sensory adaptations to the subterranean habitat such as a well developed sense of touch and binocular vision may have driven the parallel evolution of an equivalent of the mammalian neocortex and a mammal‐like lemnothalamic visual system in Kawingasaurus . The gross anatomy of the brain endocast of Kawingasaurus supports the Outgroup Hypothesis, according to which the neocortex evolved from the dorsal pallium of an amphibian‐like ancestor, which receives sensory projections from the lemnothalamic pathway. The enlarged brain as well as the absence of a parietal foramen may be an indication for a higher metabolic rate of Kawingasaurus compared to other nonmammalian synapsids.     

Cortical development and asymmetric cell divisions

The development of the mammalian neocortex involves rounds of symmetric and asymmetric cell division of neural progenitors to fulfill needs of both self-renewal of progenitors and production of differe...     

Variety of types of cortical interneurons

A most important component of the mammalian neocortex is the system of inhibitory interneurons. It is composed of cellular elements, which differ from each other in morphological, electrophysiological, and genetical features; these cells form a complex system of synaptic connections with glutamatergic cells and with each other. Some regularities that characterize the variety of types of cortical interneurons are discussed in our study. Neirofiziologiya/Neurophysiology, Vol. 39, No. 3, pp. 260–269, May–June, 2007.     

Lipids of nuclear fractions from neurons and glia of rat neocortex under conditions of artificial hypobiosis

Lipid contents were studied in tissue and nuclei isolated from neurons and glia of neocortex of rats under conditions of normothermia and in the state of artificial hypobiosis caused by hypothermia-hypoxia-hypercapnia. Compared to the neocortex tissue, both nuclear fractions were fivefold impoverished in phospholipids and cholesterol and strongly enriched with mono- and diglycerides and fatty acids. The nuclear fractions from neurons and glia contained similar amounts of phospholipids, and only the cardiolipin content in the neuronal nuclei was lower than in the glial nuclei. The state of artificial hypobiosis in rats led to an increase in the cholesterol/phospholipids ratio (mol/mol) in the nuclei from the neurons and glia; amounts of cholesterol and sphingomyelin in the nuclei from the glia were increased. The increases in the cholesterol and sphingomyelin contents and in the cholesterol/phospholipids ratio suggest an involvement of lipid-dependent signaling systems of the nuclei in the functional response of mammalian neocortex cells to artificial hypobiosis.     

Columnar organization of the optic tectum in the frog

Cobaltous lysine complex was used to label tectal cells. Cobalt soaked into a piece of filter paper and placed onto the surface of the tectum labelled neurons in the whole thickness of the tectum below the filter paper. The labelled area was sharply demarcated from the unlabelled tectal tissue. Focal cobalt injections into different tectal layers labelled small groups of cells and the cobalt-filled structures were perpendicularly oriented to the surface of the tectum. Efferent axons could be followed into layer 7, but other lateral connections were very sparse. These results support the hypothesis that the tectum has columnar organization similar to that of the mammalian neocortex.     

Identification of Cajal-Retzius cells during neocortical ontogenesis in mice with fluorescent carbocyanine

Cajal-Retzius cells, which are present transiently in the first layer of the mammalian neocortex, have been revealed in the mouse by DiI. This lipophilic fluorescent dye, locally applied over the cortex after formaldehyde fixation, allowed the global view of cortical cells. During ontogenesis, Cajal-Retzius cells retained their initial characteristic bipolar shape and orientation parallel to the meningeal surface. The bright fluorescent light emitted by this dye allowed visualization of the labelled cells by "microtomoscopy" using a confocal scanning laser microscope and analysis of the detailed aspect of these neurons and of their connections.     

How does the cerebral cortex work? Learning, attention, and grouping by the laminar circuits of visual cortex

The organization of neocortex into layers is one of its most salient anatomical features. These layers include circuits that form functional columns in cortical maps. A major unsolved problem concerns how bottom-up, top-down, and horizontal interactions are organized within cortical layers to generate adaptive behaviors. This article models how these interactions help visual cortex to realize: (i) the binding process whereby cortex groups distributed data into coherent object representations; (ii) the attentional process whereby cortex selectively processes important events; and (iii) the developmental and learning processes whereby cortex shapes its circuits to match environmental constraints. New computational ideas about feedback systems suggest how neocortex develops and learns in a stable way, and why top-down attention requires converging bottom-up inputs to fully activate cortical cells, whereas perceptual groupings do not.