Natural and lesion-induced apoptosis in the dorsal lateral geniculate nucleus during development.

We investigated natural and lesion-induced apoptosis in the developing rat dorsal lateral geniculate nucleus (dLGN). These lesions involved: i) monocular enucleation, and ii) unilateral ablation of the visual cortex at different postnatal ages before eye opening. We identified dying cells as apoptotic with light and electron microscopy, using terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick end labeling (TUNEL), and immunohistochemistry for active caspase-3. In the dLGN of normal animals, TUNEL+cells were detected during the first postnatal week, with a peak at postnatal day (P) 1. Following enucleation at birth or at P7, the frequency of apoptotic cells in the contralateral dLGN increased significantly at postlesion day (PLD) 1 and returned to normal values by PLD7. In contrast to early lesions, enucleation at P14 did not induce significant changes in apoptosis in the dLGN. Cortical lesions performed at P0, P7 or at P14 induced the death of the overwhelming majority of cells in the ipsilateral dLGN, which led to a severe reduction in size of the nucleus by PLD7 and its complete elimination by adulthood. Double labeling with TUNEL and immunofluorescence for neuronal nuclear protein (NeuN) showed that in both normal and lesioned animals, apoptotic cells were mainly neurons. We suggest that: i) apoptosis in the dLGN occurs during the precritical period of neuronal maturation; ii) developing neurons in the dLGN are more dependent on the integrity of their connections with the visual cortex than with the retina for survival; and iii) lesion-induced apoptosis in the dLGN during development depends on the type and extent of the connectivity affected.

Natural and lesion-induced apoptosis in the rat striatum during development.

We evaluated the pattern of apoptosis in the rat striatum during normal development and in two models of lesion-induced cell death. Lesions included i) unilateral ablations of the cerebral cortex at different postnatal ages, and ii) early postnatal lesions of the catecholaminergic afferent systems of the striatum with 6-hydroxydopamine (6-OHDA). Dying cells were identified as apoptotic using the TUNEL (terminal deoxynucleotidyl-transferase-mediated dUTP-biotin nick end labeling) method at the light and electron microscopic levels. Moreover, we used immunohistochemistry for the apoptotic markers active caspase-3 and fractin. TUNEL+ cells were present in the striatum during the first four postnatal weeks. Their frequency was high during the first postnatal week and peaked at postnatal day (P)5. Cortical lesions at birth, in contrast to those performed at later stages, induced a significant increase in the frequency of TUNEL+ cells in the ipsilateral striatum, which peaked at seven days postlesion. 6-OHDA lesions resulted in a similar and significant increase in the frequency of TUNEL+ cells in the striatum, which also peaked at P7. We also showed that cortical lesions at P0 and 6-OHDA lesions resulted in a reduction in the frequency, as well as in alterations of the morphology of gamma-aminobutyric acid (GABA)-immunoreactive (ir) neurons in the striatum. We suggest that: i) apoptosis in the striatum is temporally coordinated with maturation events in this area and ii) early developmental lesions of major afferent pathways to the striatum affect both the survival and phenotype of striatal neurons.

Secreted factors from ventral telencephalon induce the differentiation of GABAergic neurons in cortical cultures.

It is widely believed that the pyramidal cells and interneurons of the cerebral cortex are distinct in their origin, lineage and genetic make up. In view of these findings, the current thesis is that the phenotype determination of cortical neurons is primarily directed by genetic mechanisms. Using in vitro assays, the present study demonstrates that secreted factors from ganglionic eminence (GE) of the ventral telencephalon have the potency to induce the differentiation of a subset of cortical neurons towards gamma-aminobutyric acid (GABA)ergic lineage. Characterization of cortical cultures that were exposed to medium derived from GE illustrated a significant increase in the number of GABA-, calretinin- and calbindin-positive neurons. Calcium imaging together with pharmacological studies showed that the application of exogenous medium significantly elevated the intracellular calcium transients in cortical neurons through the activation of ionotropic glutamate receptors. The increase in GABA+ neurons appeared to be associated with the elevated calcium activity; treatment with blockers specific for glutamate receptors abolished both the synchronized transients and reduced the differentiation of GABAergic neurons. Such studies demonstrate that although intrinsic mechanisms determine the fate of cortical interneurons, extrinsic factors have the potency to influence their neurochemical differentiation and contribute towards their molecular diversity.

Tangential migration of cells from the basal to the dorsal telencephalic regions in the chick.

The evolutionary relationship between telencephalic regions of the avian and mammalian brains has been a long-standing issue in comparative neuroanatomy. Based on various criteria, a number of homologous regions have been proposed. Recent studies in mammals have shown that basal regions of the telencephalon give rise to neurons that migrate dorsally and populate the cerebral cortex. In the present study we demonstrate that, similar to mammals, neurons from a ventricular region of the palaeo-striatal complex - the dorsal subpallial sulcus - of the chick telencephalon migrate dorsally to populate the developing pallium. Further characterization of these cells revealed that they express the neurotransmitter gamma-aminobutyric acid, but not the calcium-binding protein calbindin. These findings provide evidence that the mouse and chick basal regions are not only homologous in terms of gene expression patterns and connectivity, but they both also contribute inhibitory interneurons to dorsal regions of the developing telencephalon.

Neuronal migration in the developing cerebral cortex: observations based on real-time imaging.

We have used time-lapse imaging of acute cortical slices to study the migration of neurons from their sites of origin to their positions in the developing neocortex. We found that two distinct modes of cell movement, somal translocation and glia-guided locomotion, are responsible for the radial migration of neurons generated in the cortical ventricular zone. The former is the prevalent form of radial movement of the early-born cortical neurons, while the latter is adopted by those generated later in corticogenesis. Interneurons, found to originate in the ganglionic eminence, follow tangential migratory paths to reach the developing cortex. Upon reaching the cortex, these cells seek the ventricular zone using a mode of movement that we have termed 'ventricle-directed migration', before they migrate to their positions in the cortical plate. In addition to these forms of movement, we report here a unique morphological and migratory behavior for a population of cortical neurons. These cells are multipolar in form, and are highly motile in the formation and retraction of their processes. Based on these morphological features, we refer to this type of cells as 'branching cells' and attribute the phenotype to a subset of cortical interneurons.

The origin of cortical neurons

Neurons of the mammalian cerebral cortex comprise two broad classes: pyramidal neurons, which project to distant targets, and the inhibitory nonpyramidal cells, the cortical interneurons. Pyramidal neurons are generated in the germinal ventricular zone, which lines the lateral ventricles, and migrate along the processes of radial glial cells to their positions in the developing cortex in an `inside-out' sequence. The GABA-containing nonpyramidal cells originate for the most part in the ganglionic eminence, the primordium of the basal ganglia in the ventral telencephalon. These cells follow tangential migratory routes to enter the cortex and are in close association with the corticofugal axonal system. Once they enter the cortex, they move towards the ventricular zone, possibly to obtain positional information, before they migrate radially in the direction of the pial surface to take up their positions in the developing cortex. The mechanisms that guide interneurons throughout these long and complex migratory routes are currently under investigation

The origin of cortical neurons.

Neurons of the mammalian cerebral cortex comprise two broad classes: pyramidal neurons, which project to distant targets, and the inhibitory nonpyramidal cells, the cortical interneurons. Pyramidal neurons are generated in the germinal ventricular zone, which lines the lateral ventricles, and migrate along the processes of radial glial cells to their positions in the developing cortex in an 'inside-out' sequence. The GABA-containing nonpyramidal cells originate for the most part in the ganglionic eminence, the primordium of the basal ganglia in the ventral telencephalon. These cells follow tangential migratory routes to enter the cortex and are in close association with the corticofugal axonal system. Once they enter the cortex, they move towards the ventricular zone, possibly to obtain positional information, before they migrate radially in the direction of the pial surface to take up their positions in the developing cortex. The mechanisms that guide interneurons throughout these long and complex migratory routes are currently under investigation.

Postnatal development of the dopaminergic system of the striatum in the rat.

The dopaminergic innervation of the developing caudate-putamen (patches and matrix) and nucleus accumbens (shell and core) of the rat was examined with light and electron microscope immunocytochemistry, using antibodies against dopamine. Light microscopic analysis showed, in accordance with previous studies, that early in life, dopaminergic fibers were relatively thick and present throughout the striatum. Their distribution was heterogeneous, showing dense aggregations, the so-called dopamine islands. The pattern of innervation became more uniform during the third postnatal week with most of the dopamine islands no longer detectable. For electron microscopic analysis, parts of the caudate-putamen containing dopamine islands or matrix, and of the nucleus accumbens, from the shell and the core of the nucleus, were selected. This analysis revealed that symmetrical synapses between immunoreactive profiles and unlabeled dendritic shafts predominated throughout development but, at the late stages, symmetrical axospinous synapses also became a prominent feature. These findings indicate that: (1) although the caudate-putamen and the nucleus accumbens have different connections and functions, they exhibit similar types of dopaminergic synapses, and (2) the relatively late detection of dopaminergic axospinous synapses suggests that the development of the dopaminergic system in the striatum is an active process, which parallels the morphological changes of striatal neurons and may contribute to their maturation.

Emx1 is a marker for pyramidal neurons of the cerebral cortex.

The homeobox-containing gene, Emx1, a mouse homologue of Drosophila empty spiracles, is specifically expressed in the developing telencephalic cortex. It has been reported that Emx1 transcripts and the protein product are localized in most cells of the cerebral cortex during the process of proliferation, migration, differentiation and maturation. We provide evidence here, based on a multitude of experimental approaches in developing rats, in support of the hypothesis that the expression of this gene is restricted to pyramidal neurons. Specifically, we found that, similar to pyramidal neurons, cells expressing Emx1 are distributed in all cortical layers, except layer I. Using in situ hybridization and immunocytochemistry at the light and electron microscope levels, we have shown that the density, distribution, soma shape and ultrastructural features of these cells were identical to those of pyramidal neurons. Double-labelling experiments confirmed that the vast majority of Emx1-expressing cells also contained glutamate, a marker of pyramidal neurons. We also found that this gene is expressed by most glutamate-containing neurons in dissociated cortical cell cultures and the vast majority of cells in radially arranged clones of pyramidal cells in the cortices of chimeric mice. Thus, the homeobox gene Emx1 can be reliably used as a marker of the pyramidal cell lineage.

The adhesion molecule TAG-1 mediates the migration of cortical interneurons from the ganglionic eminence along the corticofugal fiber system.

Cortical nonpyramidal cells, the GABA-containing interneurons, originate mostly in the medial ganglionic eminence of the ventral telencephalon and follow tangential migratory routes to reach the dorsal telencephalon. Although several genes that play a role in this migration have been identified, the underlying cellular and molecular cues are not fully understood. We provide evidence that the neural cell adhesion molecule TAG-1 mediates the migration of cortical interneurons. We show that the migration of these neurons occurs along the TAG-1-expressing axons of the developing corticofugal system. The spatial and temporal pattern of expression of TAG-1 on corticofugal fibers coincides with the order of appearance of GABAergic cells in the developing cortex. Blocking the function of TAG-1, but not of L1, another adhesion molecule and binding partner of TAG-1, results in a marked reduction of GABAergic neurons in the cortex. These observations reveal a mechanism by which the adhesion molecule TAG-1, known to be involved in axonal pathfinding, also takes part in neuronal migration.

Radial glial cells. are they really glia?

During the development of the cerebral cortex, radial glia serve as a scaffold to support and direct neurons during their migration. This view is now changing in the light of emerging evidence showing that these cells have a much more dynamic and diverse role. A recent series of studies has provided strong support for their role as precursor cells in the ventricular zone that generate cortical neurons and glia, in addition to providing migration guidance.

The contribution of the ganglionic eminence to the neuronal cell types of the cerebral cortex.

The principal neuronal types of the mammalian cerebral cortex are the excitatory pyramidal cells and the inhibitory interneurons, the non-pyramidal cells. It is thought that these neurons arise in the ventricular zone surrounding the telencephalic ventricles. From there, newly generated neurons migrate outward along the processes of radial glial cells to reach the cortical plate where they accumulate in an 'inside-out' sequence to form the six-layered structure of the neocortex. Here we review emerging evidence that pyramidal neurons are generated in the cortical ventricular zone, whereas the majority of the non-pyramidal cells arise in the ganglionic eminences of the ventral telencephalon. These neurons follow tangential migratory routes to reach their positions in the developing cortex.

The origin and migration of cortical neurones: new vistas.

The principal neuronal types of the cerebral cortex are the excitatory pyramidal cells, which project to distant targets, and the inhibitory nonpyramidal cells, which are the cortical interneurones. This article reviews evidence suggesting that these two neuronal types are generated in distinct proliferative zones. Pyramidal cells are derived from the neuroepithelium in the cortical ventricular zone, and use the processes of radial glia in order to migrate and take their positions in the cortex in an 'inside-out' sequence. Relatively few nonpyramidal cells are generated in the cortical neuroepithelium: the majority is derived from the ganglionic eminence of the ventral telencephalon. These nonpyramidal neurones use tangential migratory paths to reach the cortex, probably travelling along axonal bundles of the developing corticofugal fibre system.

The medial ganglionic eminence gives rise to a population of early neurons in the developing cerebral cortex.

During development of the neocortex, the marginal zone (layer I) and the subplate (layer VII) are the first layers to form from a primordial plexiform neoropil. The cortical plate (layers II-VI) is subsequently established between these superficial and deep components of the primordial plexiform neuropil. Neurons in the early zones are thought to play important roles in the formation of the cortex: the Cajal-Retzius cells of the marginal zone are instrumental in neuronal migration and laminar formation, and cells of the subplate are involved in the formation of cortical connections. Using the fluorescent tracer 1,1'-dioctodecyl-3,3,3', 3'-tetramethylindocarbocyanine (DiI), we have shown here that a substantial proportion of neurons of the marginal zone, including cells with features of Cajal-Retzius cells, and of the subplate and lower intermediate zone are not born in the ventricular neuroepithelium but instead originate in the medial ganglionic eminence (MGE), the pallidal primordium. These neurons follow a tangential migratory route to their positions in the developing cortex. They express the neurotransmitter GABA but seem to lack the calcium binding protein calretinin; some migrating cells found in the marginal zone express reelin. In addition, migrating cells express the LIM-homeobox gene Lhx6, a characteristic marker of the MGE. It is suggested that this gene uniquely or in combination with other transcription factors may be involved in the decision of MGE cells to differentiate in situ or migrate to the neocortex.

Glial cell lineages in the rat cerebral cortex.

I have traced the fates of glial cell progenitors in the rat cerebral cortex marked with a recombinant retrovirus throughout most of the period of corticogenesis, from embryonic (E) day 14 to postnatal (P) day 14. Discrete clusters of clonally related glia were examined in serially cut sections, and their phenotypes identified using reliable light and electron microscopic criteria. Analysis of a large number of clones marked with retrovirus at various stages of embryonic life contained, with very few exceptions, either all astrocytes or all oligodendrocytes. This observation suggests that the ventricular zone contains separate progenitor cells for the two glial cell types. Oligodendrocyte clones were rarely seen in the cortices injected with retrovirus at the early stages of corticogenesis (E14-E16), suggesting that there is a very small number of oligodendrocyte progenitors in the ventricular zone at these early stages. Their frequency increased significantly at later embryonic ages. At these later stages, ventricular zone cells also give rise to progenitor cells that make up the subventricular zone in early postnatal life. Injections of retrovirus in this proliferative zone shortly after birth resulted in the generation of labeled astrocyte and oligodendrocyte clones in the cortical gray and white matter, with the astrocyte clones being in the majority. Injections at increasingly later stages resulted in the presence, predominantly in the white matter of both hemispheres and in the corpus callosum, of progressively more oligodendrocyte clones and fewer astrocyte clones. Injections at P14 generated only oligodendrocyte clones in the white matter of both hemispheres. A small number of clusters (<10%) generated after subventricular zone injections contained both astrocytes and oligodendrocytes, suggesting that single subventricular zone cells can differentiate into both glial cell types.

Gap junction-mediated communication in the developing and adult cerebral cortex.

Recent cell biological and electrophysiological studies have shown that gap junctional coupling and the proteins that mediate this form of communication are present in the developing cerebral cortex from early in corticogenesis to the later stage of neuronal circuit formation. We have used electron microscopy to visualize gap junctions in the developing rat cerebral cortex, and studied the expression patterns and cellular localizations of connexin26 (Cx26; beta 2), Cx32 (beta 1) and Cx43 (alpha 1), which take part in their formation. We found that these connexins are expressed differentially during development, and their patterns of expression are correlated with important developmental events such as cell proliferation, migration and formation of cortical neuronal circuits. We also observed that gap junctions and their constituent connexins were abundant in the adult cerebral cortex. Junctions were predominantly between glial cells or between neurons and glia. The frequency and distribution of gap junctions varied in different regions of the adult cortex, possibly reflecting differences in the cellular and functional organization of these cortical areas.

The noradrenergic system influences the fate of Cajal-Retzius cells in the developing cerebral cortex.

Cajal-Retzius cells are neurons prominently located in layer I of the developing cerebral cortex. They are the first neurons to be born in the cortex reaching maturity long before any other cortical neuronal cell type; later in development they degenerate and/or change phenotype. The noradrenergic system, which originates in the locus coeruleus in the brain stem, is one of the earliest axonal systems to enter the cortex forming contacts with Cajal-Retzius cells in layer I. Here we followed the course of development of the Cajal-Retzius cells in postnatal life in animals depleted of noradrenaline in the cortex. We found that removal of this system after birth resulted in significantly more Cajal-Retzius cells during the first 2 weeks of life. This may be due to the observed decline in the number of dying cells in layer I of these animals during the same period. We speculate that the noradrenergic system regulates the development of Cajal-Retzius cells which have been implicated in neuronal migration and laminar formation in the cerebral cortex.

Basic FGF increases communication between cells of the developing neocortex.

We have found that basic fibroblast growth factor (bFGF), applied to cortical progenitor cells in vitro, produces an increase in the expression of the gap junction protein connexin (Cx) 43 and in the mRNA encoding Cx 43. This effect was evident in both proliferating and nonproliferating cells. The elevated levels of mRNA suggest that bFGF is likely to exert its effect by upregulating the rate of transcription of the Cx 43 gene. We have further shown that the increase in Cx 43 expression is mediated through the receptor tyrosine kinase pathway and is associated with enhanced intercellular dye-coupling mediated by gap junctions. These results suggest that gap junction channels provide a direct conduit for mitogens released in response to bFGF to effectively regulate proliferation during corticogenesis.

Collagen type IV promotes the differentiation of neuronal progenitors and inhibits astroglial differentiation in cortical cell cultures.

In an effort to elucidate the interactions between cells in the developing cortex and their microenvironment, we have employed dissociated cell cultures and immunocytochemistry to analyze the effect of collagen type IV (COL) on the proliferation and differentiation of rat cortical progenitor cells during the period of corticogenesis. COL, present in the proliferative zones throughout the period of neurogenesis, belongs to a group of macromolecular proteins that make up a considerable portion of the extracellular matrix (ECM). We have shown that this ECM molecule inhibits cell proliferation and glial cell differentiation while promoting neuronal differentiation. We have also demonstrated that COL, when applied to the cultures with basic fibroblast growth factor (bFGF), induces glial cell differentiation while continuing to promote neuronal differentiation. These results indicate that cortical progenitor cells respond differentially to local environmental signals, and that components of the ECM are involved in the regulation of corticogenesis.

Basic fibroblast growth factor promotes the generation and differentiation of calretinin neurons in the rat cerebral cortex in vitro.

Calretinin-expressing neurons are some of the earliest postmitotic cells to appear in the developing cerebral cortex. Lineage studies have shown that the expression of this calcium-binding protein in cortical neurons is not genetically programmed and is likely to be induced by external factors. A number of studies have clearly shown that basic fibroblast growth factor (bFGF) and a number of neurotrophins promote the proliferation and differentiation of cortical progenitor cells to a particular lineage. Here, using a culture system of dissociated rat cortical cells, we found that brain-derived neurotrophic factor and neurotrophin-3 promoted the morphological differentiation of one of the calretinin-containing neuronal subpopulations, the Cajal-Retzius cells. Another subpopulation of calretinin-expressing cells of smaller size and bipolar form was generated when cultures were treated with bFGF. The progenitors of these neurons were stimulated by bFGF to divide a number of times before initiating their differentiation programme. The number of calretinin-expressing neurons increased further when cultures were treated with a combination of bFGF and retinoic acid.