Development of local connections in ferret somatosensory cortex

Ferrets have become recognized as a useful and interesting model for study of neocortical development. Because of their immaturity at birth, it is possible to study very early events in the ontogeny of the brain. We used living slices of ferret somatosensory cortex to study the formation and development of intrinsic elements within the neocortex. A small number of fixed, hemisected brains injected with 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiI) were also used. The slices were obtained from ferret kits aged postnatal day (P)1 to P62 and maintained in a chamber; each slice received injections of fluorescent-labeled dextrans. The injections were made at different ages in several distinct sites, which included the proliferative ventricular zone, the intervening white matter (or intermediate zone), and different sites of developing cortex, including the deeper cortical plate, which incorporated the subplate in young animals, and more superficial cortical sites, depending on the age of the animal. Several animals also received injections into the ventrobasal thalamus. Injections into young animals (P1–7) produced a dominant radial pattern that extended from the ventricular zone into the cortex. Injections into the ventricular zone labeled many cells that appeared morphologically like radial glia as well as presumptive neurons. Although the predominant pattern was radial, injections in the ventricular zone often produced tangentially oriented cells and horizontally arranged fibers at the outer edge of the proliferative zone. These cells and fibers may provide a substrate for tangential dispersion of neurons within the neocortex. More superficial injections within the slice labeled lines of cells that appeared to be stacked upon one another in a radial pile in the cortex; the cortical plate received very few lateral projections. Data obtained from more mature slices indicated that, although the overall pattern of staining remained radial, the precise character of the pattern changed to include more lateral spread into surrounding cortex, which eventually refined and developed into distinct patches by P28, when the overall cortical architecture appeared adult like. The data involving thalamocortical connections were more limited, but they indicated that the thalamus projects precisely to the somatosensory cortex in a point-to-point fashion from the earliest date studied (P0) and that the ventrobasal nucleus terminates upon the somatosensory cortex in a patchy manner during the early postnatal days of development. This study of the development of the somatosensory cortex confirms the ubiquitous nature of column-like connections throughout the neocortex and provides a novel view of the radial nature of early neocortical maturation. © Wiley-Liss, Inc.     

Glutamate acting at NMDA receptors stimulates embryonic cortical neuronal migration

During cortical development, embryonic neurons migrate from germinal zones near the ventricle into the cortical plate, where they organize into layers. Mechanisms that direct neuronal migration may include molecules that act as chemoattractants. In rats, GABA, which localizes near the target destination for migrating cortical neurons, stimulates embryonic neuronal migration in vitro. In mice, glutamate is highly localized near the target destinations for migrating cortical neurons. Glutamate-induced migration of murine embryonic cortical cells was evaluated in cell dissociates and cortical slice cultures. In dissociates, the chemotropic effects of glutamate were 10-fold greater than the effects of GABA, demonstrating that for murine cortical cells, glutamate is a more potent chemoattractant than GABA. Thus, cortical chemoattractants appear to differ between species. Micromolar glutamate stimulated neuronal chemotaxis that was mimicked by microM NMDA but not by other ionotropic glutamate receptor agonists (AMPA, kainate, quisqualate). Responding cells were primarily derived from immature cortical regions [ventricular zone (vz)/subventricular zone (svz)]. Bromodeoxyuridine (BrdU) pulse labeling of cortical slices cultured in NMDA antagonists (microM MK801 or APV) revealed that antagonist exposure blocked the migration of BrdU-positive cells from the vz/svz into the cortical plate. PCR confirmed the presence of NMDA receptor expression in vz/svz cells, whereas electrophysiology and Ca2+ imaging demonstrated that vz/svz cells exhibited physiological responses to NMDA. These studies indicate that, in mice, glutamate may serve as a chemoattractant for neurons in the developing cortex, signaling cells to migrate into the cortical plate via NMDA receptor activation.     

Developmentally induced microencephalopathy in guinea pigs -embryonic glial cell activation marks selective neuronal death

We have recently shown that in utero treatment of guinea pigs with the DNA methylating substance methylazoxymethanol acetate (MAM) on gestation day (GD) 24 results in neocortical microencephalopathy, increased protein kinase C activity and altered processing of the amyloid precursor protein in neocortex of the offsprings. In order to identify the primary neuronal lesions produced by MAM-treatment, we mapped the 5-bromo-2'-deoxyuridine (BrdU)-incorporation in dividing neurons on GD 24 and we followed the effects of MAM-treatment on GD 24 on embryonic immediate early gene expression and on glial cell activation. BrdU injected on GD 24 labeled many neurons of the ventricular zone and of the intermediate zone but only scattered neurons of the cortical plate. When time-mated guinea pigs were injected intraperitoneally with MAM on GD 24, we observed the activation of microglial cells in the ventricular/intermediate zone and the appearence of astrocytes between the intermediate zone and the cortical plate, 48 h after intoxification. The activation of glial cells was accompanied by the neuronal expression of c-Fos but not of c-Jun in the ventricular/intermediate zone. Based on our observations on BrdU-incorporation and on the morphological outcome of MAM treatment in the juvenile guinea pig, our data presented here indicate that selective neurodegeneration during development induces the activation of both phagocytotic microglial cells and of astrocytes which might trophically support damaged neurons surviving this lesion procedure.     

Immunocytological localization of cell adhesion molecules L1 and N-CAM and the shared carbohydrate epitope L2 during development of the mouse neocortex

The expression of the two adhesion molecules L1 and N-CAM and their shared carbohydrate epitope recognized by monoclonal antibody L2, was studied during development of the embryonic mouse neocortex by immunohistology at light- and electron-microscopic levels between embryonic days 9 and 18. Throughout this time period N-CAM is expressed in all layers of the telencephalic anlage. L1 antigen shows a more restricted expression than N-CAM. It is not detectable at day 9. From day 10 onward it is expressed on young neurons in the marginal zone, but not in the ventricular layer. At embryonic day 13 L1 antigen appears also in the intermediate zone on afferent fibers from subcortical structures and on migrating neurons. Neuronal cell bodies in the cortical plate and subplate express L1 antigen only transiently on embryonic days 13-16. These observations suggest that L1 antigen does not play a prominent role in the initiation of neuronal migration in the ventricular zone, but could be functional during later stages of migration and in the aggregation of neuronal cell bodies at their final position in the cortical plate. The L2 epitope also shows a more restricted expression than N-CAM during the time period studied. Similar to L1 antigen, it first appears at embryonic day 10 in the marginal zone and remains undetectable in the ventricular layer also at later stages. In the marginal zone the L2 epitope is strongly expressed on neuroepithelial endfeet at the basal lamina. The basal lamina itself is L2 epitope-negative. From embryonic day 10 onward the L2 epitope is most strongly expressed in the marginal zone and subplate and more weakly in the cortical plate and intermediate zone. In the subplate it is not only associated with the surface membrane, but also with the extracellular matrix. These observations support previous biochemical data which show that the L2 epitope is not present on all N-CAM molecules of the embryonic or adult forms and suggest that the independent regulation or L2 epitope expression may have functional implications during development.     

Cell migration in the rat embryonic neocortex.

Three-dimensional reconstructions of the normal rat embryonic (E) neocortex on days E15, E17, E19, and E21, using Skandha (software designed by J. Prothero, University of Washington, Seattle), show that the neocortical ventricular zone shrinks rapidly in the medial direction during cortical morphogenesis. [3H]thymidine autoradiography indicates that the shrinkage of the ventricular zone occurs before neurons in lateral and ventrolateral parts of layers IV-II are generated. Consequently, most of these neurons originate 400-1000 microns medial to their settling sites in the cortical plate. Embryos killed at daily intervals up to E21 after a single injection of [3H]thymidine on either E17 or E18 revealed the presence of a prominent migratory path, the lateral cortical stream, used by neurons migrating to the lateral and ventrolateral cortical plate; neurons migrating to the dorsal cortical plate follow a direct radial path. Arrival times of neurons in the cortical plate depend on the migratory path and are proportional to the overall distance travelled. Neurons that migrate only radially arrive in the dorsal cortical plate in two days (shortest route). Neurons that migrate laterally arrive in the lateral cortical plate in 3 days (longer route) and in the ventrolateral cortical plate in 4 days (longest route). [3H]thymidine autoradiography also shows that cells generated in the neocortical ventricular zone migrate in the lateral cortical stream for 5 or more days and accumulate in a reservoir. Cells leave the reservoir to enter the piriform cortex and destinations (as yet undetermined) in the basal telencephalon. The lateral cortical stream is found wherever the neocortical primordium surrounds the basal ganglia and is absent behind the basal ganglia. A computer analysis of nuclear orientation in anterior and posterior parts of the intermediate zone in the dorsal neocortex between days E17 and E22 shows that horizontally oriented nuclei are more common anteriorly where many cells are migrating laterally than posteriorly where most cells are migrating radially.     

Histochemical localization of neurons with zinc-permeable AMPA/kainate channels in rat brain slices

Zinc modulates neurotransmission and may trigger neurodegeneration following brain injuries. Therefore, it is important to understand zinc dynamics in an anatomical context. Using a histochemical procedure on stimulated slices, we visualized zinc influx into neurons 'in situ'. Hippocampal, neocortical and cerebellar slices were loaded with zinc and stimulated with compounds known to open zinc-permeable channels. Zinc was then visualized by histochemical precipitation methods. Kainate stimulation labelled CA1 hippocampal pyramidal neurons, as well as subpopulations of interneurons in the hilus, CA1 and CA3 fields. Interneurons in the neocortex and many cell types of the cerebellum were also labelled. However, neither NMDA nor 50 mM K(+) stimulation resulted in comparable zinc accumulation in neurons. Immunofluorescent colocalization of parvalbumin with kainate-induced zinc staining in the hippocampus and neocortex showed a subset of zinc-sensitive neurons, positive for parvalbumin. These results confirm that zinc permeation through calcium-permeable AMPA/kainate channels constitutes a predominant route of zinc entry into different cell types. Furthermore, this technique provides a useful and versatile histochemical approach to assess the cell subpopulations of the central nervous system particularly sensitive to zinc accumulation under normal or pathological conditions.     

Evolution of cortical neurogenesis

The neurons of the mammalian neocortex are organised into six layers. By contrast, the reptilian and avian dorsal cortices only have three layers which are thought to be equivalent to layers I, V and VI of mammals. Increased repertoire of mammalian higher cognitive functions is likely a result of an expanded cortical surface area. The majority of cortical cell proliferation in mammals occurs in the ventricular zone (VZ) and subventricular zone (SVZ), with a small number of scattered divisions outside the germinal zone. Comparative developmental studies suggest that the appearance of SVZ coincides with the laminar expansion of the cortex to six layers, as well as the tangential expansion of the cortical sheet seen within mammals. In spite of great variation and further compartmentalisation in the mitotic compartments, the number of neurons in an arbitrary cortical column appears to be remarkably constant within mammals. The current challenge is to understand how the emergence and elaboration of the SVZ has contributed to increased cortical cell diversity, tangential expansion and gyrus formation of the mammalian neocortex. This review discusses neurogenic processes that are believed to underlie these major changes in cortical dimensions in vertebrates.     

Prenatal development of GABA-ergic neurons in the neocortex of the rat

The present study shows that in the prenatal rat neocortex the GABA immunoreactive neurons are not limited to the marginal, subplate, and intermediate zones, but are also found in all fetal zones of the cerebral anlage. The first GABA-ergic cells are observed on embryonic day 14 in the plexiform primordium. On embryonic day 15, a second population of GABA-ergic cells is observed in the intermediate zone. Beginning on day 16 of gestation and continuing throughout gestation, GABA-ergic neurons are observed in the marginal zone, the subplate zone, the cortical plate, and the ventricular and subventricular zones. Furthermore, while the number of GABA-ergic cells in the cortical plate increases, GABA-ergic neurons in the intermediate zone and subventricular zone decrease in number after embryonic day 19.     

Immunohistochemical identification of some plasma proteins in human embryonic and fetal forebrain with particular reference to the development of the neocortex

The histogenesis of the cerebral neocortex has been studied in human embryos and fetuses from the ventricular zone stage at 9-10 mm crown-rump length (CRL) to the well-developed neocortex at 210 mm CRL. The initial proliferation of the neuroepithelial cells in the ventricular zone stage was followed by a stage characterized by a ventricular zone covered by a primordial plexiform layer; the subventricular zone then arose before the cortical plate was formed within the primordial plexiform layer, thus dividing it into an outer marginal zone and an inner subplate zone; finally the intermediate zone appeared between the subventricular and subplate zones. The distribution of cells containing albumin, alpha-fetoprotein, transferrin, prealbumin, IgG and alpha 1-antitrypsin in the cerebral vesicle and developing neocortex was investigated by the indirect immunoperoxidase technique. Alpha-fetoprotein found in the cells of the ventricular zone was the most widespread and prominent of the plasma proteins examined in the early embryos. The cerebral vesicle was negative for all other plasma proteins investigated at this stage. By 15 and 16 mm CRL, a few cells in the ventricular zone were positive for albumin and transferrin whereas AFP exhibited a distribution similar to that of the 9 mm embryo. By 20-25 mm CRL, albumin and AFP had a similar distribution in the telencephalic wall. At 40-150 mm CRL a positive staining reaction for AFP, albumin, prealbumin and transferrin was predominant in the outer half of cortical plate. At 150-170 mm CRL only cells in the inner half exhibited positive staining and at 210 mm CRL the staining reactions were negative. The cells containing plasma proteins did not belong to a single cell line or type; thus plasma proteins were detected primarily in different types of neurons but also in glial cells. Staining with polyvalent antiserum indicated that the same cells may be positive for more than one plasma protein. Positive staining reactions were also observed in or along fiber systems. It is proposed that cells initially take up plasma protein from the CSF and migrate with it towards the cortical plate. After a certain period they lose their plasma protein but when the neuronal cells which represent the majority of the positively stained cells have reached their final position in the cortical plate they commence plasma protein synthesis which continues for a short period during which the neurons establish their pattern of connectivity.     

Effect of excess extracellular glutamate on dendrite growth from cerebral cortical neurons at 3 days in vitro: Involvement of NMDA receptors

Glutamate is an important regulator of dendrite development; however, during cerebral ischemia, massive glutamate release can lead to neurodegeneration and death. An early consequence of glutamate excitotoxicity is dendrite injury, which often precedes cell death. We examined the effect of glutamate on dendrite growth from embryonic day 18 (E18) mouse cortical neurons grown for 3 days in vitro (DIV) and immunolabeled with anti-microtubule-associated protein (MAP)2 and anti-neurofilament (NF)-H, to identify dendrites and axons, respectively. Cortical neurons exposed to excess extracellular glutamate (100 microM) displayed reduced dendrite growth, which occurred in the absence of cell death. This effect was mimicked by the ionotropic glutamate receptor agonist N-methyl-D-aspartate (NMDA) and blocked by the ionotropic glutamate receptor antagonist kynurenic acid and the NMDA receptor-specific antagonist MK-801. The non-NMDA receptor agonist AMPA, however, did not affect process growth. Neither NMDA nor AMPA influenced neuron survival. Immunolabeling and Western blot analysis of NMDA receptors using antibodies against the NR1 subunit, demonstrated that immature cortical neurons used in this study, express NMDA receptors. These results suggest that excess glutamate decreases dendrite growth through a mechanism resulting from NMDA receptor subclass activation. Furthermore, these data support the possibility that excess glutamate activation of NMDA receptors mediate both cell death in mature neurons and the inhibitory effect of excess glutamate on dendrite growth in immature neurons or in the absence of cell death.     

A radialization factor in normal cortical plate restores disorganized radial glia and disrupted migration in a model of cortical dysplasia

Treatment of pregnant ferrets on embryonic day 24 (E24) with the antimitotic methylazoxy methanol (MAM) leads to a specific constellation of effects in newborn kits, which include a very thin and poorly laminated neocortex, disruption of radial glial cell morphology with early differentiation into astrocytes, and abnormal positioning of Cajal-Retzius cells. We suggest that MAM treatment on E24 results in this model of cortical dysplasia by eliminating a population of cells that produce a factor capable of maintaining radial glia in their normal morphology. The abnormal radial glia, either alone or in combination with other abnormal features, are likely to prevent proper migration into the cortical plate. To test the possibility that normal cortex can provide the missing substance that influences radial glia, slices of E24 MAM-treated cortex were removed at postnatal day 0 (P0) and cultured adjacent to explants of P0 normal cortical plate. By labelling a small number of cells with injections of fluorescent dextrans into the cultured slices, we found that abnormal radial glia in MAM treated slices cocultured adjacent to normal cortical plate were restored toward normal, in comparison to E24 MAM treated slices cultured alone and in other control conditions. We also found that abnormally positioned Cajal-Retzius cells move into the marginal zone and that neurons are able to migrate into the cortical plate more effectively in the coculture condition. These data indicate that normal cortical plate of ferrets contains a factor causing radial glia to maintain their elongated morphology; the improved position of radial glia encourages repositioning of Cajal-Retzius cells and improved neuronal migration into the cortical plate.     

Early areal differentiation of the human cerebral cortex: Entorhinal area

The early cytoarchitectonic specialization and area-specific differentiation of the prospective entorhinal cortex were studied in the postmortem human fetal brains (9.5–13.5 postovulatory weeks). At 10 weeks, using the Golgi method, we saw the appearance of area-specific large neurons (promoter neurons) with widely bifurcating apical dendrites situated at the outer margin of the cortical plate of the prospective entorhinal cortex. The analysis of the serial Nissl-stained sections revealed the concomitant appearance of a one-cell-thick layer (monolayer) at the interface between the cortical plate and the marginal zone and multilaminated spread of the deep part of the cortical plate. This is the earliest sign of area-specific cytoarchitectonic differentiation of all pallial regions characterized by the presence of the typical cortical plate. The first subareal differentiation within the entorhinal cortex begins at 13 postovulatory weeks with uneven development of fiber-rich lamina dissecans, which separates two cellular laminae principales (externa and interna), and with the appearance of characteristic cell islands of the prospective layer II. At rostral levels, cell islands begin to develop in the rostromedial entorhinal area at the subpial depths where large promoter neurons reside. At intermediate levels, both lamina dissecans and lamina principalis interna are well delineated. At caudal levels, lamina principalis interna is continuous with the upper subplate zone of the adjacent neocortex. Both area-specific neurons (promoters) and fiber-rich (afferent) strata develop synchronously during the earliest areal differentiation of the cerebral cortex. The precocious lamination of the cortical plate is the crucial event in the histogenesis of the entorhinal cortex.     

Radial migration in the cerebral cortex is enhanced by signals from thalamus

The six layered cerebral cortex derives from cells that divide in the ventricular zone and migrate to their final destination in the cortical plate (future cortex). In the mouse, cortical layer III and IV neurons undergo their final mitotic division at around E16, at which time thalamic axons are beginning to enter the cortex. We used bromodeoxyuridine-birth dating of cells in cortical slice cultures to show that the thalamus enhances the migration out of the ventricular zone of future layer III/IV cells. When cortical slices were cultured alone, less than 35% of cells born in vitro on E16 were present in the pial half of the slice after 48 h in culture. In contrast, when cortical slices were cocultured with thalamus, 69% of these cells were found in the pial half of the slice. Explants of other developing tissues did not mimic the effect of the thalamus. The thalamus had no obvious effect on cortical radial glial cells, cortical cell viability or maintenance of cortical slice structure. We found that most precursors born at a similar age but in vivo, shortly before cortical slices were isolated, migrated to the pial half of the cultured slices in the absence of a cocultured thalamic explant. Thus, E16 cortical slices cultured without thalamus permit migration of cells born in vivo and therefore already exposed to the thalamus. Our results indicate that the thalamus provides factors to E16-born cortical precursors that enhance their directed migration out of the ventricular zone to the cortical plate.     

delta-catenin is a nervous system-specific adherens junction protein which undergoes dynamic relocalization during development

delta-catenin is a member of the Armadillo repeat family and component of the adherens junction discovered in a two-hybrid assay as a bona fide interactor with presenilin-1 (Zhou et al., [1997], NeuroReport 8:2085-2090), a protein which carries mutations that cause familial Alzheimer's disease. The expression pattern of delta-catenin was mapped between embryonic day 10 (E10) and adulthood by Northern blots, in situ hybridization and immunohistochemistry in the mouse. In development, delta-catenin is dynamically regulated with respect to its site of expression. It is first expressed within proliferating neuronal progenitor cells of the neuroepithelium, becomes down-regulated during neuronal migration, and is later reexpressed in the dendritic compartment of postmitotic neurons. In the mouse, delta-catenin mRNA is expressed by E10, increases and peaks at postnatal day (P)7, with lower levels in adulthood. In the developing neocortex, delta-catenin mRNA is strongly expressed in the proliferative ventricular zone and the developing cortical plate, yet is conspicuously less prominent in the intermediate zone, which contains migrating cortical neurons, delta-catenin protein forms a honeycomb pattern in the neuroepithelium by labeling the cell periphery in a typical adherens junction pattern. By E18, delta-catenin expression shifts primarily to nascent apical dendrites, a pattern that continues through adulthood. The dynamic relocalization of delta-catenin expression during development, taken together with previously published data which described a role for delta-catenin in cell motility (Lu et al., [1999] J. Cell. Biol. 144:519-532), suggests the hypothesis that delta-catenin regulation is closely linked to neuronal migration and may play a role in the establishment of mature dendritic relationships in the neuropil.     

Characterization of large conductance Ca2+-activated K+ channels in cerebellar Purkinje neurons

We investigated the role of large conductance, calcium-activated potassium channels (BK channels) in regulation of the excitability of cerebellar Purkinje neurons. Block of BK channels by iberiotoxin reduced the afterhyperpolarization of spontaneous action potentials in Purkinje neurons in acutely prepared cerebellar slices. To establish the conditions required for activation of BK channels in Purkinje neurons, the dependence of BK channel open probability on calcium concentration and membrane voltage were investigated in excised patches from soma of acutely prepared Purkinje cells. Single channel currents were studied under conditions designed to select for potassium currents and in which voltage-activated currents were largely inactivated. Micromolar calcium concentrations activated channels with a mean single channel conductance of 266 pS. BK channels were activated by both calcium and membrane depolarization, and showed no sign of inactivation. At a given calcium concentration, depolarization over a 60-mV range increased the mean open probability (P(O)) from < 0.1 to > 0.8. Increasing the calcium concentration shifted the voltage required for half maximal activation to more hyperpolarized potentials. The apparent affinity of the channels for calcium increased with depolarization. At -60 mV the apparent affinity was approximately 35 micro m decreasing to approximately 3 micro M at +40 mV. These results suggest that BK channels are unlikely to be activated at resting membrane potentials and calcium concentrations. We tested the hypothesis that Purkinje cell BK channels may be activated by calcium entry during individual action potentials. Significant BK channel activation could be detected when brief action potential-like depolarizations were applied to patches under conditions in which the sole source of calcium was flux across the plasma membrane via the endogenous voltage-gated calcium channels. It is proposed that BK channels regulate the excitability of Purkinje cells by contributing to afterhyperpolarizations and perhaps by shaping individual action potentials.     

N-methyl-D-aspartate (NMDA) receptors are inactivated by trypsin

Acute isolation of hippocampal CA3 pyramidal cells using trypsin produces neurons which respond to kainate and quisqualate but not N-methyl-D-aspartate (NMDA). Incubation of 6- to 12-day-old cultured hippocampal neurons or slices of pyriform cortex with trypsin irreversibly removes the NMDA responses normally present without significant effect on responses to kainate or quisqualate. These data indicate that the NMDA receptor has a trypsin-sensitive component which is necessary for agonist recognition or ion channel activation.     

Disabled-1 mRNA and protein expression in developing human cortex

Disabled-1 (Dab1) forms part of the Reelin-Dab1 signalling pathway that controls neuronal positioning during brain development; Dab1 deficiency gives rise to a reeler-like inversion of cortical layers. To establish a timetable of Dab1 expression in developing human brain, Dab1 mRNA and protein expression were studied in prenatal human cortex. The earliest Dab1 signal was detected at 7 gestational weeks (GW), the stage of transition from preplate to cortical plate, suggesting a role of the Reelin-Dab1 signalling pathway in preplate partition. From 12 to 20 GW, the period of maximum cortical migration, Dab1 expression was prominent in the upper tiers of the cortical plate, to decline after midgestation. Radially orientated apical dendrites of Dab1-expressing neurons indicated a predominant pyramidal phenotype. Pyramidal cells in hippocampus and entorhinal cortex displayed a more protracted time of Dab1 expression compared to neocortex. In addition, at later stages (18-25 GW), Dab1 was also expressed in large neurons scattered throughout intermediate zone and subplate. From 14 to 22 GW, particularly high levels of Dab1 mRNA and protein were observed in cells of the ventricular/subventricular zone displaying the morphology of radial glia. The partial colocalization of vimentin and Dab1 in cells of the ventricular zone supported a radial glia phenotype. The concentration of Dab1 protein in ventricular endfeet and initial portions of radial processes of ventricular-zone cells points to a possible involvement of Dab1 in neurogenesis. Furthermore, a subset of Cajal-Retzius cells in the marginal zone colocalized Dab1 and Reelin, and may thus represent a novel target of the Reelin-Dab1 signalling pathway.     

NMDA and AMPA receptors on neocortical neurons are differentially distributed

The distribution of glutamate receptor subtypes on the surface of neurons is highly relevant for synaptic activation and signal processing in the neocortex. As a novel approach we have used infra-red videomicroscopy in combination with photostimulation or microiontophoresis in brain slices of rat neocortex to map the distribution of N-methyl-d -aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors on pyramidal neurons of layer V. Both modes of application revealed a spatially distinct distribution of glutamate receptor subtypes: the soma and the proximal dendrite of neurons are highly sensitive to NMDA, whereas the more distal parts of the dendrite are more sensitive to AMPA. An implication is that NMDA receptors near the soma might regulate the amplification of synaptic signals resulting from AMPA receptor activation on remote dendritic sites.     

Demarcation of early mammalian cortical development by differential expression of fringe genes

Fringe has originally been found in Drosophila as a gene encoding a putative secreted protein which regulates the sensitivity of Notch signaling pathway to different ligands. We show that three members of murine fringe gene family, Lunatic fringe (L-fng), Manic fringe (M-fng) and Radical fringe (R-fng), show related patterns of expression in the developing cerebral wall. L-fng is expressed in immature cells in the ventricular zone. M-fng is upregulated transiently in maturing neurons when they leave the ventricular zone (VZ). R-fng is upregulated in more mature neurons when they enter the preplate and cortical plate. These patterns suggest that the transition from immature to mature neurons involves sequential changes in the member of fringe family genes expressed. More detailed expression analyses of fringe genes and immunohistochemistry for neuron-specific class III beta-tubulin suggest a mode of neurogenesis which might underlie the histogenesis of the cerebral cortex. A proliferative population situated outside of the VZ is defined as M-fng-positive/BrdU-positive cells, which constitutes about 10-20% of the total S-phase cells in the cerebral wall of embryonic day 10.5-12.5. We found that M-fng is expressed in mitotic figures outside the VZ and some of them react with the antibody against class III beta-tubulin. These observations suggest that a significant number of proliferative cells exist outside the VZ, which supply neurons during early cortical development.