Development of the lateral and medial limbic cortices in the rat in relation to cortical phylogeny

[3H]Thymidine autoradiography was used to investigate neurogenesis of the lateral limbic cortex and morphogenesis of the medial and lateral limbic cortices in adult and embryonic rat brains. Ontogenetic patterns in the limbic cortex are unique because some neurogenetic gradients are linked to those in neocortex, others are linked to those in paleocortex. These findings are related to hypotheses of cortical phylogeny. The experimental animals used for neurogenesis were the offspring of pregnant females injected with [3H]thymidine on 2 consecutive days: Embryonic Day (E) 13-E14, E14-E15, ...E21-E22, respectively. On Postnatal Day (P)60, the proportion of neurons originating during 24-h periods were quantified at nine anteroposterior levels and one sagittal level. Similar to neocortex, deep cells are generated earlier than superficial cells throughout the lateral limbic cortex: layer VI mainly on E14-15, layer V on E15-E16, and layers IV-II on E16-E18. There is a ventral/older to dorsal/younger neurogenetic gradient between the ventral agranular insular, dorsal agranular insular, and gustatory cortical areas and between ventral and dorsal orbital areas beneath the frontal pole. Similar to paleocortex below the rhinal sulcus, limbic cortex in the rhinal sulcus has a "sandwich" gradient: the older posterior agranular insular area is sandwiched by anterior and posterior younger areas (ventral agranular insular and perirhinal). To study morphogenesis, pregnant females were given single injections of [3H]thymidine during gestation and embryos were removed in successive 24-h intervals (sequential-survival). Neurogenesis finishes first in ventral limbic areas, later in dorsal limbic areas, and latest in neocortical areas. The cortical plate in the region of the medial and lateral limbic cortices does not have a separate subplate layer as is found in the region of the neocortex. Instead, layer VI in the limbic cortices has unusually older cells that are generated simultaneously with subplate cells.     

Development of GABA-immunoreactivity in the neocortex of the mouse.

The prenatal and postnatal development of GABAergic elements in the neocortex of the mouse was analyzed by GABA-immunocytochemistry. Radial distribution of cells and laminar numerical densities were calculated at each developmental stage to substantiate qualitative observations. The first immunoreactive neurons were observed in the cortical anlage at embryonic day 12-embryonic day 13 (E12-E13) in the primitive plexiform layer. At following prenatal stages (E14-E19), most GABA-positive neurons were present in the marginal zone, subplate, and subventricular zone. GABA-immunoreactivity in the cortical plate appeared early (E14), although the complete maturation of its derivatives was achieved postnatally. At prenatal stages we noted a well-developed system of immunopositive fibers in the subplate. As indicated by the direction of growth cones, most of these fibers had an extracortical origin and invaded the cortex laterally through the internal capsule and striatum. In rostral and middle telencephalic levels, fibers originating in the septal region contributed to the cingulate bundle. Presumably corticofugal fibers and callosal axons were also noticed. At postnatal stages the maturation of GABA-immunoreactivity appeared to be a complex, long-lasting process, in which the adult pattern was produced at the same time as the appearance of certain regressive phenomena. Thus, between postnatal day 0 and postnatal day 8 (P0-P8), GABA-positive populations disappeared from the subventricular zone, marginal zone and to a lesser extent from the subplate. At the same ages we noticed the presence of morphologically abnormal, GABA-immunoreactive neurons in the subventricular zone and subplate which are interpreted as correlates of neuronal degeneration. Most GABA-positive subplate fibers also disappeared whereas GABA-immunoreactive axons were seen in the cingulate bundle until the adult stage. In the derivatives of the cortical plate, the maturation of GABA-immunoreactive elements progressed according to the "inside-out" gradient of cortical development, with the important exception of layer IV, which was the last layer to exhibit an adult-like appearance. Within each layer deriving from the cortical plate (layers VIa to II-III), GABA-immunoreactivity showed a protracted maturation in which the first GABA-positive cells were detected a few days after cell birth but substantial numbers of neurons began to express GABA considerably later. The later phase occurred concurrently with the maturation of GABA-positive axonal plexuses. These results suggest that different GABA-positive populations show different developmental regulation of GABA expression during cortical ontogenesis.     

Origin of the cortical layer I in rodents

Using birthdating techniques, we have studied when cells that settle in the marginal zone (future layer 1) of the cortical neuroepithelium are generated in developing rat embryos. The majority of marginal zone cells are generated at embryonic day 12 (E12), E13 and E14, although some cells generated later can incorporate into this stratum after the cortical plate forms. The nature and the origin of the cell populations that colonize the preplate/marginal zone was studied by means of immunohistochemistry using cell markers for gamma-amino butyric acid (GABA), reelin and the calcium binding proteins calretinin and calbindin. At early stages of development, the preplate is formed by Cajal-Retzius cells, subplate cells, subpial granular layer cells, some interneurons and some glial cells. With the arrival of the cortical plate cells, the subplate cells descend to occupy the stratum below. Layer 1 cells are of diverse origin as some of them are generated in the ventricular zone of the cortical neuroepithelium, whereas other cell populations come from extracortical regions such as the olfactory placode or the ganglionic eminences of the basal telencephalon. The predominant cell type in the marginal zone is the Cajal-Retzius cell, which expresses reelin and calretinin, and is probably generated in the cortical neuroepithelium. These cells can be readily distinguished from cells that come from the ganglionic eminences as these later populations mainly express GABA and calbindin. Finally, our results suggest that the cells of the subpial granular layer might be generated in the rostral pole of the lateral ganglionic eminences.     

Receptor-like protein tyrosine phosphatase zeta/RPTP beta is expressed on tangentially aligned neurons in early mouse neocortex

Protein tyrosine phosphatase zeta (PTPzeta)/RPTPbeta is a chondroitin sulfate proteoglycan predominantly expressed in the brain. In this study, we examined immunohistochemical localisation of PTPzeta in the mouse telencephalon from embryonic day 9.5 (E9.5) to E15.5. During E10.5-E12.5, immunoreactivities for PTPzeta are specifically observed on the tangentially aligned neurons at the preplate (PP) of the neocortex, as well as on the neurons at the mantle layer (ML) of the ganglionic eminences (GEs). Likewise, neurons immunoreactive for CR50, a marker for Cajal-Retzius neurons, are aligned from the ML of the ganglionic eminences to the PP of the neocortex and co-express PTPzeta. During E13.5-E15.5, PTPzeta-positive neurons are present at the subplate (SP) as well as at the marginal zone (MZ) of the neocortex. These results indicate that PTPzeta is a useful marker for early-generated neocortical neurons in mice: Cajal-Retzius neurons as well as the subplate neurons.     

Neurogenetic patterns in the medial limbic cortex of the rat related to anatomical connections with the thalamus and striatum

[3H]Thymidine autoradiography was used to investigate neurogenesis in all areas of the limbic cortex in the medial wall of the hemisphere. The experimental animals were the offspring of pregnant females injected with [3H]thymidine on 2 consecutive days: Embryonic Day (E)13-E14, E14-E15, ...E21-E22, respectively. On Postnatal Day (P)60, the proportion of neurons originating during 24-h periods were quantified at nine anteroposterior levels. Three types of neurogenetic gradients are found. (i) Deep cells are older than superficial cells: layer VI is generated mainly on E15-E16, layer V on E16-E18, and layers IV-II on E18-E20. (ii) There is a ventral/older to dorsal/younger gradient between the dorsal peduncular, infralimbic, and anterior cingulate areas rostral to the genu of the corpus callosum. A ventral/older to dorsal/younger gradient is also found between superficial cells (layers II-IV) in anterior cingulate (CG3/CG1), posterior cingulate (CG2/CG1), and retrosplenial areas (RSG/RSA). (iii) An anterior/older to posterior/younger gradient is found between areas throughout the medial limbic cortex. Some of these neurogenetic patterns correlate with anatomical interconnections between the supracallosal medial limbic cortex (posterior cingulate and retrosplenial areas) and the anteroventral/anteromedial thalamic nuclei: older thalamic cells have longer axons that terminate in cortical areas containing younger cells, while younger thalamic cells have shorter axons that terminate in cortical areas containing older cells. Projections from the medial limbic cortex to the striatum also correlate with neurogenetic gradients: older cortical source cells in the infralimbic area project to the older striatal cells in the enkephalin-rich patches, while younger cortical source cells in the cingulate areas project to younger striatal cells in the surrounding matrix.     

Microtubule-associated protein 2 (MAP 2) immunoreactivity in human fetal neocortex

We used a monoclonal antibody to study the immunocytochemical distribution of microtubule-associated protein 2 (MAP 2) in human fetal neocortex between the ages of 16 and 22 weeks gestation. The staining pattern was lamina-specific. Neuronal somata and dendrites in all cortical layers and in the intermediate zone were labelled. Cajal-Retzius cells of layer I, large pyramidal neurons in the inner cortical plate and neurons in the subplate were most strongly immunoreactive. Separate from the underlying cortical plate a thin sheet of small neurons in the inner marginal zone was highlighted by MAP 2 immunoreactivity. The morphologic diversity, density and regional distribution of the interstitial neurons in the subplate was emphasized by MAP 2 staining. In general, the intensity of MAP 2 immunoreactivity in cell somata and dendrites correlated with the degree of neuronal differentiation but the pattern of intracellular staining also varied as a function of laminar position, and presumably cell type.     

Growth and targeting of subplate axons and establishment of major cortical pathways.

In the developing mammalian neocortex, the first postmitotic neurons form the "preplate" superficial to the neuroepithelium. The preplate is later split into a marginal zone (layer 1) and subplate by cortical plate neurons that form layers 2-6. Cortical efferent axons from layers 5 and 6 and cortical afferent axons from thalamus pass between cortex and subcortical structures through the internal capsule. Here, we identify in rats the axonal populations that establish the internal capsule, and characterize the potential role of subplate axons in the development of cortical efferent and afferent projections. The early growth of cortical efferent and afferent axons was studied using 1-1'-dioctodecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (Dil) as an anterograde and retrograde tracer in aldehyde-fixed brains of embryonic rats. Cortical axons first enter the nascent internal capsule on embryonic day (E) 14 and originate from lateral and anterior cortex; axons from posterior cortex extend rostrally but do not yet exit cortex. The labeled axons, tipped by growth cones with complex morphologies, take a pathway deep to the preplate. Preplate neurons extend these early cortical efferents, based on the developmental stage of the cortex, and on their location and morphology. Most of these cells later occupy the subplate. Cortical plate neurons extend axons into the internal capsule by E16. En route to the internal capsule, cortical plate axons take the same path as the earlier-growing preplate axons, through the intermediate zone deep to subplate. Subplate axons reach thalamus by E16; the first cortical plate axons enter thalamus about a day later. Thalamic axons enter cortex by E16, prior to other cortical afferents. On E15, both preplate and thalamic axons reach the midpoint of the internal capsule. To determine the subcortical distribution of subplate axons, we used Dil as a retrograde tracer in aldehyde-fixed brains and fast blue and rhodamine-B-isothiocyanate as in vivo retrograde markers in neonatal rats. Tracers were injected into the superior colliculus, the principal midbrain target of layer 5 neurons, at times before, during, and after the arrival of cortical axons, or into the subcortical pathway of primary layer 5 axons at two points, the cerebral peduncle caudal to the internal capsule, and the pyramidal decussation at the junction of the hindbrain and spinal cord, at times shortly after the passing of cortical axons. In every case, the labeled neurons are confined to layer 5; subplate neurons are not labeled.(ABSTRACT TRUNCATED AT 400 WORDS)     

Extracellular matrix distribution during neocortical wall ontogenesis in "normal" and "Reeler" mice

The spatio-temporal distribution of extracellular spaces (ECS) was analysed during prenatal corticogenesis in the normal mouse or in the recessive mutant Reeler. Colloidal iron was used to visualize extracellular anionic substances (containing glycosaminoglycans) which constitute an extracellular matrix (ECM). The spatio temporal distribution of the ECM appeared closely related to ECS localization. Before the cortical plate formation, the ECM occurred only in the marginal zone. The plate appearance resulted in a redistribution of anionic sites in the subplate of the normal mouse in addition to the marginal zone. Although, in the Reeler, the ECM remained mainly located in a broad outer cortical zone. Until birth, the distribution of the ECM in the normal mouse, followed closely the inside-out gradient of cortical neuropile maturation, while in the Reeler and outside-in gradient was observed. With EM, following standard fixation and staining an extracellular matrix (formed of 5 to 10 nm fibrils) occurred in sites where the ECM was already detected with light microscopy. The ECM fibrils were seen mainly in the vicinity of young neurons showing characters of secretory functions, such as dilated RER cisternae filled with a fibrillar material and well developed golgi complexes. Young Cajal-Retzius cells appeared especially well suited to elaborate the ECM in the marginal zone of the normal mouse or in the outer cortical zone of the Reeler. The potential role of ECM for the differentiation and formshaping of young neurons is discussed.     

Fates of the earliest generated cells in the developing murine neocortex

In mammalian species studied to date, the first-born neocortical cells normally form two layers, one above and one below the cortical plate, called the marginal zone (future layer 1) and the subplate. In primates and carnivores, many of these first-born cells die early in postnatal life. Whether this also occurs in rodents is highly controversial. In this study, we injected pregnant mice with bromodeoxyuridine on embryonic days (E) 11–14 to label the earliest generated neocortical cells, and examined their fates between birth and postnatal day 21. At birth, most cells born on embryonic day 11 were below the cortical plate, and a smaller proportion were above it. Very few of these cells remained by postnatal day 3 and there were none at any depth in the neocortex at older ages. At birth, the largest proportion of cells born on embryonic days 12 and 13 were in the subplate and smaller proportions were in the cortical plate and marginal zone. At older ages, almost all of these cells had disappeared from the marginal zone and from below the cortical plate, although some were retained in the cortical plate. The density of the remaining E12- and E13-born cells decreased more than could be explained by neocortical expansion alone. As a control, we studied cells born on embryonic day 14. These cells were restricted to the cortical plate at birth. By postnatal day 21, their density had decreased by an amount that could be explained by neocortical expansion alone. We conclude that, as in other species, many of the earliest generated cells of the murine neocortex die. J. Comp. Neurol. 377:414–422, 1997. © 1997 Wiley-Liss, Inc.     

Cellular physiology of the neonatal rat cerebral cortex

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

Expression and distribution of metabotropic GABA receptor subtypes GABABR1 and GABABR2 during rat neocortical development

To understand the possible contribution of metabotropic gamma-aminobutyric acid receptors (GABABR) in cortical development, we investigated the expression pattern and the cellular and subcellular localization of the GABABR1 and GABABR2 subtypes in the rat neocortex from embryonic day 14 (E14) to adulthood. At the light microscopic level, both GABABR1 and GABABR2 were detected as early as E14. During prenatal development, both subtypes were expressed highly in the cortical plate. Using double immunofluorescence, GABABR1 colocalized with GABABR2 in neurons of the marginal zone and subplate, indicating that these proteins are coexpressed and could be forming functional GABABRs during prenatal development in vivo. In contrast, only GABABR1 but not GABABR2 was detected in the tangentially migratory cells in the lower intermediate zone. During postnatal development, immunoreactivity for GABABR1 and GABABR2 was distributed mainly in pyramidal cells. Discrete GABABR1-immunopositive cell bodies of interneurons were present throughout the neocortex. In addition, GABABR1 but not GABABR2 was found in identified Cajal-Retzius cells in layer I. At the electron microscopic level, immunoreactivity for GABABR1 and GABABR2 was found in dendritic spines and dendritic shafts at extrasynaptic and perisynaptic sites throughout postnatal development. We further demonstrated the presynaptic localization of GABABR1 and GABABR2, as well as the association of the receptors with asymmetrical synaptic junctions. These results indicate potentially important roles for the GABABRs in the regulation of migratory processes during corticogenesis and in the modulation of synaptic transmission during early development of cortical circuitry.     

Ontogenesis of microtubule-associated protein 2 (MAP2) in embryonic mouse cortex

The developing neocortex in mice from embryonic day 13 (E13) until birth (E19) was immunoreacted with a monoclonal antibody for microtubule-associated protein 2 (MAP2) that is highly specific for neuronal somata and dendrites. In E13 neocortex there was no detectable MAP2 immunoreactivity on tissue sections or on gel blots. From E14 to birth the MAP2 immunoreactivity was present in both tissue sections and immunoblots of homogenized cortex. In the neocortex the staining pattern was lamina-specific. The molecular layer and the cortical subplate contained the most dense staining of dendrites and cell somata. The cortical plate showed weak to moderate staining at these ages while the intermediate and ventricular zones were not stained above background control levels. Gel blots correspondingly did not show detectable levels of MAP2 until E14. Ultrastructural data suggest that MAP2 is present in dendrites in each of the laminae. The laminar pattern of MAP2 immunoreactivity may be due to either the higher density of differentiating dendrites in the molecular and subplate layers or to compartmentalization of MAP2 within individual cortical neurons.     

Different origins and developmental histories of transient neurons in the marginal zone of the fetal and neonatal rat cortex

Two major classes of early-born neurons are distinguished during early corticogenesis in the rat. The first class is formed by the cortical pioneer neurons, which are born in the ventricular neuroepithelium all over the cortical primordium. They appear at embryonic day (E) 11.5 in the lateral aspect of the telencephalic vesicle and cover its whole surface on E12. These cells, which show intense immunoreactivity for calbindin and calretinin, are characterized by their large size and axonal projection. They remain in the marginal zone after the formation of the cortical plate; they project first into the ventricular zone, and then into the subplate and the internal capsule. Therefore, these cells are the origin of the earliest efferent pathway of the developing cortex. Pioneer neurons are only present in prenatal brains. The second class is formed by subpial granule neurons, which form the subpial granular layer (SGL), previously considered to be found exclusively in the human cortex. SGL neurons are smaller than pioneer neurons. They are generated in a transient compartment of the retrobulbar ventricle between E12 and E14, and we propose the hypothesis that they invade the marginal zone, through tangential subpial migration, at different moments of fetal life. SGL neurons contain calbindin, calretinin, and gamma-aminobutyric acid (GABA), but the GABA-immunoreactive group becomes inconspicuous before birth. The extracellular matrix-like glycoprotein reelin, a molecule crucial for cortical lamination, is prenatally expressed by SGL neurons; postnatally, it is present in both Cajal-Retzius cells and subpial pyriform cells, both derivatives of SGL cells. In the rat, Cajal-Retzius cells are horizontal neurons that remain only until the end of the first postnatal week. They are located in layer I at a critical distance of approximately 20 μm from the pial surface and express reelin and, only occasionally, calretinin. Subpial pyriform cells coexpress reelin and calretinin and remain in layer I longer than Cajal-Retzius cells. Both pioneer neurons and subpial granule neurons are specific to the cortex. They mark the limit between the rudimentary cerebral cortex and olfactory bulb in the rat during early corticogenesis. J. Comp. Neurol. 397:493–518, 1998. © 1998 Wiley-Liss, Inc.     

Aberrant trajectory of thalamocortical axons associated with abnormal localization of neurocan immunoreactivity in the cerebral neocortex of reeler mutant mice

We examined the molecular mechanisms underlying the formation of the thalamocortical pathway in the cerebral neocortex of normal and reeler mutant mice. During normal development of the mouse neocortex, thalamic axons immunoreactive for the neural cell adhesion molecule L1 rarely invaded the cortical plate and ran centered in the subplate which is immunoreactive for neurocan, a brain-specific chondroitin sulfate proteoglycan. On the other hand, in homozygous reeler mutant mice, thalamic axons took an aberrant course to run obliquely through the cortical plate. Injection of bromodeoxyuridine at embryonic day 11 specifically labeled subplate neurons in normal mice, whilst in the reeler neocortex it labeled cells scattered in the cortical plate as well as in the superficial layer (superplate). Neurocan immunoreactivity was associated with the bromodeoxyuridine-positive cells in the superplate, as well as being present in oblique bands within the cortical plate, along which L1-bearing thalamic axons preferentially ran. The present results support our previous hypothesis proposed for normal rats that a heterophilic molecular interaction between L1 and neurocan is involved in determining the thalamocortical pathway within the neocortical anlage [T. Fukuda et al. (1997) Journal of Comparative Neurology, 382, 141-152].     

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.     

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.     

Prenatal development of neurons in the human prefrontal cortex: I. A qualitative Golgi study

Golgi-Stensaas and rapid-Golgi staining techniques are used to study neuronal differentiation in the developing human prefrontal cortex in fetuses, premature infants, and full-term newborns from 10.5 to 40 weeks of gestation. Horizontal neurons (Cajal-Retzius neurons) above the cortical plate (in the marginal zone) and randomly oriented neurons below the cortical plate (in the primordial subplate) are more differentiated than the immature bipolar cortical plate neurons in the 10.5-week fetus. During 13.5-15 weeks of gestation the fetal subplate zone can be clearly distinguished-between the cortical plate and the intermediate zone. This subplate zone contains more mature neurons than the cortical plate, especially polymorphous neurons. The basic features of the apical and basal dendrites of pyramidal neurons develop between 17 and 25 weeks of gestation, before the thalamocortical fibres invade the cortical plate. Intensive differentiation of the subplate neurons occurs in this period, when various types of afferent fibres reside in the subplate zone. At least five neuronal types can be distinguished in the subplate, i.e., polymorphous, fusiform, multipolar, normal, and inverted pyramidal neurons. The ingrowth of afferent fibres into the cortical plate between 26 and 34 weeks of gestation coincides with intensive dendritic differentiation and the appearance of spines on dendrites of the prospective layer III and V pyramidal neurons as well as with the differentiation of the double bouquet interneurons in the prospective supragranular layers and layer IV. Multipolar nonpyramidal neurons with the dendritic features of basket neurons are observed between 32 and 34 weeks of gestation in future layer V. They are less differentiated than the double bouquet neurons. The neurons of the subplate zone continue their dendritic differentiation after 26/27 weeks of gestation and are still observed in the full-term newborn. The axonal pattern of the subplate neurons suggests a possible functional role for them as either interneurons or projection neurons.     

Studies of the earliest generated cells of the cat's visual cortex: cogeneration of subplate and marginal zones

The earliest generated cells of the cat's telencephalon that may play a role in the formation of the primary visual cortex are the subject of this study. Using [3H]thymidine autoradiography, we have found that these cells are generated between embryonic day 24 (E24) and E30 (gestation is 65 days) and that they are present in very low numbers in the white matter of the adult brain. These cells are rarely labeled by injections made after E30, when the cells destined for the cortical layers are generated. Examination of the labeling pattern in the fetal brain 10 days or more after administration of [3H]thymidine between E24 and E30 revealed a bistratified distribution of these early generated cells. Labeled cells were found in large numbers in two embryonic zones flanking the developing cortical plate: above in the marginal zone and below in the subplate. (Some if not all of the marginal zone cells constitute the population of Cajal-Retzius cells of the cat's telencephalon.). These experiments indicate that cells of the subplate and marginal zones are cogenerated in time during the days just preceding the genesis of the cortical plate. We also examined the distribution of the early generated cells shortly after their genesis--on E30, a time when cells of the cortical plate are just being generated at the ventricular zone. In this case, the labeling pattern at the occipital pole was not bistratified. Rather, labeled cells were situated within a single zone extending from the pial surface inward to the border of the ventricular zone. This finding indicates that the cells of the subplate and marginal zones are generated as a contiguous population that is subsequently split apart by the insertion of cells forming the cortical plate. A comparison between the number of early generated cells found in fetal and newborn brains with that found in adult brains suggests that these cells are generated initially in substantial numbers but then largely disappear during early postnatal life, since injections of [3H]thymidine between E24 and E30 yielded large numbers of labeled cells in the white matter and layer 1 at birth, but very few at 2 months postnatal. This significant loss contrasted with the results from injections made just a few days later (E33) that resulted in large numbers of labeled cells in cortical layer 6 not only at birth but also in adulthood.(ABSTRACT TRUNCATED AT 400 WORDS)     

The functions of the preplate in development and evolution of the neocortex and hippocampus

Recently, it has been shown that the early developmental organization of the archicortical hippocampus resembles that of the neocortex. In both cortices at embryonic stages, a preplate is present, which is split by the formation of the cortical plate into a marginal zone and a subplate layer. The pioneer neurons of the preplate are believed to form a phylogenetically ancient cortical structure. Neurons in these preplate layers are the first postmitotic neurons and have important roles in the development of the cerebral cortex. Cajal–Retzius cells in the marginal zone regulate the phenotype of radial glial cells and may direct neuronal migration establishing the inside-out gradient of corticogenesis. Furthermore, pioneer neurons form the initial axonal connections with other (sub)cortical structures. A significant difference between the hippocampus and neocortex, however, is that in the hippocampus, most afferents are guided by the pioneer neurons in the prominent marginal zone, while in the neocortex most ingrowing afferent axons enter via the subplate. At later developmental periods, most pioneer neurons disappear by cell death or transform into other neuronal shapes. Here, we review the early developmental organization of the mammalian cerebral cortex (both neocortex and hippocampus) and discuss the functions and fate of pioneer neurons in cortical development, in particular that of Cajal–Retzius cells. Evaluating the developmental properties of the hippocampus and neocortex, we present the hypothesis that the distribution of the main ingrowing afferent systems in the developing neocortex, which differs from the one in the hippocampal region, may have enabled the specific evolution of the neocortex.