Massive loss of Cajal-Retzius cells does not disrupt neocortical layer order

Perlecan controls neurogenesis in the developing telencephalon

Background

Perlecan is a proteoglycan expressed in the basal lamina of the neuroepithelium during development. Perlecan absence does not impair basal lamina assembly, although in the 55% of the mutants early disruptions of this lamina conducts to exencephaly, impairing brain development. The rest of perlecan-null brains complete its prenatal development, maintain basal lamina continuity interrupted by some isolated ectopias, and are microcephalic. Microcephaly consists of thinner cerebral walls and underdeveloped ganglionic eminences. We have studied the mechanisms that generate brain atrophy in telencephalic areas where basal lamina is intact.

Results

Brain atrophy in the absence of perlecan started in the ventral forebrain and extended to lateral and dorsal parts of the cortex in the following stages. First, the subpallial forebrain developed poorly in early perlecan-null embryos, because of a reduced cell proliferation: the number of cells in mitosis decreased since the early stages of development. This reduction resulted in a decreased tangential migration of interneurons to the cerebral cortex. Concomitant with the early hypoplasia observed in the medial ganglionic eminences, Sonic Hedgehog signal decreased in the perlecan-null floor plate basal lamina at E12.5. Second, neurogenesis in the pallial neuroepithelium was affected in perlecan deficient embryos. We found reductions of nearly 50% in the number of cells exiting the cell cycle at E12–E13. The labeling index, which was normal at this age, significantly decreased with advancing corticogenesis. Moreover, nestin⁺ or PCNA⁺ progenitors increased since E14.5, reaching up to about 150% of the proportion of PCNA⁺ cells in the wild-type at E17.5. Thus, labeling index reduction together with increased progenitor population, suggests that atrophy is the result of altered cell cycle progression in the cortical progenitors. Accordingly, less neurons populated the cortical plate and subplate of perlecan-null neocortex, as seen with the neuronal markers β-tubulin and Tbr1.

Conclusion

As a component of the basal lamina, perlecan both maintains this structure and controls the response of the neuroepithelium to growth factors. Less mitotic cells in the early medial ganglionic eminences, and impaired cell cycle progression in the late neocortex, suggests insufficient recruitment and signaling by neurogenic morphogens, such as SHH or FGF2.

The tracing study of developing entorhino-hippocampal pathway

The entorhino-hippocampal pathway is the major excitatory input from neurons of the entorhinal cortex on both ipsilateral and contralateral hippocampus/dentate gyrus. This fiber tract consists of the alvear path, the perforant path and a crossed commissural projection. In this study, the histogenesis and development of the various subsets of the entorhino-hippocampal projection have been investigated. DiI, DiO, Fast Blue tracing and calretinin immunocytochemistry as well as were carried out with pre and postnatal rats at different developmental stages. The alvear path and the commissural pathway start to develop as early as embryonic day E16, while the first perforant afferents reach the stratum lacunosum-moleculare of the hippocampus at E17 and at outer molecular layer of the denate gyrus at postnatal day 2. Retrograde tracing with DiI identifies entorhinal neurons in layer II-IV as the developmental origin of the entorhino-hippocampal pathway. Furthermore, calretinin immunocytochemistry revealed transitory Cajal-Retzius cells in the stratum lacunosum-moleculare of the hippocampus from E16. DiI labeling of entorhinal cortex fibers and combined calretinin-immunocytochemistry reveal a close relationship between Cajal-Retzius cells and entorhinal afferents. This temporal and spatial relationship suggests that Cajal-Retzius cell serves as a guiding cue for entorhinal afferents at early cortical development.

Origins and migratory routes of murine Cajal-Retzius cells

The first layer that appears in the cortical neuroepithelium, the preplate, forms in the upper part of the cortex immediately below the pial surface. In mice, this layer exists between embryonic days (E) 10 and 13, and it hosts different cell populations. Here, we have studied the first cell population generated in the preplate, the Cajal-Retzius cells. There is considerable confusion regarding these cells with respect to both their site of generation and the migratory routes that they follow. This perhaps is due largely to the different opinions that exist regarding their characterization. We have studied the site of origin of these cells, their migratory routes, and the molecular markers that may distinguish them by injecting tracers into early embryos, culturing them in toto for 24 hours, and then performing immunohistochemistry. We found that the Cajal-Retzius cells are most likely generated in the cortical hem by comparing with other cortical or extracortical origins. These cells are generated mainly at E10 and E11, and they subsequently migrate tangentially to cover the whole cortical mantle in 24 hours. From their site of origin in the medial wall of the telencephalon, they spread in a caudorostral direction, following an oblique migratory path toward the lateral part of the neuroepithelium. Prior to the splitting of the preplate, a percentage of the Cajal-Retzius cells that can be distinguished by the expression of reelin do not contain calretinin. Furthermore, there were no early-migrating neurons that expressed calbindin.

Comparative aspects of p73 and Reelin expression in Cajal-Retzius cells and the cortical hem in lizard, mouse and human

Cajal-Retzius (CR) cells of the mammalian neocortex co-express the extracellular matrix protein Reelin and p73, a transcription factor involved in cell death and survival. Most neocortical CR cells derive from the cortical hem, with minor additional sources. We analyzed the distribution of Reelin and p73 immunoreactive (ir) neurons in the telencephalon of Lacerta galloti from early embryonic stages to hatching. Numerous Reelin-ir cells appeared in the pallial MZ from the preplate stage onward. Conversely, p73-ir cells were rare in the pallial preplate and not observed in the cortical plate. Subpallial p73-ir cells spread from the septum and the telencephalic-diencephalic boundary to the pial surface of the basal forebrain and amygdala, respectively, where they co-expressed Reelin and p73. A small group of Reelin/p73-ir CR cells appeared in a rudimentary cortical hem at the interface of the medial cortex and choroid plexus. Comparison with early embryonic stages of mice and humans showed similar foci of p73-ir cells in the septum and at the telencephalic-diencephalic boundary and revealed an increasing prominence of the cortical hem, in parallel with increasing numbers of neocortical Reelin/p73 positive CR cells, which attain highest differentiation in the human brain. Our data show that Reelin-expression in the pallium is evolutionarily conserved and independent of a cortical hem, and suggest that p73 in the cortical hem may be involved in the evolutionary increase in number and complexity of the mammalian neocortical CR cells.

A Comparison Study of the Vertical Bias of Pyramidal Cells in the Hippocampus and Neocortex

In this study, we employed morphometric image analysis of the hippocampus proper and temporal lobe neocortex in postmortem tissue to determine vertical bias quantified as Deltatheta, angular dispersion, as well asan index of alignment of cellular elements relative to the radial plane. The radial alignment of cellular elements was consistent with a minicolumnar organization of the cortex. Photomicrographs were taken of the left-hemisphere hippocampal CA3/1 subfields of 13 fetal subjects ranging in gestational age from 19 weeks to 36 weeks and 19 normal individuals aged 4 months to 98 years. For comparison, micrographs from the temporal lobe (Brodmann areas 21 and 22) were similarly processed for layers III and V, where the x-axes of the transformed coordinate system were taken to be the layer III/IV and IV/V borders, respectively. Computerized image analysis measurements of the angular dispersion for the temporal lobe region and hippocampus proper differed significantly within the same brains (p < 0.001). The neocortical layer III exhibited the highest values for Deltatheta, indicating a high degree of columnar organization. Values for Deltatheta in the hippocampal CA subfields were lower but demonstrated significance for the radial alignment of neurons in this area. Values for Deltathetain layer V were intermediate between those of layer III and the hippocampus, consistent with increasing degrees of radial columnar organization of infragranular layers of the neocortex in comparison with the hippocampus and of supragranular in comparison with infragranular neocortical layers. Pyramidal cell arrays within allocortical areas and the neocortex constitute different modular arrangements. This morphological variability may be the expression of evolutionary differences in cortical development.

Karl Pribram, The James Arthur lectures, and what makes us human

Background

The annual James Arthur lecture series on the Evolution of the Human Brain was inaugurated at the American Museum of Natural History in 1932, through a bequest from a successful manufacturer with a particular interest in mechanisms. Karl Pribram's thirty-ninth lecture of the series, delivered in 1970, was a seminal event that heralded much of the research agenda, since pursued by representatives of diverse disciplines, that touches on the evolution of human uniqueness.

Discussion

In his James Arthur lecture Pribram raised questions about the coding of information in the brain and about the complex association between language, symbol, and the unique human cognitive system. These questions are as pertinent today as in 1970. The emergence of modern human symbolic cognition is often viewed as a gradual, incremental process, governed by inexorable natural selection and propelled by the apparent advantages of increasing intelligence. However, there are numerous theoretical considerations that render such a scenario implausible, and an examination of the pattern of acquisition of behavioral and anatomical novelties in human evolution indicates that, throughout, major change was both sporadic and rare. What is more, modern bony anatomy and brain size were apparently both achieved well before we have any evidence for symbolic behavior patterns. This suggests that the biological substrate underlying the symbolic thought that is so distinctive of Homo sapiens today was exaptively achieved, long before its potential was actually put to use. In which case we need to look for the agent, perforce a cultural one, that stimulated the adoption of symbolic thought patterns. That stimulus may well have been the spontaneous invention of articulate language.

Stromal-derived factor-1 (CXCL12) regulates laminar position of Cajal-Retzius cells in normal and dysplastic brains

Normal brain development requires a series of highly complex and interrelated steps. This process presents many opportunities for errors to occur, which could result in developmental defects in the brain, clinically referred to as malformations of cortical development. The marginal zone and Cajal-Retzius cells are key players in cortical development and are established early, yet there is little understanding of the factors resulting in the disruption of the marginal zone in many types of cortical malformation syndromes. We showed previously that treatment with methylazoxymethanol in rats causes marginal zone dysplasia with displacement of Cajal-Retzius cells to deeper cortical layers. Here we establish that loss of activity of the chemokine stromal-derived factor-1 (SDF1) (CXCL12), which is expressed by the leptomeninges, is necessary and sufficient to cause marginal zone disorganization in this widely used teratogenic animal model. We also found that mice with mutations in the main receptor for SDF1 (CXCR4) have Cajal-Retzius cells displaced to deeper cortical layers. Furthermore, by inhibiting SDF1 signaling in utero by intraventricular injection of a receptor antagonist, we establish that SDF1 signaling is required for the maintenance of Cajal-Retzius cell position in the marginal zone during normal cortical development. Our data imply that cortical layering is not a static process, but rather requires input from locally produced molecular cues for maintenance, and that complex syndromes of cortical malformation as a result of environmental insults may still be amenable to explanation by interruption of specific molecular signaling pathways.

Emx2 in the developing hippocampal fissure region

Mice deficient in transcription factor gene Emx2 show developmental alterations in the hippocampal dentate gyrus. Emx2, however, is also expressed in the region around the developing hippocampal fissure. The developing fissure contains a radial glial scaffolding, and is surrounded by the outer marginal zone and the dentate marginal zone, which become specifically colonized by neurons and differentiate into stratum lacunosum-moleculare and molecular layer of the dentate, respectively. In this study we show that the Emx2 mutant lacks the glial scaffolding of the fissure and has an outer marginal zone (precursor of the stratum lacunosum-moleculare), as well as a dentate marginal zone severely reduced in size while most of the reelin (Reln)-expressing cells that should occupy it fail to be generated. We have also identified a subpopulation of hippocampal Reln-expressing cells of the marginal zone, probably born in the hem, expressing a specific combination of markers, and for which Emx2 is not essentially required. Additionally, we show differential mutant phenotypes of both Emx2 and Pax6 in neocortical vs. hippocampal Reln-expressing cells, indicating differential development of both subpopulations.

Meninges control tangential migration of hem-derived Cajal-Retzius cells via CXCL12/CXCR4 signaling

Cajal-Retzius cells are critical in the development of the cerebral cortex, but little is known about the mechanisms controlling their development. Three focal sources of Cajal-Retzius cells have been identified in mice-the cortical hem, the ventral pallium and the septum-from where they migrate tangentially to populate the cortical surface. Using a variety of tissue culture assays and in vivo manipulations, we demonstrate that the tangential migration of cortical hem-derived Cajal-Retzius cells is controlled by the meninges. We show that the meningeal membranes are a necessary and sufficient substrate for the tangential migration of Cajal-Retzius cells. We also show that the chemokine CXCL12 secreted by the meninges enhances the dispersion of Cajal-Retzius cells along the cortical surface, while retaining them within the marginal zone in a CXCR4-dependent manner. Thus, the meningeal membranes are fundamental in the development of Cajal-Retzius cells and, hence, in the normal development of the cerebral cortex.

Transcription factors in glutamatergic neurogenesis: conserved programs in neocortex, cerebellum, and adult hippocampus

Glutamatergic, pyramidal-projection neurons are produced in the embryonic cerebral cortex by a series of genetically programmed fate choices, implemented in large part by developmental transcription factors. Our work has focused on Pax6, Tbr2/Eomes, NeuroD, and Tbr1, which are expressed sequentially during the neurogenesis of pyramidal-projection neurons. Recently, we have found that the same transcription factors are expressed, in the same order, during glutamatergic neurogenesis in the adult dentate gyrus, and (with modifications) in the developing cerebellum. While the precise functional significance of this transcription factor expression sequence is unknown, its common appearance in embryonic and adult neurogenesis, and in different brain regions, suggests it is part of a conserved genetic program that specifies general properties of glutamatergic neurons in these regions. Subtypes of glutamatergic neurons (e.g., layer-specific fates in the cortex) are further determined by combinations of transcription factors, superimposed on general sequential programs. These new perspectives on neurogenesis add to the conceptual framework for strategies to engineer neural stem cells for the repair of specific brain circuits.