Development of myelin in fetal and postnatal neocortex of the pig, the European wild boar Sus scrofa.

Myelination of the neocortex of altricial species is mostly a postnatal event, and the appearance of myelin has been associated with the end of the critical period for ocular dominance plasticity in rodent visual cortex. Due to their precocality, ungulates may tell a different story. Here, we analyzed the development of PDGFRα positive oligodendrocyte precursor cells and expression of myelin proteins in the laminar compartments of fetal and postnatal porcine cortex from E45 onwards. Precursor cell density initially increased and then decreased but remained present at P90. MAG and MBP staining were detectable at E70 in subventricular zone and deep white matter, ascending into gyral white matter at E85, and into the gray matter and marginal zone at E100 (birth in pig at E114). Protein blots confirmed the declining expression of PDGFRα from E65 onwards, and the increase of MBP and MAG expression from E80 onwards. Somatosensory input elicited by spontaneous activity is considered important for the formation of the body representation. Indeed, PDGFRα, MBP and MAG expression started earlier in somatosensory than in visual cortex. Taken together, myelination proceeded in white and gray matter and marginal zone of pig cortex before birth with an areal-specific time course, and an almost mature pattern was present at P5 in visual cortex.

Neocortical pyramidal neurons with axons emerging from dendrites are frequent in non-primates, but rare in monkey and human.

The canonical view of neuronal function is that inputs are received by dendrites and somata, become integrated in the somatodendritic compartment and upon reaching a sufficient threshold, generate axonal output with axons emerging from the cell body. The latter is not necessarily the case. Instead, axons may originate from dendrites. The terms 'axon carrying dendrite' (AcD) and 'AcD neurons' have been coined to describe this feature. In rodent hippocampus, AcD cells are shown to be functionally 'privileged', since inputs here can circumvent somatic integration and lead to immediate action potential initiation in the axon. Here, we report on the diversity of axon origins in neocortical pyramidal cells of rodent, ungulate, carnivore, and primate. Detection methods were Thy-1-EGFP labeling in mouse, retrograde biocytin tracing in rat, cat, ferret, and macaque, SMI-32/ßIV-spectrin immunofluorescence in pig, cat, and macaque, and Golgi staining in macaque and human. We found that in non-primate mammals, 10-21% of pyramidal cells of layers II-VI had an AcD. In marked contrast, in macaque and human, this proportion was lower and was particularly low for supragranular neurons. A comparison of six cortical areas (being sensory, association, and limbic in nature) in three macaques yielded percentages of AcD cells which varied by a factor of 2 between the areas and between the individuals. Unexpectedly, pyramidal cells in the white matter of postnatal cat and aged human cortex exhibit AcDs to much higher percentages. In addition, interneurons assessed in developing cat and adult human cortex had AcDs at type-specific proportions and for some types at much higher percentages than pyramidal cells. Our findings expand the current knowledge regarding the distribution and proportion of AcD cells in neocortex of non-primate taxa, which strikingly differ from primates where these cells are mainly found in deeper layers and white matter.

Development of microglia in fetal and postnatal neocortex of the pig, the European wild boar (Sus scrofa).

Knowledge on cortical development is based mainly on rodents besides primates and carnivores, all being altricial. Here, we analyzed a precocial animal, the pig, looking at dorsoparietal cortex from E45 to P90. At E45, most ionized calcium-binding adapter molecule 1-positive (Iba1+) cells had a macrophage-like morphology and resided in meninges and choroid plexus. Only a few cells were scattered in the ventricular and subventricular zone (VZ and SVZ). At E60/E70, all laminar compartments displayed microglia cells at a low-to-moderate density, being highest in VZ and SVZ followed by intermediate zone/white matter (IZ/WM). The cortical plate and marginal zone displayed only a few Iba1+ cells. Cells were intensely labeled, but still had poorly arborized somata and many resembled ameboid, macrophage-like microglia. Concurrent with a massive increase in cortical volume, microglia cell density increased until E85, and further until E100/E110 (birth at E114) to densities that resemble those seen postnatally. A fraction of microglia colabeled with Ki67 suggesting proliferation in all laminar compartments. Cell-to-cell distance decreased substantially during this time, and the fraction of microglia to all nuclei and to neurons increases in the laminar compartments. Eventually, of all cortical DAPI+ nuclei 7-12% were Iba1+ microglia. From E70 onwards, more and more cells with ramified processes were present in MZ down to IZ/WM, showing, for instance, a close association with NeuN+, NPY+, and GAD65/67+ somata and axon initial segments. These results suggested that the development of microglia cell density and morphology proceeds rapidly from mid-gestation onwards reaching near-adult status already before birth.

Cajal-Retzius neurons are required for the development of the human hippocampal fissure.

Cajal-Retzius neurons (CRN) are the main source of Reelin in the marginal zone of the developing neocortex and hippocampus (HC). They also express the transcription factor p73 and are complemented by later-appearing GABAergic Reelin+ interneurons. The human dorsal HC forms at gestational week 10 (GW10), when it develops a rudimentary Ammonic plate and incipient dentate migration, although the dorsal hippocampal fissure (HF) remains shallow and contains few CRN. The dorsal HC transforms into the indusium griseum (IG), concurrently with the rostro-caudal appearance of the corpus callosum, by GW14-17. Dorsal and ventral HC merge at the site of the former caudal hem, which is located at the level of the future atrium of the lateral ventricle and closely connected with the choroid plexus. The ventral HC forms at GW11 in the temporal lobe. The ventral HF is wide open at GW14-16 and densely populated by large numbers of CRNs. These are in intimate contact with the meninges and meningeal blood vessels, suggesting signalling through diverse pathways. At GW17, the fissure deepens and begins to fuse, although it is still marked by p73/Reelin+ CRNs. The p73KO mouse illustrates the importance of p73 in CRN for HF formation. In the mutant, Tbr1/Reelin+ CRNs are born in the hem but do not leave it and subsequently disappear, so that the mutant cortex and HC lack CRN from the onset of corticogenesis. The HF is absent, which leads to profound architectonic alterations of the HC. To determine which p73 isoform is important for HF formation, isoform-specific TAp73- and DeltaNp73-deficient embryonic and early postnatal mice were examined. In both mutants, the number of CRNs was reduced, but each of their phenotypes was much milder than in the global p73KO mutant missing both isoforms. In the TAp73KO mice, the HF of the dorsal HC failed to form, but was present in the ventral HC. In the DeltaNp73KO mice, the HC had a mild patterning defect along with a shorter HF. Complex interactions between both isoforms in CRNs may contribute to their crucial activity in the developing brain.

Fast prenatal development of the NPY neuron system in the neocortex of the European wild boar, Sus scrofa.

Knowledge on cortical development is based mainly on small rodents besides primates and carnivores, all being altricial nestlings. Ungulates are precocial and born with nearly mature sensory and motor systems. Almost no information is available on ungulate brain development. Here, we analyzed European wild boar cortex development, focusing on the neuropeptide Y immunoreactive (NPY-ir) neuron system in dorsoparietal cortex from E35 to P30. Transient NPY-ir neuron types including archaic cells of the cortical plate and axonal loop cells of the subplate which appear by E60 concurrent with the establishment of the ungulate brain basic sulcal pattern. From E70, NPY-ir axons have an axon initial segment which elongates and shifts closer towards the axon's point of origin until P30. From E85 onwards (birth at E114), NPY-ir neurons in cortical layers form basket cell-like local and Martinotti cell-like ascending axonal projections. The mature NPY-ir pattern is recognizable at E110. Together, morphologies are conserved across species, but timing is not: in pig, the adult pattern largely forms prenatally.

The Subpial Granular Layer and Transient Versus Persisting Cajal-Retzius Neurons of the Fetal Human Cortex.

Neurons of the subpial granular layer (SGL) in the human marginal zone (MZ) migrate tangentially from the periolfactory subventricular zone all over the neocortex. After an immature stage, from 14 to 18 gestational weeks (GW), the SGL attains maximum prominence around midgestation. At 20-25 GW, a transient miniature cell type in the MZ expresses glutamate decarboxylase (GAD) and calretinin, and extends a varicose plexus surrounding somata of large transient Cajal-Retzius neurons (tCRN), potentially modulating their activity. The compact Reelin+ horizontal axon plexus of tCRN forms a transient interface between cortical plate and MZ; it may serve as a migration substrate for cortical interneurons, and attracting NPY+ fibers from the subplate. Around 30 GW, after the disappearance of SGL and tCRN, a population of persisting Cajal-Retzius neurons (pCRN) appears and remains into adult life. pCRNs express Reelin, Tbr1, calretinin, nitric oxide synthase, and the cytokine receptor CXCR4. They are characterized by subpial location, closeness to blood vessels, and aggregation in the walls of developing sulci. Unlike tCRNs, pCRNs do not develop a compact axon plexus in the lower MZ. Occasional mitoses in the midgestation SGL suggest that CRN progenitor cells may give rise to late-appearing pCRNs populating the definitive molecular layer.

The heterogeneity of human Cajal-Retzius neurons.

The definition of a Cajal-Retzius neuron (CRN) is still controversial, in part possibly due to species differences. We review the developmental history of CRN in human neocortex and focus on two main CRN family members, transient (t) and persisting (p) CRN. They share the expression of Reelin andTbr1, complemented by p73, calretinin, CXCR4 and NOS, but differ in their moment of appearance, fate and morphology. The distinctive feature of tCRN is the axon plexus in the lower third of the marginal zone, which innervates the apical dendritic tufts of pyramidal cells and may serve as a migration substrate and waiting compartment for interneurons descending from the subpial granular layer (SGL) into the cortical plate. Around midgestation, the SGL also gives rise to a transient interneuron type, the miniature neuron, that provides the GABAergic innervation of tCRN, which eventually, through diverse signaling pathways involving p73, contribute to the demise of tCRN and the breakdown of their plexus. The pCRN appear in the last trimester of gestation and may derive from committed CRN progenitors which migrate with the SGL from the periolfactory forebrain. They lack the horizontal CR plexus, and may be implicated in cortical folding, distribution of blood vessels, and plasticity of microcircuits in the molecular layer.

A Radial Glia Fascicle Leads Principal Neurons from the Pallial-Subpallial Boundary into the Developing Human Insula.

The human insular lobe, in the depth of the Sylvian fissure, displays three main cytoarchitectonic divisions defined by the differentiation of granular layers II and IV. These comprise a rostro-ventral agranular area, an intermediate dysgranular area, and a dorso-caudal granular area. Immunohistochemistry in human embryos and fetuses using antibodies against PCNA, Vimentin, Nestin, Tbr1, and Tb2 reveals that the insular cortex is unique in that it develops far away from the ventricular zone (VZ), with most of its principal neurons deriving from the subventricular zone (SVZ) of the pallial-subpallial boundary (PSB). In human embryos (Carnegie stage 16/17), the rostro-ventral insula is the first cortical region to develop; its Tbr1+ neurons migrate from the PSB along the lateral cortical stream. From 10 gestational weeks (GW) onward, lateral ventricle, ganglionic eminences, and PSB grow forming a C-shaped curvature. The SVZ of the PSB gives rise to a distinct radial glia fiber fascicle (RGF), which courses lateral to the putamen in the external capsule. In the RGF, four components can be established: PF, descending from the prefrontal PSB to the anterior insula; FP, descending from the fronto-parietal PSB toward the intermediate insula; PT, coursing from the PSB near the parieto-temporal junction to the posterior insula, and T, ascending from the temporal PSB and merging with components FP and PT. The RGF fans out at different dorso-ventral and rostro-caudal levels of the insula, with descending fibers predominating over ascending ones. The RGF guides migrating principal neurons toward the future agranular, dysgranular, and granular insular areas, which show an adult-like definition at 32 GW. Despite the narrow subplate, and the absence of an intermediate zone except in the caudal insula, most insular subdivisions develop into a 6-layered isocortex, possibly due to the well developed outer SVZ at the PSB, which is particularly prominent at the level of the dorso-caudal insula. The small size of the initial PSB sector may, however, determine the limited surface expansion of the insula, which is in contrast to the exuberant growth of the opercula deriving from the adjacent frontal-parietal and temporal VZ/SVZ.

A Distal to Proximal Gradient of Human Choroid Plexus Development, with Antagonistic Expression of Glut1 and AQP1 in Mature Cells vs. Calbindin and PCNA in Proliferative Cells.

The choroid plexuses (ChP) are highly vascularized tissues suspended from each of the cerebral ventricles. Their main function is to secret cerebrospinal fluid (CSF) that fills the ventricles and the subarachnoid spaces, forming a crucial system for the development and maintenance of the CNS. However, despite the essential role of the ChP-CSF system to regulate the CNS in a global manner, it still remains one of the most understudied areas in neurobiology. Here we define by immunohistochemistry the expression of different proteins involved in the maturation and functionality of the ChP from the late embryological period to maturity. We found an opposite gradient of expression between aquaporin 1 (AQP1) and glucose transporter 1 (Glut 1) that define functional maturation in the ChP periphery, and proliferating cell nuclear antigen (PCNA) and calbindin (CB), present in the ChP root zone with proliferative activity. We conclude that the maturation of the ChP matures from distal to proximal, starting in the areas nearest to the cortex, expressing in the distal, mature areas AQP1 and Glut1 (related to ChP functionality to support cortex development), and in the proximal immature areas (ChP root) CB and PCNA related to progenitor activity and proliferation.

Dynamic expression of calretinin in embryonic and early fetal human cortex.

Calretinin (CR) is one of the earliest neurochemical markers in human corticogenesis. In embryos from Carnegie stages (CS) 17 to 23, calbindin (CB) and CR stain opposite poles of the incipient cortex suggesting early regionalization: CB marks the neuroepithelium of the medial boundary of the cortex with the choroid plexus (cortical hem). By contrast, CR is confined to the subventricular zone (SVZ) of the lateral and caudal ganglionic eminences at the pallial-subpallial boundary (PSB, or antihem), from where CR+/Tbr1- neurons migrate toward piriform cortex and amygdala as a component of the lateral cortical stream. At CS 19, columns of CR+ cells arise in the rostral cortex, and contribute at CS 20 to the "monolayer" of horizontal Tbr1+/CR+ and GAD+ cells in the preplate. At CS 21, the "pioneer cortical plate" appears as a radial aggregation of CR+/Tbr1+ neurons, which cover the entire future neocortex and extend the first corticofugal axons. CR expression in early human corticogenesis is thus not restricted to interneurons, but is also present in the first excitatory projection neurons of the cortex. At CS 21/22, the cortical plate is established following a lateral to medial gradient, when Tbr1+/CR- neurons settle within the pioneer cortical plate, and thus separate superficial and deep pioneer neurons. CR+ pioneer neurons disappear shortly after the formation of the cortical plate. Reelin+ Cajal-Retzius cells begin to express CR around CS21 (7/8 PCW). At CS 21-23, the CR+ SVZ at the PSB is the source of CR+ interneurons migrating into the cortical SVZ. In turn, CB+ interneurons migrate from the subpallium into the intermediate zone following the fibers of the internal capsule. Early CR+ and CB+ interneurons thus have different origins and migratory routes. CR+ cell populations in the embryonic telencephalon take part in a complex sequence of events not analyzed so far in other mammalian species, which may represent a distinctive trait of the initial steps of human corticogenesis.

Cortical hypoplasia and ventriculomegaly of p73-deficient mice: Developmental and adult analysis.

Trp73, a member of the p53 gene family, plays a crucial role in neural development. We describe two main phenotypic variants of p73 deficiency in the brain, a severe one characterized by massive apoptosis in the cortex leading to early postnatal death and a milder, non-/low-apoptosis one in which 50% of pups may reach adulthood using an intensive-care breeding protocol. Both variants display the core triad of p73 deficiency: cortical hypoplasia, hippocampal malformations, and ventriculomegaly. We studied the development of the neocortex in p73 KO mice from early embryonic life into advanced age (25 months). Already at E14.5, the incipient cortical plate of the p73 KO brains showed a reduced width. Examination of adult neocortex revealed a generalized, nonprogressive reduction by 10-20%. Area-specific architectonic landmarks and lamination were preserved in all cortical areas. The surviving adult animals had moderate ventricular distension, whereas pups of the early lethal phenotypic variant showed severe ventriculomegaly. Ependymal cells of wild-type ventricles strongly express p73 and are particularly vulnerable to p73 deficiency. Ependymal denudation by apoptosis and reduction of ependymal cilia were already evident in young mice, with complete absence of cilia in older animals. Loss of p73 function in the ependyma may thus be one determining factor for chronic hydrocephalus, which leads to atrophy of subcortical structures (striatum, septum, amygdala). p73 Is thus involved in a variety of CNS activities ranging from embryonic regulation of brain size to the control of cerebrospinal fluid homeostasis in the adult brain via maintenance of the ependyma.

Lhx2 regulates the development of the forebrain hem system.

Early brain development is regulated by the coordinated actions of multiple signaling centers at key boundaries between compartments. Three telencephalic midline structures are in a position to play such roles in forebrain patterning: The cortical hem, the septum, and the thalamic eminence at the diencephalic-telencephalic boundary. These structures express unique complements of signaling molecules, and they also produce distinct populations of Cajal-Retzius cells, which are thought to act as "mobile patterning units," migrating tangentially to cover the telencephalic surface. We show that these 3 structures require the transcription factor Lhx2 to delimit their extent. In the absence of Lhx2 function, all 3 structures are greatly expanded, and the Cajal-Retzius cell population is dramatically increased. We propose that the hem, septum, and thalamic eminence together form a "forebrain hem system" that defines and regulates the formation of the telencephalic midline. Disruptions in the forebrain hem system may be implicated in severe brain malformations such as holoprosencephaly. Lhx2 functions as a central regulator of this system's development. Since all components of the forebrain hem system have been identified across several vertebrate species, the mechanisms that regulate them may have played a fundamental role in driving key aspects of forebrain evolution.

LHX2 is necessary for the maintenance of optic identity and for the progression of optic morphogenesis.

Eye formation is regulated by a complex network of eye field transcription factors (EFTFs), including LIM-homeodomain gene LHX2. We disrupted LHX2 function at different stages during this process using a conditional knock-out strategy in mice. We find that LHX2 function is required in an ongoing fashion to maintain optic identity across multiple stages, from the formation of the optic vesicle to the differentiation of the neuroretina. At each stage, loss of Lhx2 led to upregulation of a set of molecular markers that are normally expressed in the thalamic eminence and in the anterodorsal hypothalamus in a portion of the optic vesicle or retina. Furthermore, the longer LHX2 function was maintained, the further optic morphogenesis progressed. Early loss of function caused profound mispatterning of the entire telencephalic-optic-hypothalamic field, such that the optic vesicle became mispositioned and appeared to arise from the diencephalic-telencephalic boundary. At subsequent stages, loss of Lhx2 did not affect optic vesicle position but caused arrest of optic cup formation. If Lhx2 was selectively disrupted in the neuroretina from E11.5, the neuroretina showed gross dysmorphology along with aberrant expression of markers specific to the thalamic eminence and anterodorsal hypothalamus. Our findings indicate a continual requirement for LHX2 throughout the early stages of optic development, not only to maintain optic identity by suppressing alternative fates but also to mediate multiple steps of optic morphogenesis. These findings provide new insight into the anophthalmic phenotype of the Lhx2 mutant and reveal novel roles for this transcription factor in eye development.

Dual origins of the mammalian accessory olfactory bulb revealed by an evolutionarily conserved migratory stream.

The accessory olfactory bulb (AOB) is a critical olfactory structure that has been implicated in mediating social behavior. It receives input from the vomeronasal organ and projects to targets in the amygdaloid complex. Its anterior and posterior components (aAOB and pAOB) display molecular, connectional and functional segregation in processing reproductive and defensive and aggressive behaviors, respectively. We observed a dichotomy in the development of the projection neurons of the aAOB and pAOB in mice. We found that they had distinct sites of origin and that different regulatory molecules were required for their specification and migration. aAOB neurons arose locally in the rostral telencephalon, similar to main olfactory bulb neurons. In contrast, pAOB neurons arose caudally, from the neuroepithelium of the diencephalic-telencephalic boundary, from which they migrated rostrally to reach their destination. This unusual origin and migration is conserved in Xenopus, providing an insight into the origin of a key component of this system in evolution.

Dynamic expression of the p53 family members p63 and p73 in the mouse and human telencephalon during development and in adulthood.

p63 and p73, family members of the tumor suppressor p53, are critically involved in the life and death of mammalian cells. They display high homology and may act in concert. The p73 gene is relevant for brain development, and p73-deficient mice display important malformations of the telencephalon. In turn, p63 is essential for the development of stratified epithelia and may also play a part in neuronal survival and aging. We show here that p63 and p73 are dynamically expressed in the embryonic and adult mouse and human telencephalon. During embryonic stages, Cajal-Retzius cells derived from the cortical hem co-express p73 and p63. Comparison of the brain phenotypes of p63- and p73- deficient mice shows that only the loss of p73 function leads to the loss of Cajal-Retzius cells, whereas p63 is apparently not essential for brain development and Cajal-Retzius cell formation. In postnatal mice, p53, p63, and p73 are present in cells of the subventricular zone (SVZ) of the lateral ventricle, a site of continued neurogenesis. The neurogenetic niche is reduced in size in p73-deficient mice, and the numbers of young neurons near the ventricular wall, marked with doublecortin, Tbr1 and calretinin, are dramatically decreased, suggesting that p73 is important for SVZ proliferation. In contrast to their restricted expression during brain development, p73 and p63 are widely detected in pyramidal neurons of the adult human cortex and hippocampus at protein and mRNA levels, pointing to a role of both genes in neuronal maintenance in adulthood.

Building a human cortex: the evolutionary differentiation of Cajal-Retzius cells and the cortical hem.

Cajal-Retzius (CR) cells are the most significant source of reelin, an extracellular matrix glycoprotein essential for cortical development. Strategically located in the marginal zone, CR cells control radial migration and laminar positioning of pyramidal neurons of the cortical plate. They degenerate and undergo cell death when cortical migration is completed. In human cortex development, reelin-expressing CR cells are already present in the early preplate, and continue to increase in number after the appearance of the cortical plate. In the course of the first half of gestation, the reelin signal in the marginal zone undergoes a huge amplification in parallel with the growth of the cortical plate and the expansion of the cortical surface. A significant source of CR cells is the cortical hem, a putative signalling centre at the interface of the prospective hippocampus and the choroid plexus. Hem-derived CR cells co-express reelin and p73, a transcription factor of the p53-family. They form the predominant CR cell population of the human neocortex. Characteristically, CR cells express the anti-apoptotic isoform DeltaNp73 which may be responsible for the protracted lifespan of human CR cells and the morphological differentiation of their axonal plexus. This dense fibre plexus, absent in lower mammals, amplifies the reelin-signal and establishes a physical boundary between the cortical plate and the marginal zone. In this review, we analyze the multiple sources of reelin/p73 positive CR cells at the interface of various telencephalic centres and the choroid plexus of the lateral ventricles. Additional populations of CR cells may derive from the thalamic eminence in the ventral thalamus and from the strionuclear neuroepithelium, or 'amygdalar hem'. Comparative studies in a variety of species indicate that the cortical hem is the main origin of CR cells destined for the neocortex, and is most highly developed in the human brain. The close association between cortical hem and choroid plexus suggests a concerted role in the evolutionary increase of CR cells, amplification of the reelin signal in the marginal zone, and cortical expansion.

Immunohistochemical study of Pax6 and Reissner fibre expression in the prenatal development of the mouse subcommissural organ

The subcommissural organ (SCO) releases glycoproteins into the ventricular cerebrospinal fluid (CSF), where they form Reissner's fibre (RF) and also secretes a CSF-soluble material different from RF-material. Pax6 is a transcription factor important for the regulation of cell proliferation, migration and differentiation in the developing brain. In the present work, we studied wild-type, heterozygous and homozygous Sey mice to compare the expression of RF-antibody and Pax6 in the SCO and adjacent structures. In wild-type mice between E15 to E18, we observed Pax6 expression in cells surrounding the secretory cells of the SCO, and RF-immunoreactive material only in the SCO ependymal cell layer and its basal process. In the heterozygous mice, the neuroanatomical structure of the SCO was present, but RF-antibody staining and Pax6 expression was scarce or almost undetectable; in the homozygous mice neither SCO nor other epithalamic structures were found. We suggest that Pax6 expression at the periphery of the SCO is essential for the development and activity of the organ (AU)

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DeltaNp73 regulates neuronal survival in vivo.

Apoptosis occurs widely during brain development, and p73 transcription factors are thought to play essential roles in this process. The p73 transcription factors are present in two forms, the full length TAp73 and the N-terminally truncated DeltaNp73. In cultured sympathetic neurons, overexpression of DeltaNp73 inhibits apoptosis induced by nerve growth factor withdrawal or p53 overexpression. To probe the function of DeltaNp73 in vivo, we generated a null allele and inserted sequences encoding the recombinase Cre and green fluorescent protein (EGFP). We show that DeltaNp73 is heavily expressed in the thalamic eminence (TE) that contributes neurons to ventral forebrain, in vomeronasal neurons, Cajal-Retzius cells (CRc), and choroid plexuses. In DeltaNp73(-/-) mice, cells in preoptic areas, vomeronasal neurons, GnRH-positive cells, and CRc were severely reduced in number, and choroid plexuses were atrophic. This phenotype was enhanced when DeltaNp73-positive cells were ablated by diphtheria toxin expression. However, ablation of cells that express DeltaNp73 and Wnt3a did neither remove all CRc, nor did they abolish Reelin secretion or generate a reeler-like cortical phenotype. Our data emphasize the role of DeltaNp73 in neuronal survival in vivo and in choroid plexus development, the importance of the TE as a source of neurons in ventral forebrain, and the multiple origins of CRc, with redundant production of Reelin.