Gliogenesis and ependymogenesis during embryonic development of the rat. An autoradiographic study

Developmental and functional relationships between hypothalamic tanycytes and embryonic radial glia

The hypothalamus is a key regulator of several homeostatic processes, such as circadian rhythms, energy balance, thirst, and thermoregulation. Recently, the hypothalamic third ventricle has emerged as a site of postnatal neurogenesis and gliogenesis. This hypothalamic neural stem potential resides in a heterogeneous population of cells known as tanycytes, which, not unlike radial glia, line the floor and ventrolateral walls of the third ventricle and extend a long process into the hypothalamic parenchyma. Here, we will review historical and recent data regarding tanycyte biology across the lifespan, focusing on the developmental emergence of these diverse cells from embryonic radial glia and their eventual role contributing to a fascinating, but relatively poorly characterized, adult neural stem cell niche.

Ontogeny of ependymoglial cells lining the third ventricle in mice

Introduction

During hypothalamic development, the germinative neuroepithelium gives birth to diverse neural cells that regulate numerous physiological functions in adulthood.

Methods

Here, we studied the ontogeny of ependymal cells in the mouse mediobasal hypothalamus using the BrdU approach and publicly available single-cell RNAseq datasets.

Results

We observed that while typical ependymal cells are mainly produced at E13, tanycyte birth depends on time and subtypes and lasts up to P8. Typical ependymocytes and β tanycytes are the first to arise at the top and bottom of the dorsoventral axis around E13, whereas α tanycytes emerge later in development, generating an outside-in dorsoventral gradient along the third ventricle. Additionally, α tanycyte generation displayed a rostral-to-caudal pattern. Finally, tanycytes mature progressively until they reach transcriptional maturity between P4 and P14.

Discussion

Altogether, this data shows that ependyma generation differs in time and distribution, highlighting the heterogeneity of the third ventricle.

Planar Organization of Multiciliated Ependymal (E1) Cells in the Brain Ventricular Epithelium

Cerebrospinal fluid (CSF) continuously flows through the cerebral ventricles, a process essential for brain homeostasis. Multiciliated ependymal (E1) cells line the walls of the ventricles and contribute importantly to CSF flow through ciliary beating. Key to this function is the rotational and translational planar cell polarity (PCP) of E1 cells. Defects in the PCP of E1 cells can result in abnormal CSF accumulation and hydrocephalus. Here, we integrate recent data on the roles of early CSF flow in the embryonic ventricles, PCP regulators (e.g., Vangl2 and Dishevelled), and cytoskeletal networks in the establishment, refinement, and maintenance of E1 cells' PCP. The planar organization mechanisms of E1 cells could explain how CSF flow contributes to brain function and may help in the diagnosis and prevention of hydrocephalus.

Identification of proliferative progenitors associated with prominent postnatal growth of the pons

The pons controls crucial sensorimotor and autonomic functions. In humans, it grows sixfold postnatally and is a site of paediatric gliomas; however, the mechanisms of pontine growth remain poorly understood. We show that the murine pons quadruples in volume postnatally; growth is fastest during postnatal days 0–4 (P0–P4), preceding most myelination. We identify three postnatal proliferative compartments: ventricular, midline and parenchymal. We find no evidence of postnatal neurogenesis in the pons, but each progenitor compartment produces new astroglia and oligodendroglia; the latter expand 10- to 18-fold postnatally, and are derived mostly from the parenchyma. Nearly all parenchymal progenitors at P4 are Sox2⁺Olig2⁺, but by P8 a Sox2⁻ subpopulation emerges, suggesting a lineage progression from Sox2⁺ ‘early' to Sox2⁻ ‘late' oligodendrocyte progenitor. Fate mapping reveals that >90% of adult oligodendrocytes derive from P2–P3 Sox2⁺ progenitors. These results demonstrate the importance of postnatal Sox2⁺Olig2⁺ progenitors in pontine growth and oligodendrogenesis.

Postnatal growth of the pons is not well characterized. Here the authors show that growth of the murine pons is fastest during postnatal day 0–4, a period preceding myelination, and is primarily driven by an expansion of the oligodendrocyte population that derive from Sox2⁺Olig2⁺ progenitors.

Expression of transient receptor potential channels in the ependymal cells of the developing rat brain

Cerebrospinal fluid (CSF) plays an important role in providing brain tissue with a stable internal environment as well as in absorbing mechanical and thermal stresses. From its initial composition, derived from the amniotic fluid trapped by the closure of neuropores, CSF is modified by developing and differentiating ependymal cells lining the ventricular surface or forming the choroid plexus. Its osmolarity and ionic composition brings about a change through the action of many channels expressed on the ependymal cells. Some newly discovered transient receptor potential (TRP) channels are known to be expressed in the choroid plexus ependyma. To detect additional TRP channel expression, immunohistochemical screening was performed at the choroid plexus of 13-, 15-, 17-, and 19-day embryos, using antibodies against TRPV1, TRPV3, and TRPA1, and the expression was compared with those in the adult TRP channels. The level of TRP channel expression was higher in the choroid plexus which suggests more active functioning of TRP channels in the developing choroid plexus than the ventricular lining ependyma in the 15- and 17-day embryos. All the expression of TRP channels decreased at the 19th day of gestation. TRPA1 was expressed at a higher level than TRPV1 and TRPV3 in almost all stages in both the choroid plexus and ventricular lining epithelium. The highest level of TRPV1 and TRPV3 expression was observed in association with the glycogen deposits in the cytoplasm of the choroid plexus ependymal cells of the 15- and 17-day embryos.

Novel observations on the origin of ependymal cells in the ventricular zone of the rat spinal cord

Despite extensive investigations of gliogenesis, the time of origin of ependymal cells in the spinal cord has not yet been fully elucidated. Using a single dose of 5-bromo-2-deoxyuridine combined with various survival times we monitored: mitotic activity (short survival time), the presence of newly formed cells in the ventricular zone (intermediate survival time) and the formation of ependymal cells (long survival time) during the late embryonic and early postnatal development in the ventricular zone of the spinal cord of rats. In the period of study it was found that the ependymal cells populated this region in two waves. Most of the ependymal cells originated around embryonic day 18 and then between postnatal days 8 and 15. In addition, it was observed that in the ventricular zone of the spinal cord, proliferation and production of ependymal cells continues at the slower rate at least until day 36 of postnatal development. Elucidation of the relationship between progenitors in the embryonic ventricular zone and the relative quiescent ependymal lining of the central canal in adulthood could be important in the search for the adult neural stem cell niche.

Estimation of nuclear volume as an indicator of maturation of glial precursor cells in the developing rat spinal cord: a stereological approach

Studies on nuclear volume have shown that it is an indication of the state of differentiation of cells. This study provides evidence indicating increasing nuclear volume during cell maturation. Using unbiased stereological techniques, nuclear volume of both proliferating and non-proliferating glial cells was analysed in the developing spinal cord. Proliferating glial precursor cells were identified using a 5-bromo-2'-deoxyuridine (BrdU) incorporation assay. The nuclear volume of BrdU-labelled cells and unlabelled cells was determined in both periventricular regions and the white matter of the cord at different embryonic ages. In the periventricular region BrdU-labelled nuclei were smaller than unlabelled nuclei at all ages examined. These labelled cells represent dividing undifferentiated progenitors. The unlabelled neighbouring cells with larger nuclei represent a more differentiated population. In the white matter BrdU-labelled nuclei were of similar volume to the unlabelled nuclei. Both of these groups represent glial precursor cells that have migrated from deeper regions and are at similar stages of differentiation, perhaps with different proliferative potential. These findings indicate that the nuclear volume of early glial cells increases as these cells migrate and differentiate.

Visualization of S100B-positive neurons and glia in the central nervous system of EGFP transgenic mice

S100B, the EF-hand Ca(++)-binding protein with gliotrophic and neurotrophic properties implicated in the pathogenesis of Alzheimer's disease, is coined as a glial marker, despite its documented presence in rodent brain neurons. We have generated a transgenic mouse whose EGFP reporter, controlled by the -1,669/+3,106 sequence of the murine S100B gene, allows the direct microscopic observation of most S100B-expressing cells in the central nervous system (CNS). From embryonic day 13 onward, EGFP expression was targeted to selected neuroepithelial, glial, and neuronal cells, indicating that cell-specific expression of S100B is regulated at the transcriptional level during development. In adult mice, the highest level of EGFP expression was found in ependymocytes; astrocytes; and spinal, medullar, pontine, and deep cerebellar S100B neurons. Our results, thus, agree with earlier reports suggesting that S100B is not a CNS glial-specific marker. In addition, we detected EGFP and S100B in forebrain neurons previously thought not to express S100B in the mouse, including neurons of primary motor and somatosensory neocortical areas, the ventral pallidum and prerubral field. Another interesting finding was the selected EGFP targeting to neonatal S100B oligodendrocytes and adult NG2 progenitors as opposed to mature S100B oligodendrocytes. This finding suggests that, except for oligodendrocytes at the last stage of myelin maturation, the -1,669/+3,106 sequence of the S100B gene is a useful reagent for driving expression of transgenes in most S100B-expressing cells of mouse brain.

Developmental expression of monoamine oxidases A and B in the central and peripheral nervous systems of the mouse

Monoamine oxidases A (MAOA) and B (MAOB) are key players in the inactivation pathway of biogenic amines. Their cellular localization has been well established in the mature brain, but nothing is known concerning the localization of both enzymes during development. We have combined in situ hybridization and histochemistry to localize MAOA and MAOB in the developing nervous system of mice. Our observations can be summarized as five key features. (1) MAOA is tightly linked to catecholaminergic traits. MAOA is expressed in all noradrenergic and adrenergic neurons early on, and in several dopaminergic cell groups such as the substantia nigra. MAOA is also expressed in all the neurons that display a transient tyrosine hydroxylase expression in the brainstem and the amygdala and in neurons with transient dopamine-beta-hydroxylase expression in the cranial sensory ganglia. (2) MAOA and MAOB are coexpressed in the serotoninergic neurons of the raphe from E12 to P7. During postnatal life, MAOA expression declines, whereas MAOB expression remains stable. (3) MAOA is transiently expressed in the cholinergic motor nuclei of the hindbrain, and MAOB is expressed in the forebrain cholinergic neurons. (4) MAOA- and MAOB-expressing neurons are also detected in structures that do not contain aminergic neurons, such as the thalamus, hippocampus, and claustrum. (5) Starting at birth, MAOB expression is found in a variety of nonneuronal cells, the choroid plexus, the ependyma, and astrocytes. These localizations are of importance for understanding the effects of monoaminergic transmission during development.

Regulation of tissue inhibitor of metalloproteinase-3 (Timp-3) mRNA expression during rat CNS development

To preserve tissue integrity during the structural rearrangements that occur during central nervous system (CNS) development, an intricate balance between extracellular matrix (ECM) synthesis and degradation must be maintained. The matrix metalloproteinases (MMPs) are believed to be the main mediators of ECM degradation. Because MMPs function in the turnover of a broad-spectrum of ECM proteins their activity is tightly regulated by interaction with tissue inhibitors of metalloproteinases (TIMPs). Whereas the primary function of TIMPs is to inhibit MMP activity, evidence is mounting that TIMPs are multifunctional molecules that exert diverse cell biological functions distinct from their MMP-inhibitory activities. Although the role of MMPs and TIMPs in the morphogenesis of non-neural tissues has been investigated, to date few studies have analyzed MMP or TIMP expression during CNS development. In the present report, we demonstrate the regulation of Timp-3 mRNA expression throughout the course of CNS development. In particular, Timp-3 mRNA is expressed in embryonic ventricular zones and the postnatal subventricular zone (SVZ). In addition, Timp-3 is expressed in the rostral migratory steam (RMS) to the olfactory bulb in a pattern similar to the ECM proteoglycan brevican. These data suggest that TIMP-3 and brevican may act in concert to guide neuronal migration along the RMS.

Developmental regulation of pituitary adenylate cyclase-activating polypeptide and PAC(1) receptor mRNA expression in the rat central nervous system

As the brain develops, a homogeneous population of mitotically active progenitors generates the molecularly heterogeneous post-mitotic cells of the mature brain. The balance between cell division, growth arrest and differentiation of these progenitors undoubtedly requires the activation of a vast array of genes. Pituitary adenylate cyclase-activating polypeptide (PACAP) is a member of the vasoactive intestinal polypeptide (VIP)/secretin/glucagon family. Within the nervous system, PACAP has been shown to stimulate neurite outgrowth, regulate neurotransmitter production and neuronal survival. These diverse biological actions are mediated through interaction with two types of receptors, a PACAP-selective receptor (PAC(1)-R) and receptors which interact almost equally with both VIP and PACAP. Since several lines of evidence suggest that PACAP acts as a neurotrophic factor, we sought to characterize PACAP and PAC(1)-R expression in the developing rat nervous system. The PAC(1)-R is expressed at very high levels in ventricular zones throughout the neuraxis. In addition to the embryonic enrichment in proliferative zones, PAC(1)-R expression is maintained in areas of neurogenesis in the adult central nervous system (CNS), namely, the subventricular zone of the olfactory bulb and hippocampal dentate gyrus. In contrast, PACAP is expressed primarily in the post-mitotic parenchyma. This temporal regulation and cellular distribution suggests that PACAP, through its interaction with the PAC(1)-R, may play a role in mammalian neurogenesis.

Ependymal development, proliferation, and functions: a review

A survey of the literature shows that proliferation of ependyma occurs largely during the embryonic and early postnatal periods of development in most species. Differentiation of these cells proceeds along particular regional and temporal gradients as does the expression of various cytoskeletal (vimentin, cytokeratins, glial fibrillary acidic protein) and secretory proteins (S-100). Turnover declines significantly postnatally, and only low levels of residual activity persist into adulthood under normal conditions. Although the reported response of ependyma to injury is somewhat equivocal, only limited regenerative capacity appears to exist and to varying degrees in different regions of the neuraxis. Proliferation has been most often observed in response to spinal cord injury. Indeed, the ependyma plays a significant role in the initiation and maintenance of the regenerative processes in the spinal cord of inframammalian vertebrates. In the human, however, ependyma appears never to regenerate at any age nor re-express cytoskeletal proteins characteristic of immature cells. The functions of ependyma including tanycytes, a specialized form of ependymal cell that persists into adulthood within circumscribed regions of the nervous system, are still largely speculative. Fetal unlike mature ependyma is believed to be secretory and is believed to play a role in neurogenesis, neuronal differentiation/axonal guidance, transport, and support. In the adult brain, mature ependyma is not merely an inert lining but may regulate the transport of ions, small molecules, and water between the cerebrospinal fluid and neuropil and serve an important barrier function that protects neural tissue from potentially harmful substances by mechanisms that are still incompletely understood.

Developmental expression of two rat sialyltransferases that modify the neural cell adhesion molecule, N-CAM

Polysialylation of the neural cell adhesion molecule (N-CAM) reduces the efficacy of N-CAM-mediated homophilic binding and is regulated both during development and in regions undergoing neurogenesis or remodeling in the adult. Hamster PST-1 (PST) and rat STX are two related sialytransferases that catalyze the polysialylation of N-CAM. We have isolated a cDNA clone for the rat homologue of PST and compared its amino acid and nucleotide sequence to that of rat STX. This analysis revealed regions of high sequence similarity corresponding to the enzymatic domains of the two molecules. Other regions of lower similarity were used to generate specific probes for in situ hybridization. The distribution of PST and STX mRNAs, polysialic acid, and N-CAM were analyzed at three developmental stages. PST and STX mRNAs were expressed abundantly throughout the nervous system at embryonic day 15 and postnatal day 4 and were coexpressed in most tissues examined. In the adult brain, STX expression was reduced relative to PST and expression of both mRNAs was restricted to subsets of cells in areas undergoing constant synaptic rearrangement including hippocampus and olfactory system. The results suggest that both PST and STX participate in the polysialylation of N-CAM in vivo and that their expression levels are dynamically controlled during development and regeneration.

Hypoxia-ischaemia model in the 7-day-old rat: possibilities and shortcomings

The Levene model in 7-day-old rats is the most often used model of hypoxia-ischaemia (HI) in immature animals. The rat central nervous system is immature at birth and corresponds neurodevelopmentally to the term human infant during the second postnatal week. The Levene model of HI differs from clinical asphyxia with respect to the unilateral distribution of brain injury and lack of multi-organ dysfunction. Furthermore, it does not allow cardiovascular monitoring or repeated blood sampling. On the other hand, the progressive nature of HI bears many similarities to birth asphyxia with regard to blood flow changes and cellular metabolic derangements. The model is well characterized, easy to carry out and the low cost allows inclusion of a sufficient number of animals for dose-response evaluation of neuroprotective agents. In addition, it provides the unique opportunity of long-term evaluation of neuropathological and functional outcome.

In vitro differentiation of neural progenitor cells from prenatal rat brain: common cell surface glycoprotein on three glial cell subsets

Glial progenitor cell differentiation and cell lineage relationships during brain development are complex hierarchical processes depending on genetic programming, cell-cell interactions, and microenvironmental factors. The identification of precursor cell-specific antigens provides a tool for the study of both normal development and deviations from lineage-specific differentiation associated with malignant transformation. Monoclonal antibody (mAb) RB13-6 recognizes a 130-kDa cell surface glycoprotein (gp130RB13-6) expressed by a subset of 9OAcGD3-positive glial precursor cells scattered in the rat neuroepithelium on prenatal day (PRD) 13. During prenatal development the fraction of gp130RB13-6-positive fetal brain cells (FBC) decreased from about 18% (PRD 14) to about 1.5% (PRD 22), coinciding with increasing fractions of more mature cell types, as indicated by the elevated expression of p24RB21-15, another cell surface determinant specified by mAb RB21-15 (Kindler-Röhrborn et al.; Differentiation 30:53-60, 1985) and other neural cell type-specific markers. Accordingly, gp130RB13-6 positive precursor cells were localized in the ventricular zones throughout brain development. Concomitant with their formation and in the adult rat brain, ependymal layers lining the ventricular surface, choroid plexus, and the leptomeninges were intensely labeled by anti-gp130RB13-6 mAb. As visualized by confocal laser scanning microscopy of FBC cultures from PRD 13, gp130RB13-6 was coexpressed with the RC1 antigen by progenitor cells morphologically resembling radial glia cells. In addition, a very small subpopulation of astrocytes coexpressing gp130RB13-6 and glial fibrillary acidic protein (GFAP; < 5%) occurred 3 days after seeding. Primary FBC cultures from PRD 18 contained an increased subset of astrocytes coexpressing gp130RB13-6 and GFAP (approximately 25% of all gp130RB13-6 expressing cells), apparently generated from gp130RB13-6-positive precursors. Corresponding to in vivo conditions, ciliated ependymal cells but also microglial cells/macrophages and leptomeningeal cells showed strong expression of gp130RB13-6 in culture. We thus present a new glycoprotein on the cell surfaces of a glial progenitor cell subset for further studies of cell lineage relationships between radial glia cells, astrocytes, and ependymal cells.

Restricted expression of the actin-regulatory protein, tropomyosin, defines distinct boundaries, evaginating neuroepithelium, and choroid plexus forerunners during early CNS development

In the hindbrain, rhombomeres represent morphological units that develop characteristic, segment-specific structures. Similar segments, known as prosomeres, have been proposed to exist in the forebrain. The neuroepithelial cells of the sharp boundary regions that form the borders between many segments often exhibit distinct shapes, reflecting unique cytoskeletal organization. The present investigation examined the expression of one family of actin-binding, regulatory proteins, the tropomyosins (TM), in boundaries. We found that high molecular weight TMs selectively concentrate in boundary cells and other neuroepithelial zones that exhibit unique cell shapes and movements. Specific TM expression is found at hindbrain boundaries as early as embryonic day 10 in the rat, whereas rhombomeres themselves were TM-negative. Highly restricted TM localization also defined some prosomere boundaries in the early forebrain, particularly those exhibiting unique cell shapes. Furthermore, several regions of the neuroepithelium that evaginate are TM-immunoreactive, including tuberal and preoptic neuroepithelium. Most striking, a subpopulation of neuroepithelial cells in the medial telencephalic wall expresses TM, apparently marking the neuroepithelial region that gives rise to the choroid plexus at least 2 d before its formation. This suggests that the medial cerebral wall is not entirely dedicated to generating cells that comprise allocortex. TM expression in the choroid plexus is maintained through initial evagination and appearance in all ventricles. The spatially restricted expression of TMs implicates that this actin-binding protein is involved in the dynamic regulation of cell shape or motility associated with boundary formation and morphogenesis of the neuroepithelium during critical stages of brain development.

Ventricular zone gene-1 (vzg-1) encodes a lysophosphatidic acid receptor expressed in neurogenic regions of the developing cerebral cortex

Neocortical neuroblast cell lines were used to clone G-protein-coupled receptor (GPCR) genes to study signaling mechanisms regulating cortical neurogenesis. One putative GPCR gene displayed an in situ expression pattern enriched in cortical neurogenic regions and was therefore named ventricular zone gene-1 (vzg-1). The vzg-1 cDNA hybridized to a 3.8-kb mRNA transcript and encoded a protein with a predicted molecular mass of 41-42 kD, confirmed by Western blot analysis. To assess its function, vzg-1 was overexpressed in a cell line from which it was cloned, inducing serum-dependent "cell rounding." Lysophosphatidic acid (LPA), a bioactive lipid present in high concentrations in serum, reproduced the effect seen with serum alone. Morphological responses to other related phospholipids or to thrombin, another agent that induces cell rounding through a GPCR, were not observed in vzg-1 overexpressing cells. Vzg-1 overexpression decreased the EC50 of both cell rounding and Gi activation in response to LPA. Pertussis toxin treatment inhibited vzg-1-dependent LPA-mediated Gi activation, but had no effect on cell rounding. Membrane binding studies indicated that vzg-1 overexpression increased specific LPA binding. These analyses identify the vzg-1 gene product as a receptor for LPA, suggesting the operation of LPA signaling mechanisms in cortical neurogenesis. Vzg-1 therefore provides a link between extracellular LPA and the activation of LPA-mediated signaling pathways through a single receptor and will allow new investigations into LPA signaling both in neural and nonneural systems.

Developmental fates and migratory pathways of dividing progenitors in the postnatal rat cerebellum

To investigate the developmental fates and the migratory pathways of dividing progenitors in both the white matter (WM) and the external granule layer (EGL) in the early postnatal rat cerebellum, a replication-deficient retrovirus carrying the beta-galactosidase gene (BAG) was injected into the deep cerebellar tissue or the EGL of postnatal rats to label dividing progenitors. After 1-3 days post-injection (1-3 dpi) of BAG into the deep cerebellar tissue of postnatal day 4/5 (P4/5) rats, labeled immature, unipolar cells were found mainly in the WM. From 4 to 6 dpi, similar cells appeared in the internal granule (IGL), Purkinje cell, and molecular layers, although about half of the labeled cells still resided in the WM and appeared immature. The first morphologically definable Bergmann glia, astrocytes, and oligodendrocytes were also observed. From 14 to 20 dpi, most labeled cells had developed into Bergmann glia, astrocytes, oligodendrocytes, and interneurons in their appropriate layers. When BAG injections were performed at P14, unipolar cells were initially observed, but the majority of these differentiated into myelinating oligodendrocytes in the WM and IGL by 17 dpi. Few immature cells were labeled by injections administered at P20, and these did not develop into mature glia, but into cells with lacy, fine processes, possible representing immature oligodendrocytes. In contrast, BAG-labeled progenitors of EGL produced only granule neurons. Thus, within the first 2 postnatal weeks, dividing progenitors in the WM migrate as immature cells into the cortex before differentiating into a variety of glia and interneurons. The genesis of oligodendrocytes continues through the 2nd postnatal week and largely ceases by P20. EGL cells do not produce glia, but only granule cells.

Widespread programmed cell death in proliferative and postmitotic regions of the fetal cerebral cortex

A key event in the development of the mammalian cerebral cortex is the generation of neuronal populations during embryonic life. Previous studies have revealed many details of cortical neuron development including cell birthdates, migration patterns and lineage relationships. Programmed cell death is a potentially important mechanism that could alter the numbers and types of developing cortical cells during these early embryonic phases. While programmed cell death has been documented in other parts of the embryonic central nervous system, its operation has not been previously reported in the embryonic cortex because of the lack of cell death markers and the difficulty in following the entire population of cortical cells. Here, we have investigated the spatial and temporal distribution of dying cells in the embryonic cortex using an in situ endlabelling technique called ‘ISEL+’ that identifies fragmented nuclear DNA in dying cells with increased sensitivity. The period encompassing murine cerebral cortical neurogenesis was examined, from embryonic days 10 through 18. Dying cells were rare at embryonic day 10, but by embryonic day 14, 70% of cortical cells were found to be dying. This number declined to 50% by embryonic day 18, and few dying cells were observed in the adult cerebral cortex. Surprisingly, while dying cells were observed throughout the cerebral cortical wall, the majority were found within zones of cell proliferation rather than in regions of postmitotic neurons. These observations suggest that multiple mechanisms may regulate programmed cell death in the developing cortex. Moreover, embryonic cell death could be an important factor enabling the selection of appropriate cortical cells before they complete their differentiation in postnatal life.

Pattern formation in the mammalian forebrain: striatal patch and matrix neurons intermix prior to compartment formation

The striatum of the mammalian forebrain is divided into two compartments: the patches and the matrix. Neurons of the patch compartment in the rat striatum become postmitotic earlier in neurogenesis than neurons of the matrix compartment. The selective adhesion of patch neurons to one another has been suggested previously to be an important developmental mechanism of striatal compartmentation. We asked if the selective adhesion of patch neurons is expressed before or after the migration of the majority of the matrix neurons into the striatum. Patch neurons were labelled in vivo by a fluorescent retrograde tracer injected into the substantia nigra on embryonic day 19, which almost exclusively labelled patch neurons. Matrix neurons were labelled with a maternal injection of bromodeoxyuridine at embryonic day 18. When animals were killed at embryonic day 20, the majority of the retrogradely labelled patch neurons were intermixed with the bromodeoxyuridine-labelled matrix neurons, although there appeared to be clustering of some of the patch neurons. However, by postnatal day 2 there was a complete segregation of the clusters of the retrogradely labelled patch neurons from the bromodeoxyuridine-labelled matrix neurons in the striatum. This process was modelled in vitro. The patch and matrix compartments were labelled in vivo at embryonic day 13 and 18 respectively, with different birthdate markers ([3H]thymidine or bromodeoxyuridine). At embryonic day 20 the striatal tissue was removed, dissociated and reaggregated in suspension cultures. After 1 day in vitro, labelled patch and matrix neurons were randomly intermixed within the reaggregates. Examination of the cultures at 2.5 and 4 days in vitro revealed clumping of the labelled patch neurons towards the centres of the reaggregates. Over this same period, the labelled matrix neurons did not clump and were dispersed towards the periphery of the reaggregates. The results suggest that patch neuron adhesiveness may appear relatively soon after these neurons become postmitotic, but that this adhesiveness is unable to overcome the initial force produced by the massive migration of matrix neurons into the striatum. We hypothesize that a migratory phase of embryonic striatal development exists, when fated patch and matrix neurons intermix. After this migratory phase, patch neuron adhesiveness can produce the mature segregation of the striatal compartments.

Development of ependyma in neural transplants

Under in situ conditions, the innermost (juxtaventricular) neuroepithelial layer of the embryonic brain wall develops into ependyma. No development of ependyma was usually observed, however, in transplanted embryonic brain wall. In our telencephalic transplants, however, cysts lined by epithelium resembling ependyma were observed, although only sporadically. We supposed that occasional foldings of the transplanted telencephalic wall enclosed the aforementioned cysts and so induced the formation of ependyma. This hypothesis was supported by the observation that ependyma developed frequently in a model system in which the telencephalic wall was folded artificially prior to transplantation.

Developmental expression of PC3 gene is correlated with neuronal cell birthday

We examined the developmental expression of PC3, a nerve growth factor (NGF) early induced gene in PC12 cells, in the rat central nervous system (CNS) and we found that it represents a molecular marker of ongoing postmitotic neurons production. PC3 is initially expressed in the ventral quarter of the neural tube, at the level of the presumptive cervical spinal cord just where and when (10-11 days post coitum (dpc)) the motor neurons are arising. Subsequently, the appearance of PC3 expression follows a ventro-dorsal and a rostro-caudal gradient in the spinal cord and a caudo-rostral gradient across the brain vesicles that coincide, both spatially and temporally, with the gradients of neurogenesis described in the literature. As in PC12 cells, PC3 mRNA expression appears to be transient in vivo. In all regions of the CNS, it is restricted to the ventricular zone of the neuroepithelium, while neuronal precursors cease to express PC3 as they migrate to the mantle zone. Moreover, PC3 mRNA disappears from the various regions of the CNS as neurogenesis ceases.

In vitro differentiation of E-N-CAM expressing rat neural precursor cells isolated by FACS during prenatal development

Most fetal rat brain cells expressing the embryonal, highly sialylated form of the cell adhesion molecule N-CAM (E-N-CAM) are precursor cells, as judged from the absence of marker molecules specific for mature neural cell types. However, the detection of E-N-CAM+ cells in frozen sections does not provide information on the lineage-specific differentiation of these cells during development. To investigate their differentiation behaviour in vitro, E-N-CAM+ cells were isolated at different times of brain development by fluorescence-activated cell sorting (FACS), using a monoclonal antibody (Mab RB21-7) which specifically recognizes polysialic acid (PSA) residues on E-N-CAM. Double-immunofluorescence analyses showed that the majority of E-N-CAM+ cells isolated on prenatal days 15 to 18 differentiated into neurons while a small subset of Mab RB21-7 binding cells proved to be astrocytic precursors and/or bipotential. The proportion of E-N-CAM+ astrocytic precursors increased during later development (prenatal day 22) concomitantly with the onset of gliogenesis. While conversion of E-N-CAM to mature forms of N-CAM was never observed in neurons during cultivation, E-N-CAM+ cells of the astrocyte lineage switched to N-CAM soon after the onset of GFAP expression. A lineage-specific transition of E-N-CAM to mature N-CAM expression is, therefore, suggested for these astrocytic progenitor cells during rat brain development.

Developmental expression of glial markers in ependymocytes of the rat subcommissural organ: role of the environment

The rat subcommissural organ (SCO), principally composed of modified ependymocytes (a type of glial cell), is a suitable model for the in vivo study of glial differentiation. An immunohistochemical study of the ontogenesis of rat SCO-ependymocytes from embryonic day 13 to postnatal day 10 shows that these cells express transitory glial fibrillary acidic protein (GFAP) from embryonic day 19 until postnatal day 3. However, S100 protein (S100) is never expressed in the SCO-cells, contrasting with the ventricle-lining cells of the third ventricle, which contain S100 as early as embryonic day 17. Environmental factors could be responsible for the repression of GFAP and S100 in adult rats, because GFAP and S100 are observed in ependymocytes of SCO 3 months after being grafted from newborn rat into the fourth ventricle of an adult rat. Neuronal factors might be involved in the control of the expression of S100, since after the destruction of serotonin innervation by neurotoxin at birth, S100 can be observed in some SCO-ependymocytes of adult rats. On the other hand, GFAP expression is apparently not affected by serotonin denervation, suggesting the existence of several factors involved in the differentiation of SCO-cells.

Neuronal development in vitamin B6 deficiency

The morphological changes observed in developing brain regions associated with maternal vitamin B6 deficits are summarized in Table 4. Brain development is a complex and orderly process consisting of cell division, proliferation, migration, and maturation. In the rat, vitamin B6 deficits imposed in utero and up to 30 days postnatal interfere with this orderly process. Deficits of the vitamin imposed in utero have been associated with reduced numbers of total and normal neurons in neocortex and with increased shrunken neurons (700-1500% of controls) in this region. These changes reflect the critical role of vitamin B6 in both neurogenesis and neuron longevity in neocortex. Postnatal cellular events in the neocortex, that is, neuron differentiation and synaptogenesis, were also altered by vitamin B6 deficits; higher order dendrites were reduced on stellate neurons in Layer II and on pyramidal neurons in Layer V. Synaptic density was less in the neutrophil of neocortex and in caudate/putamen, but structural integrity of the synapse was maintained. In cerebellum, both the molecular and granular areas were reduced, the monolayer organization of Purkinje cells was disrupted, and dendritic arborization of the cells was decreased. The number of myelinated axons, as determined by electron microscopy, was decreased in the mediodorsal portion of the pyramidal tract in the medulla oblongata as well as the specific activity of myelination of the total brain. Thus the functional consequences of vitamin B6 deficits during neuronal development may be through reduced connections among neurons and decreased myelination, which alter the rate and magnitude of transmission of nerve impulses.

Pattern formation in the mammalian forebrain: patch neurons from the rat striatum selectively reassociate in vitro

Mechanisms involved in the developmental organization of the rat striatum were investigated in vitro. The neurons of the patch and matrix compartments were preferentially labeled in vivo with a [3H]thymidine injection on embryonic day (E) 13 or 18, respectively. Two or 7 days later the striatum was removed, dissociated into a single cell suspension and plated on a collagen-coated substrate. After 5 days in culture the neurons had migrated into aggregates. Within an individual aggregate, neurons labeled on E13 tended to clump together, whereas neurons labeled on E18 were randomly dispersed. Comparing between aggregates, [3H]thymidine-labeled E13 cells were located in aggregates containing numerous other labeled E13 cells, whereas [3H]thymidine-labeled E18 cells were dispersed randomly between aggregates. These results suggest that early born striatal neurons (primarily patch cells) selectively associate with each other, and that this process may be crucial to the developmental compartmentalization of the rat striatum.

Gliogenesis in organotypic tissue culture of the spinal cord of the embryonic mouse. II. Autoradiographic studies

Organotypic cultures of the spinal cord of the embryonic mouse were subjected to pulses of tritiated thymidine at various times between explanation and 42 days in vitro (DIV). Autoradiography was performed both on cultures fixed immediately at the end of the pulse and on cultures maintained in radioactive-free medium for various periods after the pulse. Quantitative light autoradiographic studies showed a single peak of glial cell proliferation at 9 DIV equivalent to that demonstrated in vivo. The growth rate of glial cells (related to time in culture) decreased along an exponential decay type curve. All these observations were statistically significant when tested against the corresponding null hypothesis. Ultrastructural autoradiography shows that at early stages of the culture, radial glial cells and immature glial cells divided and eventually gave rise to astrocytes and oligodendrocytes. During the period of maximal cell proliferation, tritiated thymidine was incorporated by differentiated astrocytes and ultrastructurally recognizable immature oligodendrocytes. Oligodendrocytes did not divide beyond the stage of active oligodendrocytes (the cells initiating myelination). They were capable of producing dark oligodendrocytes within a week following the last division. These observations emphasize the similarity of the proliferation during development in organotypic culture to that in vivo, modified by the trauma of explantation and the culture conditions.

Fetal rat brain hemisphere tissue in nonadherent stationary organ culture

A simple organ culture system for brain tissue is described. Fragments of fetal rat brain hemisphere tissue are explanted to multiwell dishes base-coated with semisolid agar. In this system nonadherent organ culture can be performed for at least 50 days. Cell migration, biochemical and morphological differentiation and the formation of a layered architecture seem to mimic some of the phenomena occurring in the developing rat brain in vivo. The fragments may therefore be a useful organ culture model for nervous tissue.

Ultrastructure of Müller cells in the developing human retina

The posterior retina of human embryos from 4 to 200 mm of crown-rump length was studied by electron microscopy. At 20 mm dense inner Müller-cell processes near ganglion cells contained rough endoplasmic reticulum, free ribosomes, small matrix particles, and some intermediate filaments. These processes soon had smooth endoplasmic reticulum. By 71 mm many of these inner processes were lucent and contained many intermediate filaments and glycogen particles. Müller-cell nuclei and outer processes were observed between differentiating cone cells at 66 mm, and these outer radial-cell processes soon contained many dense matrix particles and glycogen particles. As neurons in the inner nuclear layer differentiated by 100 mm, Müller-cell cytoplasm in the mid-retina was identified by its intermediate filaments and glycogen particles. Müller cells have composite glial features that appear in the horizontal retinal layers concomitant with neuronal differentiation and maturation in each layer.

Morphology and distribution of postnatally generated glial cells in the somatosensory cortex of the rat: an autoradiographic and electron microscopic study

Postnatal cytogenesis in rat somatosensory cortex was examined by the technique combining light microscopic autoradiography with electron microscopy. Tritiated thymidine was injected intraperitoneally to rats on different days (0-21 days). All animals were sacrificed on the twenty-fifth day after birth. Coronal slices including somatosensory cortex were embedded in epoxy resin. Semithin sections for autoradiography and ultrathin ones for electron microscopy were made alternately. The labeled cortical cells were found mainly in the cases injected with [3H]thymidine during the first and the second weeks. Examination of laminar distribution of the labeled cells revealed that the cells in deeper laminae were labeled on earlier postnatal days than those in more superficial laminae. The labeled cells were examined with electron microscope to identify their nature. By this, it was revealed that ultrastructural morphology of the labeled cells were that of glial cells (either astrocyte or oligodendrocyte). Time and space pattern of this neocortical postnatal gliogenesis shows the tendency of the inside-out sequence, though the pattern is not as distinctive as that of prenatal neocortical neurogenesis. The relationships between the pattern of gliogenesis and the maturation of cortical neurons is suggested.

Neurogenesis in the brain stem of the rabbit: an autoradiographic study

With the aid of (H3)-thymidine autoradiography, neurogenesis was documented in the nuclear groups of the medulla oblongata, pons, and mid-brain, as well as in the brain stem reticular formation of the rabbit. Following single injections of (H3)-thymidine, counts were taken of intensely labeled neurons within the nuclei of the functional columns related to the cranial nerves, nuclei of several other functional classifications, and nuclei that did not fit into a functional category. In the brain stem as a whole, neurogenesis was found to occur between days 10.0 and 18.5 of gestation: however, the majority of nuclei studied contained intensely neurons only between days 12.0 and 15.0. Only in the pontine nucleus and the tectum were intensely labeled cells observed as late as day 18.5. Directional gradients of histogenesis were often observed within, as well as between, various nuclei. Within the nuclear columns related to the cranial nerves, a clear mediolateral spread of neurogenesis was observable such that nuclei of the motor columns reached a peak in neurogenesis before those in the sensory columns. Likewise, a mediolateral proliferation pattern was seen in the brain stem reticular formation. Other individual directional gradients were discernible; however, in the brain stem as a whole, distinct overall gradients were not observable. In many individual nuclei, gradients in neuron size were observed such that large neurons preferentially arose prior to smaller neurons. Information pertaining to gradients in neurogenesis, as well as to relationships among functionally related nuclei, are discussed.