Vascularization and development of the cytoarchitecture in the human neocortical rudiment

The development of cytoarchitectonics of the brain rudiments in mammals is accompanied by the formation of an intracerebral vascular network. The relationship between these two processes is insufficiently clear. We studied the development of blood vessels and cytoarchitectonics in the neocortical rudiment of 6- to 13-week old human embryos. The light and electron microscopy methods were used, as well as histochemical visualization of NADPH-diaphorase in the vessel cells. The endothelium proliferation was evaluated using antibodies to proliferating cell nuclear antigen. Starting from week 8 of development, the tangentially oriented vessels formed a intraneural network in the ventricular zone of the rudiment, which appears to restrict the motility of neuroepithelial cells. The basal membrane was initially absent, and the neuroepithelial cells were in direct contact with the endothelial cells. During week 9 of development, the tangentially oriented vessels appeared in the intermediate zone. Formations similar to glial legs with short regions of the basal membrane adjoined the walls of inter- and intraneural vessels (note that, according to the published data, glial fibrillary acidic protein is not yet visualized at this stage). Angioarchitectonics depended little on the cell population density in different zones of the rudiment; specifically, the cortical plate did not contain tangentially oriented vessels until week 12-13 of development. The data we obtained suggest that the blood vessels fulfill a special morphogenetic function in the developing neocortex.     

Inosine diphosphatase as a histochemical marker of retinal microvasculature,with special reference to transformation of microglia

Summary Nucleoside diphosphatase (IDPase), localized using inosine diphosphate as substrate, allows the selective staining of blood vessels and cells of vascular origin, such as macrophages and microglia, whereas the neuroglial, the neuronal and the pigment epithelial cells remain unstained. The staining pattern observed in the retina of mouse, rat, cat and monkey are similar; some apparent quantitative differences reflect species differences in the distribution of retinal microvasculature. At the electron-microscopic level, most of the enzyme activity in the blood vessels appears to be located along the outer wall. The cell membrane, parts of the smooth endoplasmic reticulum and the nuclear membrane in the microglial perikarya appear positive; profiles of microglial processes are intensely stained.In the developing eyes of rats and mice, the blood vessels are stainable from the earliest stage of their appearance. An array of amoeboid cells precede the growing blood vessels and spread out over the future vascularized part of the retina. These cells eventually develop characteristic microglial features, and extend many elongated and branched processes between the neuroepithelial cells while remaining in contact with, or in close proximity to, the blood vessels. Intense IDPase activity in the microglial cells, in contrast to the absence of the enzyme in the neuroglial Müller cells, suggests that microglia are involved in phosphate metabolism and indicates functional compartmentalization within the glial tissue lying between the blood retinal barrier and the retinal neurons.     

Multipotent and Stem Cells in the Developing, Definitive, and Regenerating Vertebrate Eye

This is a review of the experimental studies on the vertebrate retina neurogenesis. Data are provided on the distribution and localization of multipotent and stem cells in the developing, definitive, and regenerating eye. At the early stages of retina development, the neuroepithelial cells divide synchronously, thus leading to the accumulation of a certain number of the retinal rudiment cells. Synchronous divisions precede the asynchronous ones, when the differentiation of the retinal cells is initiated. The neuroepithelial cells are multipotent: the neuroblast is a source of the cells of different types, for example, neurons and glial cells. The proliferating multipotent cells are preserved in the ciliary-terminal zone of the retina of amphibians, fish, and chickens during their entire life. The differentiated pigment epithelium cells also proliferate in this area of the eye. The multipotent cells of the retinal ciliary-terminal zone and cells of the pigment epithelium in the eye periphery provide for the growth of amphibian and fish eyes during the entire life of these animals. In adult mammals, clonable and self-renewable cells were found among the pigmented differentiated cells in the ciliary folds. In a culture, the stem cells form spheroids consisting of depigmented and proliferating cells. Upon transdifferentiation, the cells of spheroids form rods, bipolar cells, and ganglion and glial cells, thus suggesting the possible regenerative potencies of the stem cells in the ciliary body of the mammalian eye. The main event of retinal regeneration in newts is the transdifferentiation of the pigment epithelium cells. The results of comparative analysis suggest that the stem cells of the ciliary body in the mammalian eye and pigment epithelium cells in lower vertebrates exhibit similar potencies and use similar mechanisms during the formation of the cells of the neural series.     

Immunoreactivity for laminin in the developing ventral longitudinal pathway of the brain

The first long tract to form in the brain of a vertebrate embryo is the ventral longitudinal pathway. In order to investigate what chemical cues may guide nerve growth cones along this pathway, affinity-purified antibodies to laminin and collagen type IV were used to stain sections of mouse embryos from Embryonic Days 8 through 17. A monoclonal anti-neurofilament antibody was used to show the development of the ventral longitudinal pathway in relationship to immunoreactivity for laminin and collagen type IV. At Day 8 fluorescent immunoreactivity for laminin is bright in the external limiting membrane of the neural tube, but the neuroepithelium does not show bright laminin or neurofilament immunoreactivity. At E9 the ventral longitudinal pathway is forming and punctate immunoreactivity for laminin is present on the surfaces of neuroepithelial cells in the marginal zone, through which axons of the ventral pathway extend. Punctate immunofluorescence for laminin remains concentrated in the marginal zone on Days E10 through E14, but on E16 punctate immunofluorescence was much reduced, although immunoreactivity for laminin remained bright in the maturing pial and arachnoid membranes and on blood vessels in the brain. Immunoreactivity for collagen type IV was strong in the external limiting membrane and on blood vessels, but never showed concentrated punctate immunofluorescence in the marginal zone. These results indicate that laminin may be available on cell surfaces and in extracellular spaces as an adhesive ligand for growth cones during the formation of the ventral longitudinal pathway.     

Selective expression of eGFP in mouse perivascular astrocytes by modification of the Mlc1 gene using T2A‐based ribosome skipping

Perivascular astrocyte end feet closely juxtapose cerebral blood vessels to regulate important developmental and physiological processes including endothelial cell proliferation and sprouting as well as the formation of the blood‐brain barrier (BBB). The mechanisms underlying these events remain largely unknown due to a lack of experimental models for identifying perivascular astrocytes and distinguishing these cell types from other astroglial populations. Megalencephalic leukoencephalopathy with subcortical cysts 1 (Mlc1) is a transmembrane protein that is expressed in perivascular astrocyte end feet where it controls BBB development and homeostasis. On the basis of this knowledge, we used T2A peptide‐skipping strategies to engineer a knock‐in mouse model in which the endogenous Mlc1 gene drives expression of enhanced green fluorescent protein (eGFP), without impacting expression of Mlc1 protein. Analysis of fetal, neonatal and adult Mlc1‐eGFP knock‐in mice revealed a dynamic spatiotemporal expression pattern of eGFP in glial cells, including nestin‐expressing neuroepithelial cells during development and glial fibrillary acidic protein (GFAP)‐expressing perivascular astrocytes in the postnatal brain. EGFP was not expressed in neurons, microglia, oligodendroglia, or cerebral vascular cells. Analysis of angiogenesis in the neonatal retina also revealed enriched Mlc1‐driven eGFP expression in perivascular astrocytes that contact sprouting blood vessels and regulate blood‐retinal barrier permeability. A cortical injury model revealed that Mlc1‐eGFP expression is progressively induced in reactive astrocytes that form a glial scar. Hence, Mlc1‐eGFP knock‐in mice are a new and powerful tool to identify perivascular astrocytes in the brain and retina and characterize how these cell types regulate cerebral blood vessel functions in health and disease.     

Lake-front property: a unique germinal niche by the lateral ventricles of the adult brain

New neurons and glial cells are generated in an extensive germinal niche adjacent to the walls of the lateral ventricles in the adult brain. The primary progenitors (B1 cells) have astroglial characteristics but retain important neuroepithelial properties. Recent work shows how B1 cells contact all major compartments of this niche. They share the "shoreline" on the ventricles with ependymal cells, forming a unique adult ventricular zone (VZ). In the subventricular zone (SVZ), B1 cells contact transit amplifying (type C) cells, chains of young neurons (A cells), and blood vessels. How signals from these compartments influence the behavior of B1 or C cells remains largely unknown, but recent work highlights growth factors, neurotransmitters, morphogens, and the extracellular matrix as key regulators of this niche. The integration of emerging molecular and anatomical clues forecasts an exciting new understanding of how the germ of youth is actively maintained in the adult brain.     

The morphological picture of implanted foetal rat brain tissue

The authors studied the morphological picture of implanted foetal brain tissue. Macroscopically, they found that the grafts used for morphological study had taken in every case and that the size of the graft had doubled or trebled. The surface of the implant grew above the level of the recipient's cortex and numerous branching blood vessels were observed in the graft-cortex contact area. In the light microscope, organization of the cells in circular clusters or strips was found in the implant. Large numbers of blood vessels of varying calibre were present in the vicinity of large cell concentrations. Distinct differences between the cytoarchitectonics of the normal tissue of the recipient and the graft tissue were seen. Cells with a normal appearance or an apolar form, whose nucleus contained irregularly scattered chromatin and mosaic-like nucleoli, were observed at ultrastructural level. The nuclear membrane was thrown into multiple folds which invaginated deep into the nuclear matter. The Golgi complex covered a large area. Axodendritic synapses indistinguishable from the control were found in the neuropil. No changes were observed in the walls of the capillaries. The pericapillary zone was characterized by the presence of light astrocyte processes.     

The cell biology of neural stem and progenitor cells and its significance for their proliferation versus differentiation during mammalian brain development

The switch of neural stem and progenitor cells from proliferation to differentiation during development is a crucial determinant of brain size. This switch is intimately linked to the architecture of the two principal classes of neural stem and progenitor cells, the apical (neuroepithelial, radial glial) and basal (intermediate) progenitors, which in turn is crucial for their symmetric versus asymmetric divisions. Focusing on the developing rodent neocortex, we discuss here recent advances in understanding the cell biology of apical and basal progenitors, place key regulatory molecules into subcellular context, and highlight their roles in the control of proliferation versus differentiation.     

Par-complex proteins promote proliferative progenitor divisions in the developing mouse cerebral cortex

The size of brain regions depends on the balance between proliferation and differentiation. During development of the mouse cerebral cortex, ventricular zone (VZ) progenitors, neuroepithelial and radial glial cells, enlarge the progenitor pool by proliferative divisions, while basal progenitors located in the subventricular zone (SVZ) mostly divide in a differentiative mode generating two neurons. These differences correlate to the existence of an apico-basal polarity in VZ, but not SVZ, progenitors. Only VZ progenitors possess an apical membrane domain at which proteins of the Par complex are strongly enriched. We describe a prominent decrease in the amount of Par-complex proteins at the apical surface during cortical development and examine the role of these proteins by gain- and loss-of-function experiments. Par3 (Pard3) loss-of-function led to premature cell cycle exit, reflected in reduced clone size in vitro and the restriction of the progeny to the lower cortical layers in vivo. By contrast, Par3 or Par6 (Pard6alpha) overexpression promoted the generation of Pax6+ self-renewing progenitors in vitro and in vivo and increased the clonal progeny of single progenitors in vitro. Time-lapse video microscopy revealed that a change in the mode of cell division, rather than an alteration of the cell cycle length, causes the Par-complex-mediated increase in progenitors. Taken together, our data demonstrate a key role for the apically located Par-complex proteins in promoting self-renewing progenitor cell divisions at the expense of neurogenic differentiation in the developing cerebral cortex.     

Hes genes regulate size, shape and histogenesis of the nervous system by control of the timing of neural stem cell differentiation

Radial glial cells derive from neuroepithelial cells, and both cell types are identified as neural stem cells. Neural stem cells are known to change their competency over time during development: they initially undergo self-renewal only and then give rise to neurons first and glial cells later. Maintenance of neural stem cells until late stages is thus believed to be essential for generation of cells in correct numbers and diverse types, but little is known about how the timing of cell differentiation is regulated and how its deregulation influences brain organogenesis. Here, we report that inactivation of Hes1 and Hes5, known Notch effectors, and additional inactivation of Hes3 extensively accelerate cell differentiation and cause a wide range of defects in brain formation. In Hes-deficient embryos, initially formed neuroepithelial cells are not properly maintained, and radial glial cells are prematurely differentiated into neurons and depleted without generation of late-born cells. Furthermore, loss of radial glia disrupts the inner and outer barriers of the neural tube, disorganizing the histogenesis. In addition, the forebrain lacks the optic vesicles and the ganglionic eminences. Thus, Hes genes are essential for generation of brain structures of appropriate size, shape and cell arrangement by controlling the timing of cell differentiation. Our data also indicate that embryonic neural stem cells change their characters over time in the following order: Hes-independent neuroepithelial cells, transitory Hes-dependent neuroepithelial cells and Hes-dependent radial glial cells.     

Three-dimensional organization of the developing vasculature of the kidney

Kidney function depends on a well-developed vascular system. Any impairment of the blood supply disturbs the integrity and function of the organ. The differentiation of renal vessels has been investigation for many years, but little is known about the relationship between nephrogenesis and vessel development. In the present work the spatial organization of the differentiating vessels was analyzed in precisely oriented tissue sections and in optical sections acquired by laser scan microscopy. Developing vessels as well as small capillaries were visualized with two endothelium-detecting antibodies. Small vessels running in parallel towards the organ capsule were detected in numerous cortico-medullary-oriented tissue sections. Cross-sections of the nephrogenic zone showed a regularly arranged network, which was composed of cells detected by both monoclonal antibodies. Parts of this network were localized in regions of the nephrogenic zone which have been assumed to be free of vessels or vessel-like structures for a long time. These results were confirmed by the laser-scan-microscopic analysis of complete cortex explants. The extraordinarily regular arrangement of the endothelial network in the nephrogenic zone allowed us to reconstruct the developing vascular system. The results presented here underline the close relationship between nephrogenesis and vessel development. Received: 20 May 1996 / Accepted: 11 July 1996     

Live imaging at the onset of cortical neurogenesis reveals differential appearance of the neuronal phenotype in apical versus basal progenitor progeny

The neurons of the mammalian brain are generated by progenitors dividing either at the apical surface of the ventricular zone (neuroepithelial and radial glial cells, collectively referred to as apical progenitors) or at its basal side (basal progenitors, also called intermediate progenitors). For apical progenitors, the orientation of the cleavage plane relative to their apical-basal axis is thought to be of critical importance for the fate of the daughter cells. For basal progenitors, the relationship between cell polarity, cleavage plane orientation and the fate of daughter cells is unknown. Here, we have investigated these issues at the very onset of cortical neurogenesis. To directly observe the generation of neurons from apical and basal progenitors, we established a novel transgenic mouse line in which membrane GFP is expressed from the beta-III-tubulin promoter, an early pan-neuronal marker, and crossed this line with a previously described knock-in line in which nuclear GFP is expressed from the Tis21 promoter, a pan-neurogenic progenitor marker. Mitotic Tis21-positive basal progenitors nearly always divided symmetrically, generating two neurons, but, in contrast to symmetrically dividing apical progenitors, lacked apical-basal polarity and showed a nearly randomized cleavage plane orientation. Moreover, the appearance of beta-III-tubulin-driven GFP fluorescence in basal progenitor-derived neurons, in contrast to that in apical progenitor-derived neurons, was so rapid that it suggested the initiation of the neuronal phenotype already in the progenitor. Our observations imply that (i) the loss of apical-basal polarity restricts neuronal progenitors to the symmetric mode of cell division, and that (ii) basal progenitors initiate the expression of neuronal phenotype already before mitosis, in contrast to apical progenitors.     

Haemangioblastoma: histological and immunohistological study of an enigmatic cerebellar tumour

Paraffin-embedded blocks of 36 cerebellar haemangioblastomas were reacted with a panel of antibodies including glial fibrillary acidic protein, vimentin, epithelial membrane antigen, cytokeratin, Factor VIII, a neuroendocrine marker and with Ulex europaeus. agglutinin The main histological features, apart from the characteristic large abnormal vessels, were a prominent reticulin network, a cystic architecture and cellular and nuclear polymorphism. Two cell types were identified: endothelial and stromal. Twenty tumours were positive for glial fibrillary acidic protein because of included or reactive astrocytes as well as positive stromal cells. Vimentin was positive in all tumours with a diffuse distribution and a somatic pattern; blood vessels, stromal cells and reactive astrocytes were strongly positive. Factor VIII and Ulex europaeus agglutinin reactivity were present in a similar pattern of staining in endothelium and in five cases there were stromal cells that were positive with the latter. We were not able to ascertain the histogenesis of the stromal cell, which remains enigmatic.     

Immunoreactivity for GABA,GAD65, GAD67 and Bestrophin-1 in the Meninges and the Choroid Plexus: Implications for Non-Neuronal Sources for GABA in the Developing Mouse Brain

Neural progenitors in the developing neocortex, neuroepithelial cells and radial glial cells, have a bipolar shape with a basal process contacting the basal membrane of the meninge and an apical plasma membrane facing the lateral ventricle, which the cerebrospinal fluid is filled with. Recent studies revealed that the meninges and the cerebrospinal fluid have certain roles to regulate brain development. γ-aminobutyric acid (GABA) is a neurotransmitter which appears first during development and works as a diffusible factor to regulate the properties of neural progenitors. In this study, we examined whether GABA can be released from the meninges and the choroid plexus in the developing mouse brain. Immunohistochemical analyses showed that glutamic acid decarboxylase 65 and 67 (GAD65 and GAD67), both of which are GABA-synthesizing enzymes, are expressed in the meninges. The epithelial cells in the choroid plexus express GAD65. GABA immunoreactivity could be observed beneath the basal membrane of the meninge and in the epithelial cells of the choroid plexus. Expression analyses on Bestrophin-1, which is known as a GABA-permeable channel in differentiated glial cells, suggested that the cells in the meninges and the epithelial cells in the choroid plexus have the channels able to permeate non-synaptic GABA into the extracellular space. Further studies showed that GAD65/67-expressing meningeal cells appear in a manner with rostral to caudal and lateral to dorsal gradient to cover the entire neocortex by E14.5 during development, while the cells in the choroid plexus in the lateral ventricle start to express GAD65 on E11–E12, the time when the choroid plexus starts to develop in the developing brain. These results totally suggest that the meninges and the choroid plexus can work as non-neuronal sources for ambient GABA which can modulate the properties of neural progenitors during neocortical development.     

Ultrastructure of the area postrema of the monkey, Macaca fascicularis

The area postrema of the monkey, Macaca fascicularis, were a pair of oval organs at the caudal end of the floor of fourth ventricle. Their ependymal lining was covered by well-developed microvilli with occasional overlying supraependymal cells. Two types of lining cells were present: pyramidad- and flattened cells. The pyramidal cell showed a long extending basal process resting on the underlying blood vessels. In transmission electron microscopy, the organ showed numerous fenestrated sinusoids characterized by a distinct perivascular space containing mast cells, macrophages and collagen fibrils. The parenchyma of the organ was composed of neurons and glial elements. Only one type of neuron ranging from 9.5 to 15 microns could be distinguished. The neurons contained an indented nucleus surrounded by organelle rich cytoplasm. The soma of the neuron was enclosed by glial element resembling astrocyte. The glial processes terminated on the blood vessel where they were "tunnelled" by a variable number of nerve fibres some of which gained a direct access to the external basal lamina of the perivascular space. Synapses in the neuropil predominantly of the axodendritic variety were observed. Axon terminals containing round agranular vesicles were seen to make synaptic contacts with the neuronal soma. No structural changes were observed in the area postrema following bilateral cervical vagotomy. However, degenerating axon terminals were observed in the subpostremal zone 7, 14 and 21 days after vagotomy suggesting a direct afferent projection into this region.     

A Functional Requirement for Astroglia in Promoting Blood Vessel Development in the Early Postnatal Brain

Astroglia are a major cell type in the brain and play a key role in many aspects of brain development and function. In the adult brain, astrocytes are known to intimately ensheath blood vessels and actively coordinate local neural activity and blood flow. During development of the neural retina, blood vessel growth follows a meshwork of astrocytic processes. Several genes have also been implicated in retinal astrocytes for regulating vessel development. This suggests a role of astrocytes in promoting angiogenesis throughout the central nervous system. To determine the roles that astrocytes may play during brain angiogenesis, we employ genetic approaches to inhibit astrogliogenesis during perinatal corticogenesis and examine its effects on brain vessel development. We find that conditional deletion from glial progenitors of orc3, a gene required for DNA replication, dramatically reduces glial progenitor cell number in the subventricular zone and astrocytes in the early postnatal cerebral cortex. This, in turn, results in severe reductions in both the density and branching frequency of cortical blood vessels. Consistent with a delayed growth but not regression of vessels, we find neither significant net decreases in vessel density between different stages after normalizing for cortical expansion nor obvious apoptosis of endothelial cells in these mutants. Furthermore, concomitant with loss of astroglial interactions, we find increased endothelial cell proliferation, enlarged vessel luminal size as well as enhanced cytoskeletal gene expression in pericytes, which suggests compensatory changes in vascular cells. Lastly, we find that blood vessel morphology in mutant cortices recovers substantially at later stages, following astrogliosis. These results thus implicate a functional requirement for astroglia in promoting blood vessel growth during brain development.     

Comparative characteristics of the cyto- and angioarchitectonics of the inferior olives and the dentate nuclei of the cerebellum during human prenatal ontogenesis]

The development of the inferior olives and the denticulate nuclei of the cerebellum and their capillary network were studied in human fetuses 4--10 months old. General regularities in the formation of the inferior olives and the denticulate nuclei of the cerebellum were stated. During prenatal ontogenesis, nuclear cytoarchitectonics becomes more complex that is especially evident after 7th month of the intrauterine development: the density of the neural cells arrangement decreases, while the density of the glia arrangement and that of the glial index increases. At the same time, essential alterations occur in the capillary network: int acquires three-dimensional structure, becomes longer with more complicated interrelations between the neural cells and the capillaries.     

Nestin expression in pancreatic stellate cells and angiogenic endothelial cells

Nestin is an intermediate filament protein expressed by neuroepithelial stem cells and which has been proposed to represent also a marker for putative islet stem cells. The aim of this study was to characterize the cell type(s) expressing nestin in the rat pancreas. By immunohistochemistry, nestin positivity was localized exclusively in mesenchymal cells of normal and regenerating adult pancreas. In the latter condition, the number of nestin-positive cells and the intensity of nestin immunoreactivity were greatly increased. Most nestin-positive cells had the morphology of stellate cells, a type of pericyte associated with blood vessels which has been previously reported to occur in liver and pancreas. In addition, nestin positivity was present in endothelial cells from neocapillaries during pancreas regeneration, and in all blood vessels during morphogenesis in fetal pancreas. Nestin expression was not found in the ductal epithelial cells from which islet cells originate in fetal and regenerating pancreas. In primary pancreatic tissue explants, nestin-positive mesenchymal cells rapidly attached to plastic and proliferated. These cells also expressed desmin, vimentin, and glial fibrillary acidic protein which are known to represent stellate cell markers. In summary, nestin in the pancreas is primarily a marker for reactive stellate cells, or pericytes, and endothelial cells during active angiogenesis.