Lack of evidence for glial cells originating from the external granular layer in mouse cerebellum

Origin, lineage and function of cerebellar glia

The glial cells of the cerebellum, and particularly astrocytes and oligodendrocytes, are characterized by a remarkable phenotypic variety, in which highly peculiar morphological features are associated with specific functional features, unique among the glial cells of the entire CNS. Here, we provide a critical report about the present knowledge of the development of cerebellar glia, including lineage relationships between cerebellar neurons, astrocytes and oligodendrocytes, the origins and the genesis of the repertoire of glial types, and the processes underlying their acquisition of mature morphological and functional traits. In parallel, we describe and discuss some fundamental roles played by specific categories of glial cells during cerebellar development. In particular, we propose that Bergmann glia exerts a crucial scaffolding activity that, together with the organizing function of Purkinje cells, is necessary to achieve the normal pattern of foliation and layering of the cerebellar cortex. Moreover, we discuss some of the functional tasks of cerebellar astrocytes and oligodendrocytes that are distinctive of cerebellar glia throughout the CNS. Notably, we report about the regulation of synaptic signalling in the molecular and granular layer mediated by Bergmann glia and parenchymal astrocytes, and the functional interaction between oligodendrocyte precursor cells and neurons. On the whole, this review provides an extensive overview of the available literature and some novel insights about the origin and differentiation of the variety of cerebellar glial cells and their function in the developing and mature cerebellum.

Postnatal cerebellar granule cells of the white rat (Rattus norvegicus): a quantitative study, using design-based stereology

A stereological study was carried out on postnatal cerebellar granule cells of rats aged 6 and 10 days, for detecting whether and how much they would differ from those of young adult rats. The following parameters were estimated: number-weighted mean volume of the nucleus and of the soma; mean total surface area of the soma; mean absolute volumes per cell of total cytoplasm, mitochondria, Golgi apparatus, and cytosol; mean surface density of the rough endoplasmic reticulum (RER); mean total surface area of the RER. These values were compared between the two postnatal ages. In addition, those values were also analysed in comparison to the ones depicted in young adult rats (60 days), already published by our team, in order to detect similarities between them. It was noticed that, between 6 and 10 days, the mean surface density of the RER was the only parameter that did not change significantly. The comparison of each of the postnatal ages with 60 days revealed that, with the exception of the absolute volume of Golgi apparatus, significant differences were displayed concerning other organelles and cellular compartments. It was concluded that, although fine structural differences have been disclosed, from the stereological point of view postnatal granule cells at 10 days were practically similar to the young adult ones at 60 days. Some potential physiological implications have been considered.

Prenatal ontogeny of the epidermal growth factor receptor and its ligand, transforming growth factor alpha, in the rat brain

Transforming growth factor alpha (TGF alpha) interacts with the epidermal growth factor receptor (EGF-R) to produce its biological effects. TGF alpha induces the proliferation and differentiation of central nervous system (CNS) astrocytes and pluripotent stem cells, as well as the survival and differentiation of postmitotic CNS neurons. Both TGF alpha and EGF-R have been localized to the postnatal CNS. As the majority of CNS neuronal proliferation and migration occurs antenatally, we have examined the ontogeny of TGF alpha and EGF-R in the embryonic rat brain by in situ hybridization. EGF-R mRNA was expressed in the brain as early as embryonic day 11 (E11; the earliest age examined). It was initially detected in the midbrain, with subsequent expression first in multiple germinal zones, followed by expression in numerous cells throughout the brain. In many brain areas, EGF-R mRNA appeared in germinal centers during the later stages of neurogenesis and the early stages of gliogenesis. In the midbrain, the distribution of EGF-R mRNA overlapped extensively with that of tyrosine hydroxylase mRNA, suggesting that fetal dopaminergic neurons express EGF-R. Immunocytochemistry was used to demonstrate the presence of EGF-R-immunoreactive protein in brain areas that expressed EGF-R mRNA on E15 and E20. The expression of TGF alpha in many brain structures preceded that of EGF-R mRNA. TGF alpha mRNA was distributed throughout many non-germinal centers of the brain on E12 and later. Some brain areas, such as the external granule cell layer of the cerebellum, expressed EGF-R, but not TGF alpha mRNA. Northern blot analysis demonstrated that mRNA species for both TGF alpha and EGF-R were similar in embryos and adults. These data indicate that TGF alpha and EGF-R are positioned to have a role in the genesis, differentiation, migration, or survival of numerous cell populations in the embryonic brain.

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.

Dysembryoplastic neuroepithelial tumour of the cerebellum

A case of dysembryoplastic neuroepithelial tumour of the cerebellum occurring in a 28-year-old woman is presented. The lesion extended from the cortex of the inferior vermis upwards into the white matter. Histologically, it exhibited areas of microcystic cerebellar astrocytoma and glial regions with hamartomatous blood vessels as well as areas with oligodendrocyte-like cells (OLC) with a delicate, fibrillary stroma lying in a mucinous, often microcystic matrix. The OLC showed prominent rosette formation and immunohistochemical features suggesting neuronal, i.e. granule cell, differentiation.

Medulloblastoma: II. A pathobiologic overview

The pathobiology of medulloblastoma is reviewed in light of emerging data regarding its immunocytochemical and cytobiologic, as well as molecular biologic, characteristics. The nature of the lesion, particularly its nosologic relation to primitive neuroectodermal tumor, is discussed, as is its place in the World Health Organization classification of tumors of the central nervous system.

Morphological changes of astroglia in the cerebellar cortex of developing mice after neonatal administration of cytosine arabinoside

Neonatal administration of cytosine arabinoside (Ara-C) induced marked hypoplasia of the mouse cerebellum. In 17-20-day-old mice with neonatal Ara-C injections, the external granular layer was still preserved as an irregular cell laminae in the superficial part of the cerebellum, in contrast to its complete disappearance in normal animals at these stages. Immunohistochemical examination using anti-glial fibrillary acidic protein and monoclonal antibody-1D11 (Ono et al: Dev. Brain Res., 50:154-159, 1989) demonstrated malformation of a palisade-like arrangement of labeled glial fibers in the superficial part of the Ara-C treated mouse cerebellum. Furthermore, the immunoreactive astroglia were observed just beneath the pial membrane and their processes were oriented to the deeper part of the molecular layer.

Development of the cerebellum with particular reference to cellular differentiation in the external granular layer

Immunocytochemical evidence of differentiation in developing human cerebellum is presented in this study. Antibodies to neuron specific enolase, neurofilament protein, glial fibrillary acidic protein, vimentin, cytokeratin, epithelial membrane antigen and lymphoid markers, DLC and Leu 7 were used. The external granular layer showed positivity with neuronal markers between 27 weeks gestation and 4 months postnatal, but was negative for all other markers including glial fibrillary acidic protein. Characteristic staining reactions were noted in the other cerebellar layers. Monoclonal antibodies, UJ13A (pan-neuroectodermal marker) and G10 (localising microtubule-associated protein MAP1x) were also used in a limited number of cryostat sections and were positive and negative, respectively, in the external granular layer. The results of this study are discussed in relation to the theory that the external granular layer may be one source of medulloblastomas.

Ectopic glial cells in rat cerebella following neonatal administration of methylazoxymethanol acetate

Immunological and ultrastructural studies of adult cerebella following neonatal injection of methylazoxymethanol acetate revealed ectopic glial cells in the molecular layer and at the pial surface. This finding strengthens the view that the external granular layer might give rise to Bergmann glia.

Brain tumours in man

Brain tumours occur at all ages but they differ in type depending upon the age of the patient. In adults, probably more than 50% of tumours in the brain are metastatic carcinomas or melanomas. The pathological classification of primary brain tumours depends largely upon the cell type involved. Recently, immunocytochemical identification of cell-specific proteins by the use of polyclonal or monoclonal antibodies has greatly enhanced the accuracy of cell identification within tumours. Primary brain tumours in children arise mainly in the brain stem and cerebellum and are astrocytomas, primitive neuroectodermal tumours (medulloblastomas) and ependymomas. Gliomas form the largest group of primary brain tumours in adults, with an annual incidence of 3.94/100,000 in Southern England. In young adults, well differentiated astrocytomas and oligodendrogliomas arise in the cerebral hemispheres. Poorly differentiated, malignant glial tumours include anaplastic astrocytomas and glioblastoma multiforme; these tumours are most common in older adults with a peak annual incidence of 7.3/100,000 in the sixth decade. The major complication of brain tumours is due to their mass effect from tumour growth and from peritumoral oedema. Surgical excision of gliomas is difficult and usually incomplete due to the infiltrative nature of the tumour. As yet these tumours respond poorly to irradiation and chemotherapy.

Fetal origin of the medulloblastoma: evidence from growth analysis of two cases

Growth analysis of medulloblastomas was performed in two children. They initially manifested symptoms at the age of 3 years and 9 months and at the age of 2 months respectively. Computerized tomography (CT) scans were obtained at different points in each case. The growth curves were drawn on a semilogarithmic graph by calculating the tumor volume on CT on the assumptions that the tumor started from a single tumor cell and that the growth rate was constant. By extrapolating the curves back, tumor inception was estimated to have occurred respectively at the 14-23rd week and at the 16-17th week of gestation. Additional cell kinetic data were obtained from DNA analysis of surgical pathology specimens. Calculated cell-cycle times were 22-32 h for both cases. The S phases comprised 26.3% and 27% and the G0G1 phases 66.8% and 62% of the cell cycle, respectively, for case 1 and 2. Assuming a labelling index of 14%, the cell loss factors were estimated to be 97% and 74% (case 1 and case 2 respectively). The seventeenth week of gestation in humans corresponds to the timing of events occurring postnatally at days 3-18 in the developing cerebella of rodents, i.e., at the time of maximal activity in the migration and differentiation of the cells of the fetal external granular layer. Medulloblastomas have been experimentally induced in rodents by the injection of oncogenic viruses during the neonatal period, and statistical data on the epidemiology of human medulloblastomas have suggested a possible association with the contamination of polio vaccine by the SV 40 virus.(ABSTRACT TRUNCATED AT 250 WORDS)

A Golgi and morphometric study of the ectopic granule cells in the molecular layer of the rat cerebellum

The morphological types of isolated ectopic granule cells (EGCs) were examined with Golgi-Rio Hortega staining in the cerebellar molecular layer of adult normal rats. The EGCs showed a significant reduction in the number of dendrites (mean 2.42 +/- 0.07) with respect to the controls (mean 3.97 +/- 0.09), and they usually exhibited poorly developed dendritic terminals. The karyometric analysis on semithin sections indicated that the average nuclear area in EGCs was significantly smaller (16.62 +/- 0.20 micron 2) than in normally positioned granule cells (21.08 +/- 0.24 micron 2).

Embryonal central neuroepithelial tumors and their differentiating potential. A cytogenetic view of a complex neuro-oncological problem

The embryonal central nervous system (CNS) neoplasms are reviewed with special reference to their differentiating potential and in the light of current neuro-oncogenetic concepts partly derived from the experimental induction of neural tumors. The conceptual (and, occasionally, practical) distinction between adult-type and embryonal CNS tumors raises a complex problem, because neoplastic transformation essentially involves replicating stem cells in tissues of renewal and because in the human brain such cells are found mostly in the course of CNS development. A cytogenetic scheme is therefore needed to serve as a frame of reference for a classification of embryonal CNS tumors that will account for the different histological entities documented so far and for the range and the restrictions of their differentiating capabilities. Most embryonal CNS tumors can be fitted into such a scheme. The cerebral medulloepithelioma, the cerebral and cerebellar neuroblastomas, the primitive polar spongioblastoma, and the ependymoblastoma show characteristic morphological features and a correspondingly distinctive cellular differentiating potential. The differentiating capabilities of the cerebellar medulloblastoma, the pineoblastoma, and the retinoblastoma are also distinctive, and are apparently determined by the cytogenesis of the area of the CNS in which the tumors originate. The indiscriminate application of a simplistic concept that would include all the so-called "primitive neuroectodermal tumors" into a single neuroepithelial tumor entity is unlikely to bring further understanding to the problem.

Derivation of cerebellar Golgi neurons from the external granular layer: evidence from explantation of external granule cells in vivo

The present report provides evidence to challenge the traditional view that cerebellar Golgi cells are derived from the ventricular neuroepithelium, postulating instead that they originate from external granule cells. Supporting evidence for this assertion comes from three sources: 1) Typical Golgi cells are found in ectopic granule cell colonies, both outside the cerebellum (in the subarachnoid space) and also within the cerebellar cortex between fused folia. Because ectopic granule cell colonies are derived from external granule cells, which become displaced after treatment with 6-hydroxydopamine (6-OHDA), it was assumed that the ectopic Golgi cells also stem from such displaced external granule cells. 2) In order to demonstrate that Golgi cell precursors migrate from the external granular layer into the Purkinje cell plate, the development of the cerebellar cortex was studied over the period of Golgi cell genesis. On E19 the external granular layer in the rat is subdivided into an outer proliferative and an inner subproliferative zone. At the inner margin of the external granular layer, and in the marginal zone, radially oriented, darkly staining cells are present that exhibit all the characteristics of migrating neurons possessing a leading process oriented toward the Purkinje cell plate, a somatic cilium, and a close association with radial glia fibers. In later stages, these cells are also found deep to the Purkinje cell plate. Because Golgi cells arise during the period between E19 and postnatal day 2 in the rat (Altman and Bayer, '77, '78) and as the basket cells, the first neurons of proven origin from the external granular layer, are not produced before the second postnatal day (Altman, '72), the earlier migrating neurons are presumed to be Golgi cells. 3) Available data from cell kinetic 3H-thymidine studies show that there is no unequivocal evidence for Golgi cell genesis from the ventricular neuroepithelium, because, at the time of Golgi cell birth, ventricular and external granular stem cell populations are proliferating, and with the present methods it is not possible to decide which of these are the precursors of Golgi cells. Thus, taken together, the findings of this study show that Golgi cells are more likely to arise from the external granular layer than from the ventricular neuroepithelium. This concept would unify cerebellar histogenesis by proposing that projection neurons arise from the ventricular neuroepithelium, whereas all interneurons of the cerebellar cortex are descendants of the external granular layer.

6-Hydroxydopamine induced ectopia of external granule cells in the subarachnoid space covering the cerebellum. II. Differentiation of granule cells: a scanning and transmission electron microscopic study

The present report describes the morphological differentiation of ectopic granule cells from external granule cells that have been induced to escape from the cerebellar cortex into the subarachnoid space by injecting neonatal rats with 100 microgram 6-hydroxydopamine (6-OHDA) into the cisterna magna. The following cell types were observed in the period between 5 and 25 days postinjection (dpi): (1) unipolar cells with one process bearing a growth cone at its tip; (2) bipolar cells with two thin beaded processes originating from opposite cell poles, bearing growth cones at their tips; (3) bipolar cells with a T-like process at one pole and a short process lacking a terminal growth cone at the opposite pole; (4) multipolar cells with one thin beaded process and two or more short processes bearing growth cones of a different morphology at their tips; (5) intermediate stages. In the late second week p.i., cell aggregates were observed that continually increased in size up to 30 dpi. On the basis of our light, transmission, and scanning electron microscopic findings, we interpret these cell types to be equivalent to the individual stages of granule cell differentiation that characterize axon formation, migration, and aggregation. In the period between 30 and 365 dpi, granule cells were almost exclusively organized into cell colonies of different sizes, but small cell clusters and single granule cells exhibited the scanning electron microscopic features of adult granule cells, i.e., a small spherical cell body, a single axon with parent axonal stem, T-junction, and parallel fiber, and dendrites engaged in synaptic glomeruli. The parallel fibers ran in fasciculi of different sizes, often parallel to each other, but without preferential orientation over the cerebellar surface. During migration and aggregation, the granule cells and their processes were associated with a substrate of glial sheets that in turn were connected to intracortical Bergmann glia fibers. Our findings indicate that (1) granule cells differentiate normally in an ectopic environment in the presence of glia, (2) ectopic Bergmann glia contain no directional information to guide aberrant migratory granule cells to their correct destination, (3) granule cells can survive outside the brain parenchyma for periods up to one year (the longest postinjection interval studied).

Divergent glial and neuronal differentiation in a cerebellar medulloblastoma in an organ culture system: in vitro occurrence of synaptic ribbons

A cerebellar medulloblastoma from a 2-year-old boy was maintained in vitro in an organ culture system for 6.5 months, and the explants studied by light and electron microscopy at different time intervals. The tumor cells progressively demonstrated divergent differentiation into astrocytes and neuroblasts. Astrocytic differentiation, confirmed by immunohistochemistry for GFA protein, became maximal after about 7 weeks in vitro and was thereafter maintained in different areas of the explants. Concomitantly, neuroblastic differentiation was expressed in other cells, with the progressive development of cell processes filled with many microtubules, of neuroblastic rosettes, of increased numbers of dense-core and clear-centered vesicles, of occasional 9 + 0 cilia, and of synaptic ribbons appearing in vitro. Neuroblastic differentiation was most pronounced in 4- and 6-month-old explants, but synapses were not found. The differentiating features reported are in contrast to those of the original tumor, which was largely undifferentiated. The alternative interpretation of a divergent glial and pineocytic differentiation is also considered. These findings support the concept of the differentiating bipotential of the cerebellar medulloblastoma.

Primitive neuroectodermal tumors of the central nervous system

Primitive neuroectodermal tumors are morphologically similar malignant tumors arising in intracranial and peripheral sites of the nervous system, showing varying degrees of cellular differentiation with a tendency to disseminate along cerebrospinal fluid pathways. They occur primarily in children and young adults. Under the designation primitive neuroectodermal tumors are included medulloblastomas and tumors that may differentiate in other directions, such as medulloepithelioma, neuroblastoma, polar spongioblastoma, pineoblastoma, ependymoblastoma, retinoblastoma, and olfactory neuroblastoma. From a practical, histologic point of view, these tumors are often indistinguishable from one another and are best thought of as primitive neuroectodermal tumors with or without differentiating features.

Cell proliferation and DNA-synthesis in the external granular layer of the postnatal rat cerebellum: a quantitative study

The external granular layer (EGL) of the rat cerebellum can be subdivided into two morphologically distinct zones, the outer of which consists of proliferating cells, while the inner has generally been considered to contain post-mitotic cells exclusively. Two criteria, the detection of mitotic figures and the observation of nuclei labelled shortly after an injection of [3H]-thymidine, were used to locate proliferating cells. Their distribution across the layer was quantified for several ages of animal. Though most frequent superficially, cell proliferation was found to occur at all levels of the EGL. Possible explanations for the existence of a gradient of proliferative activity across the EGL are discussed.

Morphological identification and biochemical characterization of isolated brain cell nuclei from the developing rat cerebellum

Cell nuclei from developing rat cerebellum were isolated and the various types of nuclei were characterized and quantified. Nuclear pellets appeared to be both quantitatively and qualitatively representative of the entire cerebellum, and of sufficient purity to perform biochemical studies as well as morphological comparison with histological sections. Isolated nuclei were classified into 6 groups based on nuclear size and shape, heterochromatin aggregations, and nucleoplasmic density. The total population of cerebellar cells primarily consisted of two types of nuclei after day 10. One group of nuclei, resembling those of internal granule neurons or external germinal cells, contributed at least 70% of the total isolated cell nuclei from day 1 to day 90, whereas another nuclear group that was identified as dark oligodendrocytes constituted 8-9% of the total population on days 45 and 90. Nuclear DNA, RNA, and protein content of the cerebellum also were determined throughout postnatal development. DNA concentration markedly declined after day 15, while the RNA/DNA ratio increased until day 3 and remained constant to day 90. The nuclear protein/DNA ratio increased from birth to day 3, decreased to its lowest value on day 10, and increased to day 90. Utilizing DNA values, the total cell population as well as contributions of different cell types were calculated. At birth the cerebellum was estimated to contain 5.9 million cells, increasing to 94 million by day 21. By day 90, 107 million cells were present, of which 8.6 million oligodendrocytes and 93.6 million internal granule cells were estimated.

Presence of radial glia in foetal mouse cerebellum

The ventricular layer (VL) of foetal mouse cerebellum at days 13--15 of gestation was studied by light and electron microscopy. In Golgi-stained material, round or ovoid cells are located in the VL. These cells have ascending processes, which extend to the pial surface. Ultrastructurally, the ascending processes are electron-lucent, contain microfilaments, some smooth endoplasmic reticulum and scant free ribosomes. They appear to be immature glial processes, oriented radially away from the ventricle. The perikarya of these glial cells lie either in the ventricular or subventricular zones. Juxtaposed along the length of these radially oriented glial processes are unidentified cells, some of which are attached to the immature glial fibres by puncta adhaerentia. These cells are elongated or ovoid with a thin rim of cytoplasm containing few organelles. These unidentified cells may represent neuroblasts (Purkinje cells, Golgi cells, cells of the deep cerebellar nuclei) or glioblasts, (precursors of astrocytes and/or oligodendrocytes) at very early stages of development.

On the development of the cerebellum of the trout, Salmo gairdneri. V. Neuroglial cells and their development

The neuroglia of the cerebellum of Salmo gairdneri Richardson, 1836, has been studied in mature and developing specimens with light and electron microscopy. The light microscopic observations were largely carried out on Golgi material. The cerebellum of the trout contains all of the neurologlial cell types described for the mammalian cerebellum, viz. ependymal cells, Golgi epithelial cells, velate protoplasmic astrocytes, smooth protoplasmic astrocytes and oligodendrocytes. In addition two types of glial elements, which combine characteristics of ependymal cells and of velate astrocytes, are found. These elements are designated as ependymoid astrocytes and astrocytoid ependymal cells. Smooth astrocytes and oligodendrocytes were observed only in later stages of development and possibly arise from the secondary matrix. The other glial cell types, as well as transitional forms between these types, are present in rather early stages, and show a similar ultrastructure. It is plausible that all these types develop from the glioblasts produced by the ventricular matrix layer. Many glial cells are radially oriented and keep in contact with the meningeal surface throughout development. The lattice formed by matrix cells in the earliest stages, and by glial cells and the axons of granule cells later on, plays a role in directing the migration of cells. Other functions of the glia, such as dividing the cerebellar cortex in synaptic compartments, are suggested. It may be concluded that the high degree of differentiation of the teleostean cerebellum is also reflected by the morphology of the neuroglia.