Eduardo Bascó

Radial glia in the developing mouse cerebral cortex and hippocampus.

SummaryThe regional distribution of radial glia in the developing cerebral cortex and the hippocampus of the mouse was studied using silver impregnation and immunocytochemical staining for glial fibrillary acidic protein (GFAP). Whilst the former technique revealed radial fibres at a slightly earlier age, immunocytochemistry gave a better picture of their general distribution and enabled systematic study of the appearance and disappearance of GFAP-positive radial glia throughout the cortex. Although a clear association between migrating neurones and radial glia was evident in the later stages of cortical plate formation, this relationship was not apparent in all cortical regions nor at the very early stages of the formation of the cortical plate. Even after allowing for a delayed appearance of GFAP immunoreactivity in relatively mature radial glia, the uneven distribution of these cells, their appearance after the cortical plate has already been formed, and their regional development in a pattern dissynchronous with that of the cortical plate argue against a general role of these structures in neuronal migration in the mouse, although there are notable phylogenetic differences.

Immunocytochemical demonstration of glial fibrillary acidic protein in mouse tanycytes

SummaryImmunohistochemical techniques were used to stain for the astrocytespecific glial fibrillary acidic protein (GFAP) in the cells lining the third ventricle of the developing and mature mouse brain. Before birth immunoreactive tanycytes were only observed in the infundibular recess of the median eminence, where they could first be seen at embryonic day 17. They possessed long processes running towards the ventral surface of the brain. During the early postnatal period GFAP-positive tanycytes gradually appeared throughout the third ventricle, although the ependymal cells themselves remained unstained. The tanycytes retained their immunoreactivity for anti-GFAP serum in the adult, and were also evident in the adult rat third ventricle. It is suggested that the presence of GFAP in these specialised cells of the third ventricle indicates that they, the transient radial glia of the developing cerebral cortex, the persistent Bergmann glia of the cerebellum, similar astrocytes with radial processes in the hippocampal dentate gyrus and conventional astroglia are all closely related cell types.

The surface-contact glia.

1 General Introduction.- 1.1 Brief History.- 1.2 Development of the Glia: Current Views and Problems.- 2 Materials and Methods.- 3 Postnatal Cell Proliferation at Nongerminal Sites of the Brain.- 3.1 Introductory Remarks.- 3.2 Mitotic Activity at Nongerminal Sites of the Immature Cerebellar Cortex.- 3.3 Mitotic Activity at Nongerminal Sites of the Immature Forebrain: Time Course and Regional Distribution.- 3.4 Comments.- 3.4.1 Cerebellum.- 3.4.2 Forebrain.- 3.5 Light and Electron Microscopic Description of [3H] Thymidine-Labeled Cells at Nongerminal Sites of the Postnatal Brain.- 3.5.1 Light Microscopic Autoradiography.- 3.5.2 Electron Microscopic Autoradiography.- 3.6 Comments.- 3.6.1 Cerebellum.- 3.6.2 Forebrain.- 4 Radial Glia in the Pre- and Postnatal Brain.- 4.1 Introductory Remarks.- 4.2 GFAP Immunocytochemistry of the Developing Forebrain.- 4.2.1 Cerebral Cortex.- 4.2.2 Hippocampus and Dentate Gyrus.- 4.2.3 Diencephalon.- 4.3 Silver Impregnation of the Forebrain Radial Glia.- 4.4 Comments.- 5 Demonstration of Proliferative Capacity of the GFAP-Immunoreaetive Radial Glia.- 5.1 Introductory Remarks.- 5.2 [3H]Thymidine Uptake into the GFAP-Immunopositive Radial Glia.- 5.3 Comments.- 6 Transport of Material by Glial Processes.- 6.1 Introductory Remarks.- 6.2 Transport of HRP by the Forebrain Radial Glia and Cerebellar Bergmanu Glia.- 6.2.1 Transport in the Forebrain.- 6.2.2 Transport in the Cerebellum.- 6.3 Comments.- 7 Discussion.- 7.1 Proliferating Cells at Nongerminal Sites in the Early Postnatal Period.- 7.2 Persistence of the Radial Glia.- 7.3 Does Postnatal Glial Proliferation Involve Dormant Stem Cells or the Differentiated Glia?.- 7.4 Derivatives of and Mechanism of Derivation from the Radial Glia.- 7.4.1 Astrocytes.- 7.4.2 Cerebellar Bergmann Glia.- 7.4.3 Diencephalic Tanycytes and Retinal Muller Cells.- 7.5 Common Properties of Radial Glial Derivatives.- 7.5.1 Capability of Transporting Material Between Various Brain Fluid Spaces.- 7.5.2 Guidance of Neuronal Migration.- 7.5.3 Morphological Similarities.- 8 Concept of the Surface-Contact Glia.- 9 Current Approaches to the Glia and Some Perspectives.- 10 Summary.- 11 References.- 12 Subject Index.

Proliferation of bergmann-glia in the developing rat cerebellum

SummaryMitotic cells in the ganglionic layer of the infant rat cerebellum were studied between 3 to 12 postnatal days. The connection of these cells with the radial glial fibers of the primitive molecular layer could be established. On this basis it was assumed that the mitotic cells studied were immature Bergmann-glial cells whose proliferative activity seemed to continue even after the formation of their characteristic radial fibers. This phenomenon might offer an explanation for the divergent views on the generation time of Bergmann-glia.

The types of proliferating glioblasts in the immature mouse neocortex and dentate gyrus as revealed by electron microscopic autoradiography.

Proliferating glial elements of 3 to 12-day-old mouse parietal cortex and dentate gyrus were investigated by the electron microscopic autoradiography of 3H-thymidine. The presence in these regions of three types of proliferating glial precursors was verified and related to astrocyte development. Comparing the two areas studied it was concluded that postnatal glial proliferation is independent from neurogenesis and that astroglia is capable of proliferation even in advanced stages of differentiation.

Radial Glia in the Pre- and Postnatal Brain

The morphological similarity between the cerebellar Bergmann glia and the embryonic radial glia suggests a possible developmental relationship, even in the adult cerebellum. Less obvious is such a relationship in the mature forebrain where embryonic radial fibers virtually do not persist. Therefore, in the following the forebrain radial fiber system and its fate during development shall be our concern rather than its counterpart in the cerebellum, where quite a number of studies have been carried out on this issue (Rakic 1971b; Bignani and Dahl 1973).

Current Approaches to the Glia and Some Perspectives

The development of knowledge on the glia, as outlined briefly in Sect. 1.1, has now reached a stage where it has become evident that no one method used on its own can extend our understanding of glial structure and function. On the other hand, powerful new techniques such as autoradiography, immunocytochemistry at both light and electron microscopic levels, and tracer administrations have added substantially to the classical histological methods, which have thus reached a high level of sophistication. The combination of this new type of histology with biochemical and functional investigations is thought to determine glial research of the near future.

Postnatal Cell Proliferation at Nongerminal Sites of the Brain

Postnatal cell-proliferation in the central nervous system has been well established since the beginning of this century (Allen 1912; Smart 1961; Altman 1962, 1966 a, b; Altman and Das 1967; Angevine 1965; Taber Pierce 1966, 1967, 1973). In mammals most of the neurons are formed during embryonic life; only a few of them continue proliferating after birth, particularly in the forebrain. In the mouse forebrain the granule cells of the olfactory bulb and hippocampal dentate gyrus are generated from the last third of gestation until day 20 after birth (Altman and Das 1965; Altman 1966 b; Angevine 1965; Hinds 1968 a, b). During the course of normal development postnatal proliferation of other forebrain neurons has not been verified. Even for these areas the early postnatal period is the latest time of neuronal proliferation; this is followed by their differentiation, after which, as stated by Greenfield et al. (1958), they undergo no further divisions.

Demonstration of Proliferative Capacity of the GFAP-Immunoreactive Radial Glia

Our earlier findings in the cerebellum (Basco et al. 1977) and now in the fore- brain suggest that the proliferation of the glia involves considerably differentiated cells as judged by morphological criteria such as position, ramification pattern, and cytoplasmic and nuclear structure. A spectacular example of this phenomenon was presented in Sect. 3.2, where the cerebellar Bergmann glia were shown to undergo divisions while having a well-developed fiber system extended to the pial surface.

Transport of Material by Glial Processes

In the foregoing we pointed out the developmental, morphological, and immunocytochemical similarities between cells thought so far to belong to distinct classes. As far as functional similarities of these cell types are concerned no common features are mentioned in the literature. Most glial functions known to date, such as K+ transport (Kuffler and Nicholls 1966; Kuffler 1967; Orkand 1977), uptake of amino acid transmitters (Henn and Hamberger 1971; Hosli and Hosli 1976; Schousboe et al. 1977), and compartmentation of energy metabolism (Hamberger and Sellstrom 1975) have not been comparatively investigated in all cell types related to the glia. As a matter of fact, such comparative investigations of functions of various glial populations would require further sophistication in bulk cell separation techniques, most of which, at least for the time being, suffer from the drawback of uncertainty and a high degree of cross contamination.