A qualitative and quantitative ultrastructural study of glial cells in the developing visual cortex of the rat

Astrocyte subtypes in the rat olfactory bulb: morphological heterogeneity and differential laminar distribution

Despite increased recognition of the importance and heterogeneity of astrocyte functions throughout the central nervous system (CNS) relatively little attention has been paid to morphological diversity among astrocytes. Recent studies have indicated that subsets of astrocytes are involved in glial-axonal interactions critical to both development and reinnervation of the rat olfactory bulb. Here, we have characterized the morphologies and distribution of astrocytes within anatomically and functionally distinct layers of the adult main olfactory bulb (MOB). Using a known immunohistochemical marker for astrocytes, glial fibrillary acidic protein (GFAP), and the classic gold sublimate method, we identified six astrocyte subtypes based on their morphology and distribution: (1) unipolar, (2) irregular, (3) wedge-shape, (4) circular, (5) semicircular, and (6) elongate. Unipolar, irregular and wedge-shape astrocytes have not been previously described in the CNS. The unipolar and irregular types are located exclusively in the olfactory nerve layer. Wedge-shape astrocytes are unique to, and are the major subtype in, the glomerular layer. These three morphologically unique astrocyte subtypes may correspond to olfactory nerve layer (ONL) and glomerular layer (GL) astrocytes, which express molecules that regulate axonal growth or synaptogenesis during development and/or regeneration of the olfactory nerve. In the glomerular layer, astrocytes are highly organized with respect to the glomeruli. Individual astrocytes are loyal to a single glomerulus. In the external plexiform layer, astrocytes are spaced relatively uniformly. In the granule cell layer, astrocytes appear to compartmentalize granule cell aggregates, recently shown to be coupled by tight junctions. The distribution and patterns of astrocyte processes and the density of GFAP immunoreactivity are distinctive for each of the layers of the olfactory bulb. The spacing of astrocytes and the organization of their processes may be important to compartmentalization of neuronal functions. High levels of GFAP immunoreactivity correlated with layers of high neuronal plasticity. The morphological diversity and differential distribution of astrocytes in the olfactory bulb reported here support growing evidence for functional diversity of astrocytes and important interactions among specific astrocyte and neuron subtypes. It is reasonable to hypothesize, therefore, that as for neurons, morphologically distinctive astrocyte subtypes may correspond to functionally specific classes.

Multiple restricted lineages in the embryonic rat cerebral cortex

We have labelled precursor cells in the embryonic rat cerebral cortex using BAG, a retroviral vector that expresses beta-galactosidase. We had previously reported that labelled precursor cells generate clusters of labelled cells that could be classified into four types by their morphological appearance and anatomical distribution (Price and Thurlow, 1988). In this study, we have used immunohistochemistry and intracellular dye labelling to identify the cell types that make up these clusters. We discovered that clusters are almost always composed of a single cell type. In addition to clusters composed entirely of neurones, we found four different types of glial cell clusters. In the grey matter, glial clusters are composed either of protoplasmic astrocytes, or of cells that have an astrocyte morphology, but no glial filaments. In the white matter, clusters are composed of either fibrous astrocytes or oligodendocytes. Our results indicate that each of these different cortical cell types is generated from a separate population of precursor cells.

Regeneration of adult rat CNS axons into peripheral nerve autografts: ultrastructural studies of the early stages of axonal sprouting and regenerative axonal growth

If one end of a segment of peripheral nerve is inserted into the brain or spinal cord, neuronal perikarya in the vicinity of the graft tip can be labelled with retrogradely transported tracers applied to the distal end of the graft several weeks later, showing that CNS axons can regenerate into and along such grafts. We have used transmission EM to examine some of the cellular responses that underlie this regenerative phenomenon, particularly its early stages. Segments of autologous peroneal or tibial nerve were inserted vertically into the thalamus of anaesthetized adult albino rats. The distal end of the graft was left beneath the scalp. Between five days and two months later the animals were killed and the brains prepared for ultrastructural study. Semi-thin and thin sections through the graft and surrounding brain were examined at two levels 6-7 mm apart in all animals: close to the tip of the graft in the thalamus (proximal graft) and at the top of the cerebral cortex (distal graft). In another series of animals with similar grafts, horseradish peroxidase was applied to the distal end of the graft 24-48 h before death. Examination by LM of appropriately processed serial coronal sections of the brains from these animals confirmed that up to several hundred neurons were retrogradely labelled in the thalamus, particularly in the thalamic reticular nucleus. Between five and 14 days after grafting, large numbers of tiny (0.05-0.20 microns diameter) nonmyelinated axonal profiles, considered to be axonal sprouts, were observed by EM within the narrow zone of abnormal thalamic parenchyma bordering the graft. The sprouts were much more numerous (commonly in large fascicles), smoother surfaced, and more rounded than nonmyelinated axons further from the graft or in corresponding areas on the contralateral side of animals with implants or in normal animals. At longer post-graft survival times, the number of such axons in the parenchyma around the graft declined. At five days, some axonal sprouts had entered the junctional zone between the brain and the graft. By eight days there were many sprouts in the junctional zone and some had penetrated the proximal graft to lie between its basal lamina-enclosed columns of Schwann cells, macrophages and myelin debris. Within the brain, sprouts were in contact predominantly with other sprouts but also with all types of glial cell.(ABSTRACT TRUNCATED AT 400 WORDS)

Astrocytes in cat visual cortex studied by GFAP and S-100 immunocytochemistry during postnatal development

A monoclonal antibody to glial fibrillary acidic protein (GFAP) and a polyclonal antiserum to the S-100 protein were used to study the expression of these astrocytic proteins in the postnatal visual cortex of the cat. Three changes in antigen expression of these astroglial markers could be distinguished over development. First, the density of cells in the white matter, which are heavily labelled with both antibodies from birth until adulthood, diminishes after the third postnatal weeks. By intracellular filling with Lucifer Yellow the reduction of the cell density can be attributed to the disappearance of large astrocytes with a morphology of transforming radial glia, present only in early postnatal development. Second, heavily labelled, large cells present in the grey matter at the seventh postnatal day have disappeared by the fifth postnatal week. On the basis of their morphology these cells can also be classified as radial glial cells. Finally, astroglial cells of the adult-like stellate form appear to be labelled in the cortical layers between the third and seventh postnatal weeks. While the density of these cells and the S-100 immunoreactivity of the cell bodies is adult-like at the fourth postnatal week, there is a gradual increase of the staining intensity with the GFAP antibody up to the seventh postnatal week. This developmental period is paralleled by the appearance of S-100-positive astrocytic processes. The gradual expression of GFAP immunoreactivity and the increased expression of S-100 is interpreted as reflecting the time course of astrocytic maturation. A possible relation of the maturation of astrocytes and cortical development, both of which are prominent in the time period between the third and seventh postnatal week, is discussed.

Postnatal development of dye-coupling among astrocytes in rat visual cortex

Intercellular coupling among astrocytes was studied in rat visual cortex slices from animals aged 1 week to 4 months. Cell coupling via gap junctions was determined by the dye spread of the low molecular weight dye Lucifer Yellow CH injected into electrophysiologically identified cells to adjacent cells. Coupling among glial cells was first detected at postnatal day 11 and was thereafter consistently observed until adulthood. Dye spread was observed up to 300 microns radially from the injected cell covering multiple cortical layers. Following dye injection into a single cell up to several hundred Lucifer Yellow-positive cells could be observed. Quantitative analysis revealed a similar extent of dye spread at different developmental stages including a quite constant number of dye-coupled astrocytes from the end of the second postnatal week to adulthood. Double labelling of Lucifer Yellow-filled cells with an antiserum against the glial fibrillary acidic protein confirmed the astrocytic nature of the injected and coupled cells. Comparison of the density of dye-coupled cells in a given area and the total number of astrocytes as revealed by immunocytochemical staining suggests that dye-coupling includes the entire local astrocytic population. It is concluded that coupling among astrocytes via gap junctions in rat visual cortex occurs shortly after birth and reflects one of the first steps in astroglial maturation.

Pre- and postnatal developmental changes of adrenoceptor subtypes in rat brain

beta-Adrenergic receptor subtypes, beta 1 and beta 2, were studied during pre- and postnatal development in the rat brain. [125I]Iodocyanopindolol (6-300 pmol/L) binding assays in the presence of 5-hydroxytryptamine (0.6-6 mumol/L) were used to measure exclusively beta-adrenergic receptors. In forebrain tissue, saturable and stereoselective binding was detected on gestational day 13. The amount of beta-adrenergic binding increased until postnatal day 23, when adult values were reached. The dissociation constants of [125I]iodocyanopindolol binding remained the same throughout development, as did the affinity of several beta-adrenergic and non-beta-adrenergic compounds. The proportion of the beta 2-adrenergic receptors was determined using the beta 1-selective antagonist ICI-89406 (7-150 nmol/L) and was found to change from 65% in prenatal forebrain tissue to 28% in adulthood. In cerebellum/medulla pons tissue, however, the proportion of beta 2-receptor binding (80%) remained unchanged during the whole developmental period.

Distribution of glial fibrillary acidic protein and vimentin immunoreactivity during rat visual cortex development

The postnatal maturation of astrocytes in the rat visual cortex was analysed by immunostaining the astroglial proteins vimentin and glial fibrillary acidic protein with poly- and monoclonal antibodies. Vimentin immunoreactivity was present in the visual cortex up to the third postnatal week, whereas immunolabelling first disappeared in the cortical layers and then in the white matter. In the early postnatal period, vimentin antibodies labelled radial glial fibres. After the first postnatal week staining of radial glial fibres gradually disappeared and vimentin immunoreactivity was localized in a few protoplasmic astrocytes in the grey matter and fibrous astrocytes in the white matter. The development of glial fibrillary acidic protein-positive astrocytes was not fully complete until postnatal day 50. Glial fibrillary acidic protein-positive radial glial fibres were present after birth and disappeared towards the end of the third postnatal week. Staining of astrocytes in the white matter and in cortical layers I and VI reached an adult density at postnatal days 8 and 20, respectively. A progressively later development of glial fibrillary acidic protein-positive astrocytes was observed in cortical layers II-V which was completed between postnatal days 47 and 50. In the adult rat visual cortex glial fibrillary acidic protein-positive astrocytes were especially dense in layers I and VI, moderate in layers II/III and V and nearly absent in layer IV and lower layer III. The time course of the loss of vimentin and the gradual appearance of glial fibrillary acidic protein immunoreactivity in the visual cortex is considered as an index of astrocytic maturation and the spatiotemporal sequence of this maturation pattern is discussed in terms of reciprocal neuron-astrocyte interactions during brain development.

The response of the cerebral hemisphere of the rat to injury. II. The neonatal rat

The response to injury of the cerebrum of the neonatal rat was studied in knife wounds by using both light and electron microscopical, and immunohistochemical, techniques. The rats were injured at 2, 4, 8, 12, 16 and 20 dayspost natumand the tissues examined 8 days later. A mature scar, that is, a layer of fibrous tissue separated from the injured neuropile by a glia limitans, is not formed in the brains of rats lesioned before 8 dayspost natum. Before this time, the neuropile of the severed hemisphere grows together and both the glia limitans externa and ventricular lining are repaired. The only evidence of the wound, 20 days after injury, is a subpial and periventricular accumulation of astrocytes and occasional groups of blood vessels; elsewhere glial and neuronal processes traverse the wound obliterating all signs of the original lesion. After 8 dayspost natum, scar tissue is deposited. The scar first appears in the superficial cortex as fibroblasts and macrophages invade from the meninges. With increasing age at injury, these cells penetrate more deeply and, after 16 dayspost natumat injury, the entire lesion contains these cells. Concomitantly, a glia limitans is formed over the walls of the lesion, firstly in the superficial cortex continuous with the glia limitans externa, and successively in the deeper cortex, white matter and corpus striatum as the meningeal fibroblasts and macrophages invade these regions. In the developing cerebrum, injured before 8 dayspost natum, the failure to form a scar is unrelated to the maturity of the astrocytes and fibroblasts, because both interact to regenerate the glia limitans externa. The development of a scar, in animals injured after 8 dayspost natum, is correlated with the failure of both axonal and dendritic regeneration. Because there are few oligodendrocytes, and no myelin, it appears that inhibition of axonal and dendritic growth is linked to scar formation, and not to putative inhibitory substrates such as those on the surface of oligodendrocytes, CNS scarring may be initiated by the invasion of fibroblasts and macrophages from the meninges into the injured neuropile. The possible reasons why these mesenchymal cells fail to penetrate before 8 dayspost natumare discussed.

Postnatal Development of Protein Kinase C-like Immunoreactivity in the Cat Visual Cortex

The postnatal development of protein kinase C isozymes II and III (PkCII/III) was investigted in the cat visual cortex by applying immunohistochemical methods with a monoclonal antibody against PkC(II/III). PkC(II/III)-like immunoreactivity was found in astrocytes and neurons. All astrocytes but only a few of the immunoreactive neurons were homogeneously labelled. The majority of the latter exhibited a punctate distribution of reaction product. The staining pattern of neurons and glial cells showed developmental changes until at least 18 months of age. These were characterized by (1) a gradual increase of immunolabelled astrocytes, (2) an abrupt appearance of immunopositive neurons at 4 weeks of age, (3) an aggregation of immunolabelled neurons in a well-delineated band in lower layer IV between 4 weeks and 12 months of age, and (4) a decrease in number of PkC(II/III)-positive neurons after 12 months of age. These developmental changes in the expression of PkC(II/III)-like immunoreactivity correlate well with the time course and the laminar selectivity of experience-dependent malleability. Moreover, they correspond closely to changes in several systems that contribute to PkC-activation and are thought to be involved in use-dependent neuronal plasticity. Thus, we consider these results as compatible with the hypothesis that the PkC isozymes II and III participate in cellular mechanisms underlying use-dependent plasticity.

Boundaries defined by adhesion molecules during development of the cerebral cortex: the J1/tenascin glycoprotein in the mouse somatosensory cortical barrel field

The distribution of the 200/220 KDa J1 glycoprotein (J1-200/220), within the developing vibrissae-related barrel field of the mouse somatosensory cortex, was studied by immunocytochemistry using a monoclonal antibody. J1-200/220, a member of the L2/HNK-1 family of adhesion molecules, also appears to be the mouse homologue of tenascin. J1/tenascin-positive barrel-like structures are visible in the somatosensory cortex between 24 and 48 hr after birth, with the molecule present in prospective barrel boundaries. Immunoelectronmicroscopy reveals labeling that is associated with glial and neuronal plasma membranes, as well as glial end-feet on blood vessels. A possible major source of J1/tenascin expression at this time is astrocyte precursor cells and radial glia. In the putative astrocyte precursor cells, immunolabeling was observed within organelles including the Golgi apparatus. At P6-7 J1/tenascin is most prevalent within prospective interbarrel septae. J1/tenascin-positive barrel boundaries are barely visible on P9 and not observed on P16. The findings indicate that J1/tenascin represents a major component of previously described "hidden" boundaries that we have seen during development using other methodologies. The expression of adhesion molecule-rich boundaries during the critical stages of barrel field formation indicates roles for such molecules during specific cerebral cortical pattern formation events.

Role of cell death in the topogenesis of neuronal distributions in the developing cat retinal ganglion cell layer

The neurons of the developing and adult ganglion cell layer of the cat retina may be morphologically divided into two major populations. One population, the classic neurons, is mainly composed of ganglion cells, and of a small percentage of displaced amacrines, the bar cells. The remaining neurons are microneurons, which make up the majority of the displaced amacrine population. The loss of ganglion cells during the development has been attributed to cell death. It has alternatively been suggested that some ganglion cells may lose their axon and be transformed into displaced amacrine cells, without degeneration of the cell soma. Reexamination of foetal and postnatal cat retinas confirms the presence of degenerating cells in the ganglion cell layer. Their number appears to be at a maximum on embryonic day (E) 57 but declines rapidly until birth. The peak of cell death thus coincides with the decline in optic nerve fibre counts and classical neuron or ganglion cell numbers. Some cells in early stages of degeneration resemble classical neurons, but the original morphology of those advanced stages of degeneration could not be identified, nor was it possible to identify pyknotic microneurons at any stage. Substantial degeneration of the microneurons is not suggested but if it occurs, it is masked by an overall increase in the population of these cells before birth. Cell death in the microneuron population thus cannot yet be ruled out. It has been argued in the literature that fragments of degenerating cells in developing neural tissue are cleared by microglia within 10-14 hours. In order to test the hypothesis that operation of cell death can alone account for the observed loss of classical neurons in the foetal cat retina, we have modelled the effect of various presumed clearance times on corresponding neuronal population magnitudes. It is found that a constant clearance time of 10-24 hours would be consistent with the observed loss of classical neurons before birth. If this is true, then no ganglion cells would remain for transformation into amacrine cells. The absolute density of degenerating or pyknotic cells is found to be relatively constant across the retina. However their density expressed as a percentage of the local population of classical neurons is markedly higher in peripheral than central retina. In the former region, they compose more than 10% of classical neurons at stage E57. On the same day, the percentage distribution maps define an elongated central area containing only 3-5% pyknotic profiles. This region corresponds to the location of the future visual streak.(ABSTRACT TRUNCATED AT 400 WORDS)

Developing neuronal populations of the cat retinal ganglion cell layer

An improved flat-mount procedure demonstrates that the developing ganglion cell layer of the cat retina contains two morphologically distinct populations of presumed neurons at all ages between embryonic day 36 (E36) and adulthood. One population resembles the adult "classical neurons" composing the ganglion cells and bar-cells of Hughes, while the remaining cells, which are smaller and possess much less Nissl substance, presumably correspond to precursors of the adult microneurons. Although the total neuron population of the retinal ganglion cell layer remains quite constant at all studied ages, its component subpopulations alter significantly during prenatal development; some 50% of classical neurons disappear before birth and the microneuron population doubles during the same period. An obvious centroperipheral gradient exists for classical neurons by stage E47, but the microneuron density gradient only becomes apparent at birth. A 2:1 centroperipheral ratio for the total neuron population is also apparent at E47. Centroperipheral neuronal density gradients continue to increase during postnatal growth. Loss of classical neurons during prenatal life as a result of cell death or transformation into microneurons, has been postulated as a mechanism for determining neuron density gradients. Cell death does occur in the ganglion cell population but it is not yet established whether microneurons of the ganglion cell layer originate from ganglion cell transformation or migrate as a differentiated class from the ventricular layer. However, it can be concluded that not all microneurons originate from ganglion cell transformation, because the total loss of classical neurons is less than the increase in microneuron numbers during development. The population magnitudes of both neuronal classes in the ganglion cell layer stabilise after birth. However, it is during the postnatal period that the adult cruciate density topography is achieved by both populations. It is concluded that differential areal growth is the prime mechanism for postnatal cell redistribution.

Distribution of neurons and glia in the visual cortex (area 17) of the adult albino rat: a quantitative description

The neuronal and glial cell composition of the rat visual cortex (area 17) has been determined quantitatively using stereological techniques. The volume numerical densities (number of cells per mm3 of cortex) of neurons and of the principal glial cell types (astroglia, oligodendroglia, and microglia) were calculated from tangential semithin resin sections spaced at regular intervals 50 micron apart throughout the entire depth of the visual cortex. From measurements of cortical and laminar thickness the separate volume numerical densities of neurons and glial cells were derived for each lamina in the cortex. In addition, the absolute numbers of cells in each lamina under 1 mm2 of cortical surface were calculated. The mean cortical volume numerical density of neurons was 60,020 +/- 3840/mm3 (mean +/- SEM; n = 8), and 49,040 +/- 2610/mm3 for the combined glial cell types. Astroglia, oligodendroglia, and microglia were present in a ratio of 6:3:1 respectively. It was determined from neuronal and glial somatic volume estimates that the somata of these cells occupied approximately 13.5% of unit cortical volume, with 81.3% of the unit volume being occupied by cortical neuropil. Using previously published reports that described the laminar composition of neurons in terms of the relative proportions of pyramidal and non-pyramidal cells, the laminar volume numerical densities for these neuronal categories have been derived. In addition, it has been estimated that under 1 mm2 of cortical surface there are 79,500 pyramidal and 7790 non-pyramidal neurons distributed throughout layers 1-6 of the rat visual cortex.

The morphology, number, and distribution of a large population of confirmed displaced amacrine cells in the adult cat retina

The presence of a large population of some 730,000 displaced amacrines is confirmed in the ganglion cell layer of the cat retina. These cells correspond to the microneurons of Hughes and Wieniawa-Narkiewicz (Nature 284:468-470, '80) and the bar-cells of Hughes (J. Comp. Neurol. 197:303-339, '81): a population of profiles of which the majority had previously been presumed to be glia (Stone: J. Comp. Neurol. 12:337-352, '65; J. Comp. Neurol 180:753-772, '78; Hughes: J. Comp. Neurol. 163: 107-128, '75). A sample of such nonganglion cells was identified by Nissl criteria in an area of retina subsequently subjected to serial sectioning and electron microscopy. Such cells form synapses with other processes in the inner plexiform layer. Members of each morphological subclass were found to bear synapses. In some instances, synapses occurred both onto and from the soma and processes of a cell, which is strong evidence for their being displaced amacrines, or preferably, "amacrines of the ganglion cell layer." In confirmation of their amacrine nature, it was established that the microneurons and bar-cells survive optic nerve section for up to 2.5 years. Ganglion cells underwent retrograde degeneration and completely disappeared in a much shorter time. Injection of kainic acid, a neurotoxin, into an eye whose optic nerve had been cut over 2 years previously resulted in the pyknosis of all morphologically classified microneurons and bar-cells without influence on conventional glial cells. These results further support the conclusion that microneurons and bar-cells are neurons and that they collectively form the displaced amacrine population of the cat ganglion cell layer. The topographic distribution of the displaced amacrines resembles that of the ganglion cells in form; their density peaks at 4,500-5,000 cells mm-2 in the area centralis and falls to less than 1,000 mm-2 in peripheral retina. A ganglion cell distribution map based on the latest morphological criteria derived from this study confirms that there are 170,000 ganglion cells in the cat retina. Displaced amacrines form some 80% of the total neuron population of the cat ganglion cell layer. The large population magnitude of these confirmed displaced amacrines implies their nonectopic origin and now provides a fresh insight into the ontogeny of the cat retinal ganglion cell layer.

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.

Gliogenesis in organotypic tissue culture of the spinal cord of the embryonic mouse. I. Immunocytochemical and ultrastructural studies

The technique of organotypic tissue culture offers an opportunity to observe in vitro complex interactions among glial cells and neurons, leading to the formation of myelin. In the present and accompanying work a combined ultrastructural, immunocytochemical and autoradiographic approach was used in a detailed study of the process of gliogenesis. Using immunocytochemical and ultrastructural criteria, differentiation along the oligodendroglia cell line is seen to be initiated a few days later than along the astroglial line. The sequence and timing of oligodendroglial differentiation both ultrastructurally and chemically follow those described in vivo. Formation of myelin has been demonstrated only by oligodendrocytes in which there is continuity between the perikaryal plasmalemma and myelin membranes. Oligodendroglial maturation culminated with the formation of light, medium and dark oligodendrocytes. The periodic acid Schiff-positive, glial fibrillary acidic protein (GFAP)-negative process of radial glial cells at explantation become GFAP-positive within 3 days, as described in vivo. Many of the astrocytes appear to have been derived from radial glial cells. Large numbers of dark glial cells, similar to the so-called 'intermediate glial cells', were seen. These were found to be astrocytes whose appearance probably reflected reaction to explantation-induced injury.

The generation and regeneration of oligodendroglia. A short review

During postnatal development of the higher vertebrate CNS, large populations of oligodendroglia are generated from precursor cells in a very dependable way. In adult lesioned CNS tissues, local populations of oligodendroglia are replenished by proliferation of this replenishment varies from one species to another and also from one lesion type another. Studies on the developmental generation of oligodendroglia are reviewed here, delineating what is known of the early relationships between the CNS glial lineages and of what regulates this development. Contributions from recent cell biological work are considered against the background of morphological and radioautographic results. The quiescent condition of extremely slow turnover in the normal adult CNS is noted, and the dramatic effects of lesions on the neural cell environment are considered. Lesions can trigger proliferation at a much greater rate in the mature oligodendroglial population, as observed both in situ and in tissue culture; in addition to persisting stem cells, the mature cells participate in replenishing the local oligodendroglial population. This regeneration from cells already committed to the oligodendroglial lineage may minimise such disturbing effects of the lesion environment as might distort replenishment of the population from precursor cells.