Division of astroblasts and oligodendroblasts in postnatal rodent brain: evidence for separate astrocyte and oligodendrocyte lineages

Cocultures of meningeal and astrocytic cells--a model for the formation of the glial-limiting membrane

The glial-limiting membrane at the border of the central nervous system (CNS) consists of glial endfeet covered by a basal lamina. The formation of the glia limitans seems to be controlled by adjacent meninges but only little is known about this interaction. In the present study astrocytes and meningeal cells were investigated in vitro to see if cocultures of these cells can serve as a suitable model for the differentiation of the glial-limiting membrane and can be used to define the conditions under which the glial-limiting membrane develops. The following observations were made in cocultures of meningeal and astrocytic cells of two-day-old rats: (i) epithelioid astrocytes were transformed into stellate cells; (ii) single colonies of proliferating epithelioid astrocytes were generated; (iii) the area around these colonies becomes devoid of meningeal cells, which seem to form a circular border around the astroglial islands; (iv) from the glial colonies long thin glial processes grow towards the surrounding meningeal cells, terminating at the site of contact; (v) in the contact zone between meningeal cells and astrocytes irregular shaped deposits of electron dense material resembling a basal lamina were seen. These observations indicate that indeed a structure resembling a glial-limiting membrane develops in cocultures of meningeal and astrocytic cells. Its formation depends on the balance of growth promoting effects of meningeal cells on astrocytes and growth inhibiting effects of astrocytes on meningeal cells. Both activities can be enriched from conditioned media of pure astrocytic or meningeal cell culture. The proposed model of meningo-astrocytic cocultures may be a helpful instrument for further investigations on the formation of the glia limitans.

Development of the chiasm of a marsupial, the quokka wallaby

We have previously shown that the mature optic chiasm of a marsupial is divided morphologically into three regions, two lateral regions in which ipsilaterally projecting axons are confined and a central region containing only contralaterally projecting axons. By contrast, in the chiasms of eutherian (placental) mammals studied to date, there is no tripartite configuration. Ipsilaterally and contralaterally projecting axons from each eye are mixed in the caudal nerve and in each hemichiasm and encounter axons from the opposite eye near the midline of the chiasm. Here, we show that, unlike eutherians, marsupials have astrocytic processes in high concentrations in lateral regions of the nerve and rostral chiasm. Early in development, during the period when optic axons are growing through the chiasm, many intrachiasmatic cells are seen with densities five to eight times higher in lateral than in central chiasmatic regions. Such cells continue to be added to all chiasmatic regions; later in development, considerably more are added centrally, as the chiasm increases in volume. In the mature chiasm, cell densities are similar in all regions. By contrast to the marsupial, cell addition in the chiasm of a placental mammal, the ferret, is almost entirely restricted to later developmental stages, after axons have grown through the chiasm, and there are no obvious spatial variations in the distribution of cells during the period examined. During development, similar to the adult marsupial, ipsilaterally projecting axons do not approach the chiasmatic midline but remain confined laterally. We propose that the cells generated early and seen in high densities in the lateral chiasmatic regions of the marsupial may play a role in guiding retinal axons through this region of pathway selection. These data suggest that there is not a common pattern of developmental mechanisms that control the path of axons through the chiasm of different mammals.

Glial differentiation: a review with implications for new directions in neuro-oncology

Major advances in cell culture techniques, immunology, and molecular biology during the last 10 years have led to significant progress in understanding the process of normal glial differentiation. This article summarizes our current understanding of the cellular and molecular basis of glial differentiation based on data obtained in cell culture and reviews current hypotheses regarding the transcriptional control of the gene switching that controls differentiation. Understanding normal glial differentiation has potentially far-reaching implications for developing new forms of treatment for patients with glial neoplasms. If oncogenesis truly involves a blockage or a short circuiting of the differentiation process in adult glial progenitor cells, or if it results from dedifferentiation of previously mature cells, then a clear understanding of differentiation may provide a key to understanding and potentially curtailing malignancy. Differentiation agents represent a relatively new class of drugs that effect cellular gene transcription at the nuclear level, probably through alterations in chromatin configuration and/or differential gene induction. These exciting new agents may provide a means of preventing the dedifferentiation of low-grade gliomas or inducing malignant glioma cells to differentiate with minimal toxicity. In the future, genetic therapy has the potential of more specifically rectifying the defect in genetic control that led to oncogenesis in any given tumor.

Patterns of reactivity with anti-glycolipid antibodies in human primary brain tumors

Antibodies against carbohydrates of three glycolipids were used to determine patterns of immunohistochemical reactivity of histologically identifiable cell subpopulations in 101 human primary brain tumors. For all tumor types fibrillary cells, polar cells, and gemistocytes (commonly seen in astrocytomas and ependymomas) stained more frequently for galactosylcerebroside with mAbO1 than small tumor cells and macrophages. Frequency of staining for sulfatide with mAbO4 was fibrillary > polar > small cells = macrophages. Gemistocytes stained more frequently with mAbO4 than polar cells in all tumors except low grade astrocytomas. These data indicate that tumors classified on histological grounds as astrocytic are often stained with antibodies that recognize oligodendrocytes and their progenitors. Thus, anti-glycolipid antibodies used in the study of developmental lineage may offer useful tools for classification of human brain tumors. Staining of fibrillary cells, polar cells, and gemistocytes for paragloboside directly with mAb F1H11 was much less common than with mAbO1, but this increased by pretreatment of the tissues with neuraminidase (F1H11 + N). Of particular note was the finding that small tumor cells frequently stained with F1H11 + N. Evidence that these were not macrophages was obtained using double immunostaining with F1H11 + N and anti-macrophage antibodies. In astrocytomas the frequency of small tumor cells immunostained with F1H11 + N was high grade > anaplastic > low grade, demonstrating a correlation of this tumor cell population with more aggressive astrocytomas. Thus, immunostaining with F1H11 + N may be of value in identifying small, anaplastic tumor cells, especially in small biopsies or tissue taken adjacent to the main tumor mass.

Problems encountered when immunocytochemistry is used for quantitative glial cell identification in autoradiographic studies of cell proliferation in the brain of the unlesioned adult mouse

We have used sections of adult mouse brain to determine whether antibodies specific for oligodendroglia (anti-carbonic anhydrase II, CA II; anti-galactocerebroside, GC; anti-myelin basic protein, MBP) and astroglia (anti-glial fibrillary acidic protein, GFAP; anti-S 100 protein) are suitable for quantitative studies of the proliferation and subsequent differentiation of these cells. Unlesioned adult mice received a single injection of 3H-thymidine (TdR) and were killed between 1 h and 70 days later. Quantitative evaluations of autoradiographs of 2-microns-thick serial sections stained immunocytochemically with the antibodies mentioned above or with Richardson's method for histological control led to the following conclusions. Anti-GC and anti-MBP stained only the oligodendrocytic processes and, thus, cannot be used in well-myelinated brain areas. Anti-CA II stained only a portion of the differentiated oligodendrocytes, but no proliferating cells. Anti-S 100 protein recognized all the astrocytes, but also many (interfascicular) oligodendrocytes. Anti-GFAP stained only a few astrocytes in the unlesioned mouse; all astrocytes may become GFAP-immunopositive only after wounding the brain. Thus, in contrast to in vitro studies, immunocytochemical studies with these antibodies on sections of adult animals cannot be recommended for the quantitative analysis of cell proliferation. In addition, our results show that differentiated glial cells proliferate in adult mice. Astro- and oligodendrocytes divide with the same cell cycle parameters and mode of proliferation up to about 1 month after 3H-TdR injection. In contrast to oligodendrocytes, some astrocytes might re-enter the cycle after a few weeks of quiescence.

Cortical type 2 astrocytes are not dye coupled nor do they express the major gap junction genes found in the central nervous system

The O-2A progenitor cell first described from the rat optic nerve is a bipotential precursor of oligodendrocytes and type 2 astrocytes. Each cell expresses specific markers that distinguish them as unique cell types. O-2A progenitors cultured in high serum preferentially differentiate into type 2 astrocytes and when exposed to defined medium or low serum develop along the oligodendrocyte lineage. We analyzed the gap junction gene expression of type 2 astrocytes to determine if they are coupled to form a syncytium, like their type 1 astrocyte counterparts. Dye coupling experiments demonstrated that cortical type 2 astrocytes are not coupled, while type 1 astrocytes in the same culture dish are highly coupled. Immunocytochemistry revealed the presence of Cx43 in type 1 astrocytes but we could not detect Cx26, 32, or 43 protein in type 2 astrocytes. In situ hybridization did not detect mRNA for any of the three connexin genes in type 2 astrocytes. These data demonstrate that type 2 astrocytes do not express the major gap junction genes found in the central nervous system. The precise function of type 2 astrocytes is not known but the lack of gap junction genes expression suggests that their functions are different from the spatial buffering capacity of type 1 astrocytes.

Postmitotic oligodendrocytes generated during postnatal cerebral development are derived from proliferation of immature oligodendrocytes

The phenotype of proliferating glia is examined during postnatal rodent development by combining immunocytochemistry (ICC) with 3H-thymidine autoradiography (ARG) to identify cells in the S phase of the cell cycle. Antibodies (ABs) which are specific for cells in the oligodendrocyte (OL) lineage were utilized, with emphasis placed upon the proliferation of OLs as it remains unclear whether this cell type divides in situ. The results show that proliferating cells stain with ABs which are specific for OLs and myelin glycolipids. The proliferating OLs (oligodendroblasts), although they do not appear to have formed myelin sheaths, have quite elaborate and distinctive morphologies. These oligodendroblasts give rise to very long, thin processes which in turn have additional branches. Their cytoarchitecture corresponds closely to cells described as oligodendroblasts with electron microscopy and whose processes often appear to be in the initial phase of myelination (Skoff et al: J. Comp. Neurol. 169:291-312, 1976a). These proliferating OLs are still quite immature because the expression of myelin specific proteins is only occasionally observed in 3H-thymidine labeled cells. The phenotype of the oligodendroblasts is quite different from that of proliferating astrocytes (astroblasts). As shown in previous studies (Skoff; Dev. Biol. 139:149-163, 1990), the astroblasts, which are identified by the presence of glial fibrillary acidic protein (GFAP), usually have thick, stubby processes, and both their nucleus and cytoplasm are larger and of lighter density than those found in oligodendroblasts. In early myelinating regions of the cerebrum, glycolipid positive cells account for the majority of the 3H-thymidine labeled cells. This data, when combined with the quantification of proliferating astrocytes (ASs) from previous immunocytochemical and electron microscopic studies, indicate that oligodendroblasts and astroblasts constitute the vast majority of the proliferating glia in the brain and in optic nerve at times when ASs and OLs are being generated. In normal postnatal cerebral development, the immature ASs and OLs which proliferate are the direct, immediate precursors for most postmitotic ASs and OLs.

Three-dimensional morphology of astrocytes and oligodendrocytes in the intact mouse optic nerve

The three-dimensional morphology of astrocytes and oligodendrocytes was analysed in the isolated intact mature mouse optic nerve, by correlating laser scanning confocal microscopy and camera lucida drawings of single cells, dye-filled with lysinated rhodamine dextran or horseradish peroxidase, respectively. These techniques enabled the entire process field of single dye-filled cells to be visualized in all planes and resolved the fine details of glial morphology. Morphometric analysis showed that the processes of all astrocytes had branches ending at the pial surface, on blood vessels, and freely in the nerve; branches ending in the nerve were described to end at nodes of Ranvier in the accompanying paper. Astrocytes were classified into a single morphological population in which each cell subserved multiple functions. The results of this study do not support the contention that astrocytes can be subdivided into two morphological and functional subtypes, namely type-1 and type-2, which have process ending either at the glia limitans or at nodes, respectively. Three-dimensional analysis of oligodendrocyte units, defined as the oligodendrocyte, its processes and the axons it ensheaths, showed the provision of single myelin segments for an average of 19 nearby axons (range 12-35) with a mean internodal length of 138 microns (range 50-350 microns). Mouse optic nerve oligodendrocytes were a homogeneous population and were markedly similar to those in the rat optic nerve. The results of our analysis of oligodendrocyte morphology are consistent with the view that the number and internodal length of myelin sheaths supported by a single oligodendrocyte are related to the diameter of the ensheathed axons.

Divergent lineages for oligodendrocytes and astrocytes originating in the neonatal forebrain subventricular zone

Although previous studies have revealed that the prenatal rat ventricular zone contains separate progenitor cells for neurons, astrocytes, and oligodendrocytes during the development of the cerebral cortex as early as the beginning of neurogenesis (Luskin et al., 1993; Grove et al., 1993), it is still unclear whether there are bipotential progenitor cells in the neonatal telencephalic subventricular zone which give rise to both astrocytes and oligodendrocytes during the peak of gliogenesis. To investigate this possibility, discrete groups of clonally related cells, generated by infecting progenitor cells of the neonatal subventricular zone with a retroviral lineage tracer, were analyzed ultrastructurally. An intracerebral injection of retrovirus encoding the reporter gene E. coli beta-galactosidase (lacZ) was made into the subventricular zone of newborn rats. Two weeks later their brains were perfused, sectioned, and histochemically reacted with X-Gal to identify at the light microscopic level clones of lacZ-positive cells. The sections were processed for electron microscopy to enable the identity of clonally related cells to be assessed at the ultrastructural level. All of the clones analyzed contained cells of the same phenotype and could be divided into four distinct types: immature cell clones situated in the subependymal zone surrounding the lateral ventricle, oligodendrocytes clones, and white or gray matter astrocyte clones. Not all of the cells in every clone displayed ultrastructural features of a mature cell. Rather, in some glial clones the lacZ-positive cells appeared to be at different stages of differentiation. However, we never encountered clones which contained both macroglial subtypes or clones containing neurons. Although the existence of bipotential progenitor cells cannot be completely dismissed, our results indicate the absence of progenitor cells in vivo in the neonatal subventricular zone which divide and generate astrocytes and oligodendrocytes.

Basic fibroblast growth factor improves recovery after chemically induced breakdown of myelin-like membranes in pure oligodendrocyte cultures

The putative role of growth factors in remyelination was investigated in pure oligodendrocyte (OL) secondary cultures derived from newborn rat brain. These cells form myelin-like membranes and were used as a model system for toxic attack. A 24 hr treatment with 2.10(-5) M lysophosphatidylcholine (LPC) induced a loss of 59% of the cells in these cultures, with a 64% reduction in [125I]-iododeoxyuridine incorporation compared to untreated controls. An absence of processes and myelin-like sheaths was observed in the remaining cells. Numerous intracytoplasmic inclusions were observed on transmission electron microscopy. Immunocytochemical studies with A2B5 monoclonal antibody (mAb), which recognizes oligodendrocyte-type 2 astrocyte (O-2A) precursors, OL-1 mAb (directed against cell surface sulfatides), and anti-myelin basic protein (anti-MBP) antibody showed that the entire OL lineage was affected at all stages of maturation. A 3 day treatment with 10 ng/ml basic fibroblast growth factor (bFGF) induced reconstruction of myelin-like membranes, albeit less compacted than in untreated controls. The doubling in number of cells and the 46% increase in [125I]-iododeoxyuridine incorporation was due essentially to proliferation of O-2A progenitors. These results indicate that if bFGF release occurs during demyelination, it may participate in myelin repair mechanisms in the central nervous system.

Astrocytes: form, functions, and roles in disease

Astrocytes, once relegated to a mere supportive role in the central nervous system, are now recognized as a heterogeneous class of cells with many important and diverse functions. Major astrocyte functions can be grouped into three categories: guidance and support of neuronal migration during development, maintenance of the neural microenvironment, and modulation of immune reactions by serving as antigen-presenting cells. The concept of astrocytic heterogeneity is critical to understanding the functions and reactions of these cells in disease. Astrocytes from different regions of the brain have diverse biochemical characteristics and may respond in different ways to a variety of injuries. Astrocytic swelling and hypertrophy-hyperplasia are two common reactions to injury. This review covers the morphologic and pathophysiologic findings, time course, and determinants of these two responses. In addition to these common reactions, astrocytes may play a primary role in certain diseases, including epilepsy, neurological dysfunction in liver disease, neurodegenerative disorders such as Parkinson's and Huntington's diseases, and demyelination. Evidence supporting primary involvement of astrocytes in these diseases will be considered.

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

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

Ciliary neurotrophic factor and leukemia inhibitory factor promote the generation, maturation and survival of oligodendrocytes in vitro

We have found that CNTF and LIF are pleiotropic modulators of development in the O-2A lineage. Both molecules enhanced the generation of oligodendrocytes in cultures of dividing O-2A progenitors. CNTF and LIF also promoted oligodendrocyte maturation, as determined by expression of myelin basic protein, and could promote oligodendrocyte survival to an extent comparable with insulin-like growth factor-1 or insulin. In addition, LIF and CNTF both promoted the differentiation of O-2A progenitors into type-2 astrocytes but only when applied in the presence of extracellular matrix (EnMx) derived from cultures of endothelial cells. The ability of CNTF and LIF to enhance differentiation of O-2A progenitors along either of the alternative pathways of oligodendrocyte and astrocyte differentiation suggests that these proteins are able to enhance the process of differentiation per se, while the actual path of differentiation promoted is determined by the presence or absence of additional molecules in the extracellular environment.

Morphology of astrocytes and oligodendrocytes during development in the intact rat optic nerve

The detailed three-dimensional morphology of macroglial cells was determined throughout postnatal development in the intact rat optic nerve, a central nervous system white matter tract. Over 750 cells were analyzed by intracellular injection of horseradish peroxidase or Lucifer Yellow to provide a new perspective of glial differentiation in situ. Retrograde analysis of changes in glial morphology allowed us to identify developmental timetables for three morphological subclasses of astrocytes and oligodendrocytes, and to estimate their time of emergence from undifferentiated glial progenitors. Glial progenitors were recognised throughout postnatal development and persisted in 35-day-old nerves, where we suggest they represent adult progenitor cells. Astrocytes were present at birth, but the majority of these cells developed over the first week as three morphological classes emerged having either transverse, random, or longitudinal process orientation. Several lines of evidence led us to believe that the majority of astrocytes in the rat optic nerve were morphological variations of a single cell type. Young oligodendrocytes were first observed 2 days after birth, indicating that they diverged from progenitors at or near this time. During early development these cells extended a large number of fine processes, which then bifurcated and extended along axons. Later, as myelination proceeded, oligodendrocytes exhibited fewer processes which grew symmetrically and uniformly along the axons, resulting in a highly stereotypic mature oligodendrocyte form. Our analysis of oligodendrocyte growth suggests that these cells did not myelinate axons in a random manner and that axons may influence the myelinating processes of nearby oligodendrocytes.

Neuron-oligodendroglial interactions during central nervous system development

It is well established that a variety of growth factors influence the differentiation of oligodendroglial lineage cells in culture, although little information is available concerning the role and source of these factors in vivo. Developing oligodendroglia are almost constantly in a neuronal environment and would be expected to respond to a variety of signals from neurons that affect their survival, migration, division, maturation and myelin production. However, very little is known about the specific interactions that occur between these two cell types. Here we review the experimental evidence for the influence of neurons on oligodendroglial differentiation, including studies on the effects of both soluble factors and contact dependent events. We also propose a scheme for the control of myelinogenesis via both internal and external signals.

Lines of glial precursor cells immortalised with a temperature-sensitive oncogene give rise to astrocytes and oligodendrocytes following transplantation into demyelinated lesions in the central nervous system

Immortalised lines of murine glial precursor cells expressing the neomycin resistance gene and a temperature-sensitive mutation of the SV 40 T oncogene were established from cultures of oligodendrocytes and precursor cells infected with a replication-incompetent, helper-free retrovirus. At the permissive temperature (33 degrees C), they could be continually propagated in vitro and cells were present expressing the 04 antigen specific for glial precursor cells and oligodendrocytes. At 38 degrees C, where the expression of the T antigen is down regulated, cell division largely ceased. During early passage in vitro, limited differentiation to a more mature phenotype, as evidenced by expression of GFAP and the oligodendrocyte marker 01 was observed at both 33 degrees C and 38 degrees C. When transplanted into demyelinating lesions in the spinal cords of adult rats early passages of the lines yielded myelin-forming oligodendrocytes and astrocytes. Cells from later passages of the lines although failing to synthesise myelin still associated specifically with the demyelinated axons. These experiments demonstrate the retention of physiological properties of these oncogene-carrying glial cells when transplanted in vivo and suggest that such immortalised populations can be used for the isolation of molecules regulating glial cell function.

Expression of janusin (J1-160/180) in the retina and optic nerve of the developing and adult mouse

We have analyzed the expression of the oligodendrocyte-derived extra-cellular matrix molecule janusin (previously termed J1-160/180) in the retina and optic nerve of developing and adult mice using indirect light and electron microscopic immunocytochemistry, immunoblot analysis, and enzyme-linked immunosorbent assay. In the optic nerve, janusin is not detectable in neonatal and only weakly detectable in 7-day-old animals. Expression is at a peak in 2- or 3-week-old animals and subsequently decreases with increasing age. In the retina, expression increases until the third postnatal week and then remains at a constant level. In immunocytochemical investigations at the light microscopic level, janusin was found in the myelinated regions of the nerve with spots of increased immunoreactivity possibly corresponding to an accumulation of the molecule at the nodes of Ranvier. At the electron microscopic level, contact sites between unmyelinated axons, between axons and glial cells, and between axons and processes of myelinating oligodendrocytes were immunoreactive. Cell surfaces of astrocytes at the periphery of the nerve and forming the glial-limiting membrane, in contrast, were only weakly immunopositive or negative. In cell cultures of young postnatal mouse or rat optic nerves, oligodendrocytes and type-2 astrocytes, but not type-1 astrocytes were stained by janusin antibodies. In the oligodendrocyte-free retina, janusin was detectable in association with neuronal cell surfaces, but not with cell surfaces of Müller cells or retinal astrocytes. Our observations indicate that expression of janusin in the optic nerve and in the retina is developmentally differentially regulated and that other cell types, in addition to oligodendrocytes, express the molecule. Since the time course of janusin expression in the optic nerve coincides with the appearance of oligodendrocytes and myelin and since janusin is associated with cell surfaces of oligodendrocytes and outer aspects of myelin sheaths and is concentrated at nodes of Ranvier, we suggest that janusin is functionally involved in the process of myelination.

Dipeptidyl peptidase II in astrocytes of the rat brain. Meningeal cells increase enzymic activity in cultivated astrocytes

Astrocytes grown in media conditioned by meningeal cells (MCM) develop cellular processes and markedly increased protein per cell. One protein component affected is the dipeptidyl peptidase II (DPP II). The increase of DPP II activity is dose- and time-dependent and can also be elicited by the second messenger cAMP. More mature astrocytes express higher levels of DPP II than immature proliferating astrocytes. The rate of proliferation of astrocytes is markedly enhanced by enriched MCM. These observations lead to the assumption that DPP II has a function within the catabolic processes of cellular differentiation. To assess whether the in vitro results may reflect in vivo conditions, we investigated the postnatal development of DPP II in the rat brain. Differentiating astrocytes in vivo are especially found early postnatally and, indeed, during this period high specific activities are found in brain. Depending on the region investigated DPP II activities decrease within the first ten days to one fourth of their P2 level and finally reach at about similar levels in all brain regions. Exceptions are the hypothalamus, where the activity is generally 1.5- to 3-fold higher than elsewhere in brain, and pons and mesencephalon, where the perinatal activity peak is lacking. The bulk activity of DPP II in immature rat brains is attributed to differentiating astrocytes loosing it in later postnatal stages due to a neuronal influence.

Immunotyping of radial glia and their glial derivatives during development of the rat spinal cord

The differentiation of glia in the central nervous system is not well understood. A major problem is the absence of an objective identification system for involved cells, particularly the early-appearing radial glia. The intermediate filament structural proteins vimentin and glial fibrillary acidic protein have been used to define the early and late stages, respectively, of astrocyte development. However, because of the non-specificity of vimentin and the temporal overlap in expression patterns of both proteins, it is difficult to refine our view of the process. This is especially true of the early differentiation events involving radial glia. Using the developmentally-expressed intermediate filament-associated protein IFAP-70/280 kD in conjunction with vimentin and glial fibrillary acidic protein markers, a comprehensive investigation of this problem was undertaken using immunofluorescence microscopy of developing rat spinal cord (E13-P28 plus adult). The phenotypes of the cells were defined on the basis of their immunologic composition with respect to IFAP-70/280 kD (I), vimentin (V) and GFAP (G). A definitive immunotype for radial glia was established, viz, I+/V+/G-; thus reliance upon strictly morphological criteria for this early developmental cell was no longer necessary. Based upon the immunotypes of the cells involved, four major stages of macroglial development were delineated: (1) radial glia (I+/V+/G-); (2) macroglial progenitors (I+/V+/G+); (3) immature macroglia (I-/V+/G+); and (4) mature astrocytes (I-/V+/G+ primarily in white matter and I-/V-/G+, the predominant type in gray matter). It is of interest to note that the cells of the floor plate were distinguished from radial glia by their lack of IFAP-70/280 kD immunoreactivity. Introduction of the IFAP-70/280 kD marker has therefore provided a more refined interpretation of the various differentiation stages from radial glia to mature astrocytes.

Urokinase-type plasminogen activator expression by neurons and oligodendrocytes during process outgrowth in developing rat brain

The expression of tissue- and urokinase-type plasminogen activators has been studied in developing cerebellum, hippocampus, cerebral cortex, olfactory bulb and olfactory mucosa of the rat by in situ hybridization. All identifiable neurons express urokinase mRNA from an early stage in their development, and this expression appears to coincide with the onset of axogenesis. For cerebellar granule cells, both axonal growth and urokinase expression are initiated before they migrate from the external granule layer; for the majority of neocortical neurons, however, both processes are commenced after the cells have migrated to the cortical plate. Neurons continue to express this protease in the adult. The large projection neurons exhibit the highest levels of message, the smaller interneurons having much lower levels except for hippocampal granule cells, which have notably high levels of expression. Glial cells generally do not express urokinase message, except for transient expression by oligodendrocytes in developing fibre tracts during the period of myelination. Thus for both neurons and oligodendrocytes, the onset of urokinase-type plasminogen activator expression coincides with their initiation of major process outgrowth, although neurons maintain this expression in the adult, possibly to retain a degree of synaptic plasticity. In contrast, although high levels of message for the related protease, tissue plasminogen activator, are found in the embryonic floor plate, in postnatal brain it is abundantly expressed only by ventricular ependymal cells and by cells in connective tissue surrounding the olfactory nerve.

Inflammation and the brain

Inflammation in the brain selectively damages the myelin sheath resulting in a variety of clinical syndromes of which the most common is multiple sclerosis. In these disorders, the areas of inflammation and demyelination can be identified in life by magnetic resonance imaging. Events occurring at the blood-brain barrier depend on T-cell activation, which increases immune surveillance within the central nervous system. T-cells activated against brain antigens persist to establish the conditions needed for inflammatory demyelination and this depends on local release of cytokines, culminating in removal of oligodendrocytes and their myelin lamellae by macrophages or microglia. These interactions involve binding between receptors present on microglia for the Fc portion of antibody and complement components to corresponding ligands on target cells. Taken together, the evidence from clinical and experimental studies provides a rationale for the issue of immunological treatments in patients with multiple sclerosis.

Rat cerebral cortical neurons in primary culture release a mitogen specific for early (GD3+/04-) oligodendroglial progenitors

The development of cells of the oligodendroglial lineage, from immature progenitor to myelinating cells, occurs largely in a neuronal environment, yet little is known about specific interactions between these 2 cell types. We have tested the effects of medium conditioned by cultures of rat cerebral cortical neurons (CCM), cerebellar granule interneurons (GCM), and a dorsal root ganglion derived cell line (NDCM) on cells of the oligodendroglial lineage in culture. Different stages of the lineage were defined using the cell surface antigens GD3, 04, and GalC. CCM and NDCM were mitogenic for the early GD3+/04- oligodendroglial progenitor, whereas GCM was mitogenic for the later GD3+/04+ stage. Neutralising antibodies to PDGF and bFGF were able to eliminate the mitogenic activity of NDCM and GCM, respectively, but did not abolish the mitogenic effect of CCM. We have demonstrated that neurons in primary culture from distinct CNS regions exert different influences on cells of the oligodendroglial lineage, and specifically that cortical neurons release an unknown mitogen for GD3+/04- oligodendroglial progenitors.

Development and differentiation of glial precursor cells in the rat cerebellum

The development and differentiation of bipotential glial precursor cells has been studied extensively in tissue culture, but little is known about the distribution and fate of these cells within intact animals. To analyze the development of glial progenitor cells in the developing rat cerebellum, we utilized immunofluorescent, immunocytochemical, and autoradiographic techniques. Glial progenitor cells were identified with antibodies against the NG2 chondroitin-sulfate proteoglycan, a cell-surface antigen of 02A progenitor cells in vitro, and the distribution of this marker antigen was compared to that of marker antigens that identify immature astrocytes, mature astrocytes, oligodendrocyte precursors, and mature oligodendrocytes. Cells expressing the NG2 antigen appeared in the cerebellum during the last 3-4 days of embryonic life. Over the first 10 days of postnatal life, the NG2-labeled cells incorporated 3H-thymidine into their nuclei and their total number increased. At all ages examined, the NG2-labeled cells did not contain either vimentin-like or glial fibrillary acidic protein (GFAP)-like immunoreactivity, suggesting that they do not develop along an astrocytic pathway. NG2-labeled cells of embryonic animals expressed GD3 ganglioside antigens, a property of oligodendrocyte precursors, whereas NG2-positive cells of postnatal animals did not express GD3 immunoreactivity. Nevertheless, the NG2-labeled cells of the nascent white matter expressed oligodendrocyte-specific marker antigens. Cells lying outside of the white matter continued to express the NG2 antigen. In adult animals, the NG2-labeled cells incorporated 3H-thymidine. Glial cells isolated from adult animals and grown in tissue culture express the NG2 antigen and display the phenotypic plasticity characteristic of 02A progenitor cells. These findings demonstrate that a population of glial progenitor cells is extensive within both young and adult animals.

Distinct effects of bFGF and PDGF on oligodendrocyte progenitor cells

We have compared the effects of platelet-derived (PDGF) and basic fibroblast (bFGF) growth factors on the shape, migration, and differentiation of oligodendrocyte progenitor cells, the precursors of myelin-forming cells in the CNS. In the presence of bFGF, oligodendrocyte progenitors purified from rat neonatal brain cultures were stellate, non-motile, and had a morphological complexity of 1.26 +/- 0.03 as measured by fractal dimension (D). These cells expressed transcripts encoding the POU-homeodomain transcription factor Oct-6, but not myelin genes. Upon addition of PDGF, bFGF-treated cells became motile and twofold less complex in shape (D = 1.19 +/- 0.03). These changes occurred within 6 +/- 4 h and were dependent on de novo transcription and translation, but not DNA synthesis. Upon removal of PDGF the cells reverted to their stellate shape (D = 1.26). Removal of both bFGF and PDGF resulted in oligodendrocyte differentiation after 3 days, with a fourfold increase in complexity of shape (D = 1.55 +/- 0.08), loss of Oct-6 transcripts, and gain of myelin transcripts. Thus PDGF is both necessary and sufficient to induce a motile state in progenitor cells growing in the presence of bFGF. Together with our previous data (McKinnon et al.: Neuron 5:603, 1990), our results suggest that bFGF and PDGF may control distinct phases of proliferation and migration of oligodendrocyte progenitor cells in vivo.

Expression of myelin protein genes and other myelin components in an oligodendrocytic cell line conditionally immortalized with a temperature-sensitive retrovirus

We have conditionally immortalized oligodendrocytes isolated from normal and shiverer primary mouse brain cultures through the use of the retroviral vector ZIPSVtsA58. This vector encodes an immortalizing thermolabile simian virus 40 large T antigen (Tag) and allows for clonal selection by conferring neomycin (G418) resistance. We isolated 14 shiverer and 10 normal lines that expressed the early oligodendrocyte marker 2',3'-cyclic nucleotide 3'-phosphodiesterase mRNA. These cell lines grew continuously at the permissive temperature (34 degrees C) and displayed Tag nuclear immunostaining. On shifting to nonpermissive temperatures (39 degrees C), the cells showed rapid arrested cell growth and loss of Tag staining. One line (N20.1) engineered from normal oligodendrocytes also expressed myelin basic protein (MBP) and proteolipid protein (PLP) mRNAs, genes normally expressed by mature, differentiated oligodendrocytes. No differences in any of the myelin-specific protein mRNA levels were observed in N20.1 cells grown at 39 degrees C for > 9 days compared with cells maintained at 34 degrees C. Immunocytochemical staining revealed N20.1 cells to be positive for the oligodendrocyte surface markers--galactocerebroside, A007, and A2B5. However, MBP and PLP polypeptides could not be detected by western blot or immunocytochemical staining at either the permissive or nonpermissive temperature. Cell-free protein synthesis experiments indicated that the MBP mRNAs isolated from N20.1 cells were translatable and directed the synthesis of the 17-, 18.5-, and 21.5-kDa MBP isoforms. Analysis of the PLP/DM20 gene splice products by polymerase chain reaction indicated that the expression of DM20 mRNA predominated over that of PLP mRNA in this cell line. Because the cell line expressed the MBP and PLP genes, it represents a "mature" oligodendrocyte, but the splicing patterns of these genes indicate that it is at an early stage of "maturation." This cell line has now been passaged > 40 times with fidelity of phenotype and genotype.

Both oligodendrocytes and astrocytes develop from progenitors in the subventricular zone of postnatal rat forebrain

The developmental fates of subventricular zone (SVZ) cells of the postnatal rat forebrain were determined by retroviral-mediated gene transfer and immunolabeling for glial antigens. A beta-galactosidase-containing retrovirus injected stereotactically into the SVZ infected small, immature cells. By 28 days post-injection labeled cells had appeared in both gray and white matter of the ipsilateral hemisphere. White matter contained labeled oligodendrocytes, but few astrocytes, while neocortex and striatum contained both glial types, often appearing in tightly knit clusters. An analysis after simultaneously injecting alkaline phosphatase- and beta-galactosidase-containing retroviruses showed that cells in each cortical cluster were related. Most clusters contained a single cell type, but approximately 15% contained both astrocytes and oligodendrocytes. These observations strongly suggest that a single SVZ cell can differentiate into both glial types.

O2A progenitor cells transplanted into the neonatal rat brain develop into oligodendrocytes but not astrocytes

The differentiation of the bipotential O2A progenitor cell into an oligodendrocyte or a type 2 astrocyte has been well documented in cell cultures of various regions of the central nervous system. The appropriate tools to prove its existence in vivo have been lacking. We report on an in vitro-in vivo approach that combines stable labeling of an enriched population of cultured O2A progenitors by the fluorescent dye fast blue, followed by their transplantation into neonatal rat brains, which allowed us to study the influence of the brain microenvironment on their lineage decision. The grafted cells survived well and 21 days after grafting nearly all were positive for the oligodendroglial marker galactocerebroside. Surprisingly, the fast blue-positive grafted cells did not stain for the astroglial marker glial fibrillary acidic protein. These results indicate that the O2A progenitor's plasticity is restricted by the in vivo environment, resulting in the developmental exclusion of the type 2 astrocyte initially described in vitro.

Differential myelinogenic capacity of specific developmental stages of the oligodendrocyte lineage upon transplantation into hypomyelinating hosts

The capacity of oligodendrocytes (OLs) and their progenitors to migrate, proliferate, and differentiate in vivo was evaluated by transplanting highly enriched populations of sequential stages of the OL lineage (A2B5+O4-, O4+GalC-, and GalC+) into the telencephalon of the hypomyelinating mouse, shiverer. The shiverer mouse neither expresses the major myelin basic protein (MBP) nor makes normal myelin due to a large deletion in the gene for MBP. Thirty days after transplantation, serial 225 micron sections of the host brain were immunostained with antiserum to MBP and analyzed by confocal microscopy. The presence of MBP+ patches of myelin in the otherwise MBP- host brain allowed a retrospective analysis of the myelinogenic activity of the transplanted progenitors cells. Both the extent of MBP+ myelin and the location of MBP+ structures relative to the initial site of cell deposition were highly dependent on the developmental stage of the transplanted cells. Specifically, A2B5+O4- OL progenitors migrated distances of > or = 600 microns and produced MBP+ patches in nearly every slice of the host brain. An average of over 250 separate patches were found per host brain, some of which had cross-sectional areas of > 250,000 microns2 containing as many as 60 MBP+ OL cell bodies, and with densities of myelination rivaling that of normal brain. In marked contrast, transplantation of O4+GalC- cells produced only small (1,000-25,000 microns2), scattered (25-40 per brain) patches of MBP+ myelin containing one to five cell bodies, all of which were within 50 microns of the needle track or the nearest ventricular surface. GalC+ cells produced MBP+ myelin at a level similar to that of O4+CalC- cells. These data suggest that the developmental transition of OL progenitors from the O4- to the O4+ phenotype is accompanied by a dramatic reduction in the innate capacity of the cells to migrate and survive in vivo. The use of developmentally identified, enriched populations of OL progenitor cells offers the opportunity for more precise analyses of transplantation and remyelination behavior, and relates to clinically relevant studies indicating that contaminant cell types can seriously interfere with the stable integration of donor tissue into the host.

The O-2A(adult) progenitor cell: a glial stem cell of the adult central nervous system

Systematic comparison of the properties of oligodendrocyte-type-2 astrocyte (O-2A) progenitor cells derived from optic nerves of perinatal and adult rats has revealed that these two populations differ in many fundamental properties. In particular, O-2A(perinatal) progenitor cells are rapidly dividing cells capable of generating large numbers of oligodendrocytes over a relatively short time span. Oligodendrocyte differentiation generally occurs synchronously in all members of a clone, thus leading to elimination of that clone from the pool of dividing cells. However, some O-2A(perinatal) progenitors are also capable of giving rise to O-2A(adult) progenitors. These latter cells express many of the characteristics of stem cells of adult animals, including the capacity to undergo asymmetric division and differentiation. We suggest that precursors which function during early development give rise to terminally differentiated end-stage cells and to a second generation of precursors with properties more appropriate for later developmental stages. It is this second generation of precursors which express the properties of stem cells in adult animals, and we therefore further suggest that our work offers novel insights into the possible developmental origin of stem cells.

Development and regeneration in the O-2A lineage: studies in vitro and in vivo

This brief review discusses selected aspects of our studies on the control of division and differentiation of the glial precursor cells which give rise to oligodendrocytes. For more extensive reviews on this topic, the reader is referred to recent reviews by Raff (1989), Richardson et al. (1991), Noble (1991) and Noble et al. (1991).

Proliferation and differentiation of O4+ oligodendrocytes in postnatal rat cerebellum: analysis in unfixed tissue slices using anti-glycolipid antibodies

We report the study of the in vivo morphology, differentiation, and proliferation of oligodendrocytes (OLs) and their progenitors identified by the antiglycolipid antibodies O4, R-mAb, and O1 in postnatal rat cerebellum, using a novel immunocytochemical staining protocol which allows the analysis of the expression of OL-specific glycolipids in live, unfixed brain slices. An analysis of the individual cells identified in double label immunocytochemistry indicated that the order of antigen expression in OLs during in vivo development is, first, antigens recognized by O4, second, antigens recognized R-mAb, and third, antigens recognized by O1. This order of antigen expression is correlated with increasing morphological complexity and is a pattern mimicked in many culture systems. In vivo O4 identified 3 distinct stages of the OL lineage: (1) morphologically simple proligodendrocyte antigen+ (POA+) R-mAb- blast cells localized at the leading edge of myelinogenesis; (2) morphologically more complex R-mAb+O1- cells; and (3) actively myelinating O1+ [i.e., galactocerebroside+ (GalC)] OLs residing within the white matter. Only the POA+R-mAb- cells incorporated BrdU in animals that were prelabeled 3 hr before immunocytochemistry. We have demonstrated in vivo the subdivision of pre-GalC+ OL progenitors into shorter, biologically noteworthy, stages of maturation. A spatial comparison of the cell populations identified by O4, R-mAb, and O1 demonstrated a progressive wave of OL maturation from the base of the cerebellum toward the folia. The data are consistent with the hypothesis that multiprocessed O4+GalC- progenitors are the most mature stage of the OL lineage with significant proliferative capacity and the first postmigratory stage in normal development.

Effects of acidic and basic fibroblast growth factors (aFGF, bFGF) on glial precursor cell proliferation: age dependency and brain region specificity

Acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF) are present in high levels in most areas of the embryonic rodent brain. To begin to understand the role of these growth factors in brain development, the effects of aFGF and bFGF on dissociated cell cultures prepared from embryonic and neonatal rat brain were studied. Addition of aFGF and heparin or bFGF alone to serum-free cultures of the dissociated Embryonic Day (E) 14.5 mesencephalon stimulates cell proliferation, as judged by [3H]thymidine autoradiography, leading to a maximal 75-fold increase in the total number of cells. This effect is dose-dependent with half-maximal increases at concentrations of about 5-6 ng/ml of aFGF or bFGF and is inhibited by the FGF antagonist HBGF-1U. The effect of aFGF on cell proliferation in cultures prepared from E14.5 mesencephalon is similar to that in cultures prepared from E14.5 cortex. However, in cultures prepared from E14.5 rhombencephalon or diencephalon, the proliferative effect of aFGF is much reduced. In all brain areas studied, the proliferative effect of aFGF declines with increasing age. Immunocytochemical analysis of E14.5 mesencephalic cultures demonstrated that the aFGF-induced increase in cell number is due to the proliferation of A2B5-immunoreactive (IR) glial precursor cells, but not of neuronal precursors, fibroblasts, or microglial cells. Moreover, differentiated glial fibrillary acidic protein-IR astrocytes and 2',3'-cyclic nucleotide 3'-phosphohydrolase-IR oligodendrocytes were not observed in cultures continuously treated with aFGF or bFGF, but were observed in high numbers after removal of the growth factors. These results suggest (1) that aFGF and bFGF are potent mitogens for glial precursor cells in all embryonic brain regions, (2) that the magnitude of the effects of aFGF depends on embryonic age and brain region, and (3) that both growth factors inhibit the differentiation of astrocyte or oligodendrocyte precursors. These observations made in vitro strongly support the hypothesis that FGF plays a critical role in gliogenesis and the timing of glial differentiation in the brain.

Localization of Pi class glutathione-S-transferase in the forebrains of neonatal and young rats: evidence for separation of astrocytic and oligodendrocytic lineages

The Yp isoform (Pi class) of glutathione-S-transferase has recently been localized in oligodendrocytes in the brains of mature rats. To examine at what postnatal age Pi first appears in oligodendrocytes or precursor cells, antibodies against Pi were used to immunostain tissue sections from the forebrains of neonatal rats and young rats up to 17 days of age. In the brains of neonates Pi immunofluorescence was observed in ovoid cells in the subependymal layer, and in ovoid cells and cells bearing short, thick processes in the corpus callosum and cingulum. These cells did not immunostain for vimentin. During the first postnatal week Pi-positive cells showed positive immunostaining for ganglioside GD3, which is characteristic of oligodendrocyte precursors, and process-bearing Pi-positive cells appeared in the cingulum and at the lateral borders of the corpus callosum in increasing numbers. During the second postnatal week the cytoplasm of Pi-positive cells became more compact, and the processes thinner, and the Pi-positive cells and their processes began to immunostain for 2',3'-cyclic nucleotide-3'-phosphohydrolase, which is characteristic of immature and mature oligodendrocytes and myelin sheaths. By age 17 days Pi was observed in relatively mature oligodendrocytes. The observations suggest that Pi occurs in oligodendrocyte precursors, immature oligodendrocytes, and mature oligodendrocytes in the postnatal through 17 day old rat forebrain. In the accompanying paper (Cammer and Zhang, '92)--if references are permitted in the Abstract a different glutathione-S-transferase isoform, Yb (Mu class), was localized in cells of the astrocyte lineage, beginning in the forebrains of neonatal rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Localization of mu class glutathione-S-transferase in the forebrains of neonatal and young rats: implications for astrocyte development

The Yb (Mu class) isoform of glutathione-S-transferase has recently been localized in ependymal cells, subependymal cells, and astrocytes in the forebrains of rats 3 weeks to adult in age. It was not known, however, at what age Mu might first be observed during postnatal development and whether the first cells in which it was found would be immature astrocytes or some less differentiated glial precursor cell, if the latter were present in vivo. Tissue sections from the forebrains of neonatal to 16 day old rats were immunostained with antibodies against Mu. In neonates Mu was observed in vimentin-positive cells and their processes adjacent to the lateral ventricles, and in the corpus striatum. The colocalization with vimentin suggested that these were subependymal cells and radial glia. In the corpus striatum the radial glia, while still vimentin-positive, rapidly lost Mu from their radial cell processes, whereas the cell-bodies remained Mu-positive. During the first postnatal week the Mu-positive, glial-fibrillary-acidic-protein (GFAP)-positive cell bodies of immature astrocytes appeared in the corpus striatum. The earliest Mu-positive cells in the immature white matter of the corpus callosum were vimentin-positive and had striking longitudinal processes that also were vimentin- and Mu-positive. Like the processes of radial glia, the longitudinal processes lost their Mu-immunoreactivity, only later and more gradually. Mu-positive, GFAP-positive cells appeared later in the corpus callosum than in the corpus striatum.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of a glial associated antigen GA-1 by monoclonal antibody

A glial antigen (GA-1) was identified by monoclonal antibodies (MAb) raised against C6 rat glioma cells. MAb-7D3 (IgG2a kappa) revealed GA-1 as a single protein band with a Rf value of 0.09 by the use of basic-PAGE Western blot. SDS-PAGE Western blot and radioimmunoprecipitation (RIP) further resolved GA-1 into two subunits with a molecular weight of 200 and 78 kDa respectively. Subcellular localization by immunocytochemical staining revealed its cytosolic presence with a punctate pattern perinuclearly. Significant expression of GA-1 may be detected in 4 glioma or glial cell lines derived from rat brain. However, no expression may be detected in the rest of the 18 mammalian cell lines or primary neural cell cultures examined. All of the above data thereby suggest that GA-1 may be glial specific whereas the epitope of GA-1 defined by MAb-7D3 is species (rat) specific.

Carbonic anhydrase in distinct precursors of astrocytes and oligodendrocytes in the forebrains of neonatal and young rats

Carbonic anhydrase is present in oligodendrocytes and astrocytes in the mature rat brain. Whereas carbonic anhydrase-positive oligodendrocyte precursors had been identified during the first postnatal week, no information was available about the earliest occurrence of carbonic anhydrase in the astrocytic cell line, nor had carbonic anhydrase been detected in astrocytes in neonatal rat brains. Beginning on the first postnatal day, rat brains were double immunostained with anti-carbonic anhydrase II and respective 'markers' for immature and mature astrocytes and oligodendrocytes. During the first postnatal week there were intensely carbonic anhydrase-positive cells which were ovoid or had broad processes. On the basis of their shapes and antigen contents these were considered to be precursors of oligodendrocytes. Beginning on the first postnatal day carbonic anhydrase II was also observed in some vimentin-positive radial glia and in other vimentin-positive cells that differed in their appearance from the immature oligodendrocytes. The vimentin-positive, carbonic anhydrase-positive cells were less smooth-surfaced, and had much finer processes, than the oligodendrocyte precursors. By the third postnatal day there appeared carbonic anhydrase-positive, glial fibrillary acidic protein (GFAP)-positive cells that resembled the vimentin-positive cells. It is concluded that the latter are immature astrocytes and that carbonic anhydrase is in distinct precursors of oligodendrocytes and astrocytes as early as the first postnatal day.

Proligodendroblast antigen (POA), a developmental antigen expressed by A007/O4-positive oligodendrocyte progenitors prior to the appearance of sulfatide and galactocerebroside

Evidence is presented for the immunological identification of a developmental antigen appearing at a critical point in the oligodendroglial lineage. Specifically, monoclonal antibody A007 recognizes cells in the oligodendrocyte lineage at two distinct stages. Analyses of purified lipid standards and lipid extracts from galactocerebroside-positive (GalC+) oligodendrocytes by enzyme-linked immunosorbent assay, lipid dot blot, and immuno-TLC demonstrated that A007 recognizes sulfatide (SUL) and seminolipid. However, neither 35SO4 incorporation into SUL nor SUL accumulation could be detected in A007-positive cells lacking galactocerebroside (i.e., A007+GalC- progenitor cells) present early in development. These data suggest that A007 also recognizes an antigen, named proligodendroblast antigen (POA), that appears during the late stage of oligodendrocyte progenitor development prior to the expression by oligodendrocytes of SUL and GalC. We have previously reported that monoclonal antibody O4 also recognizes not only SUL and seminolipid, but in addition an antigen that appears prior to the expression of SUL and galactocerebroside. In the present study all A007+ cells were also O4+ (and vice versa), and the developmental patterns of the two antibodies appeared to be identical. We conclude that (1) A007 is similar or identical to O4 with respect to its antigenic specificity, and (2) during oligodendrocyte lineage progression both antibodies react first with antigen POA on the surface of the oligodendrocyte progenitor cell prior to the expression of SUL [i.e., A007+O4+(POA+)SUL-GalC- proligodendroblasts], and only later with SUL as terminally differentiating oligodendrocytes emerge (i.e., A007+O4+SUL+GalC+ oligodendrocytes).

Cellular organisation of the optic nerve and the implications for optic neuritis

Opportunities for studying growth, degeneration and repair in the central nervous system have altered over the last decade with the development of techniques for culturing neurones and glia and the availability of immunological or molecular markers that identify separate lineages and their progeny. Much pioneering work has been carried out in the rodent optic nerve but the principles that emerge are representative for other parts of the nervous system; development of neurones and glia may differ substantially in rats and man, so that assumptions must be made in extrapolating from properties of the rat optic nerve to diseases of the human central nervous system.

Differential regulation of cerebroside sulfotransferase and 2',3'-cyclic nucleotide 3'-phosphodiesterase by basic fibroblast growth factor in relation to proliferation in rat oligodendrocyte cultures

Previous results (Fressinaud, C., Sarliève, L.L., and Labourdette, G. J. J. Cell. Physiol., 141:667-674, 1989b) have shown that cerebroside sulfotransferase (CST; EC 2.8.2.11) is enriched in pure rat oligodendrocyte (OL) cultures and that its activity is increased by factors mitogenic for OL precursors and galactocerebroside (GC) expressing OL, such as basic fibroblast growth factor (bFGF), platelet-derived growth factor, and high insulin concentrations. In contrast, transforming growth factor beta or low insulin concentrations were found to be ineffective in this culture system. As bFGF mainly enhanced the proliferation of OL precursors (GC negative cells) rather than that of differentiated (GC+) cells, a relationship between OL precursor proliferation and CST increase was suggested. This hypothesis was first tested in 20-day-old OL cultures grown in chemically defined medium. The dose-response curve of [125I] Iododeoxyuridine ([125I]dUrd) incorporation toward bFGF was parallel to that of CST specific activity, and maximal stimulation was reached at 5 ng/ml bFGF for both. In contrast, 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP; EC 3.1.4.37) specific activity decreased after bFGF treatment. To determine if CST increase was linked to the proliferation of OL precursors induced by bFGF, cell proliferation was blocked by cytosine arabinoside (ARA-C). From 10(-8) to 10(-5) M ARA-C there was a dose-dependent inhibition of cell proliferation and a decrease in CST specific activity, whereas CNP specific activity was enhanced. When the cells were treated with bFGF and 10(-6) M ARA-C together, the proliferation was completely blocked and CST activity decreased by 72% below control values, whereas CNP activity was not significantly decreased. Immunocytochemical studies showed that the number of sulfatide-expressing cells and the number of cycling cells were increased after bFGF treatment, but that there was no overlapping between these two populations. Taken together these results suggest that CST activity and sulfatide expression appear shortly after the arrest of OL precursor division.

Clonal segregation of oligodendrocytes and astrocytes during in vitro differentiation of glial progenitor cells

To study the clonal lineage of the glial progenitor population, isolated from newborn rat brain (Lubetzki et al. J Neurochem 56:671, 1991), we combined somatic transgenesis using a retroviral vector encoding a modified bacterial beta-galactosidase with nuclear localization, and triple immunofluorescence labeling with A2B5, anti-galactosylceramide, and anti-glial acidic fibrillary protein antibodies. This allowed clonal analysis of the postnatal glial lineage with precise phenotypic identification of each cell within the lacZ-positive clones. When infected cells were cultivated under constant conditions, in the presence of either 1% or 10% fetal calf serum (FCS)-containing medium, all the 250 lacZ-positive clusters examined were homogeneous, i.e., either oligodendroglial or astroglial. Mixed astrocyte-oligodendroglial clones were observed when cells cultivated in the presence of 1% FCS were switched to a 10% FCS-containing medium, confirming the bipotentiality of glial progenitor cells (Temple and Raff Nature 313:223, 1985). However, even under the switch culture conditions, segregation into homogeneous clones of either oligodendrocytes or astrocytes still predominated, and the percentage of mixed clones dropped from 25 to 8 or to 3, when the switch took place at 8, 16, or 22 days in vitro, respectively. Two additional observations lead us to suggest that microenvironmental factors are responsible for the clonal segregation of glial progenitor cells: 1) the uneven distribution of oligodendrocyte and astrocyte clusters, the latter being seen mostly on the edge of the coverslips; and 2) the presence, in the vicinity of an homogeneous lacZ-positive clone, of some lacZ-negative cells expressing the same phenotype.

Studies of glial lineage and proliferation in vitro using an early marker for committed oligodendrocytes

The potential of immature glial cells to differentiate into astrocytes (ASs) or oligodendrocytes (OLs) has been examined using a monoclonal antibody (007) that is specific for OLs in vivo. Cells were dissociated from 2-day postnatal mouse cortex and labeled with the 007 antibody 2 hr after plating. The cells which were labeled during this single, brief exposure to the antibody retained the antibody on their surfaces over the course of the experiments. Cells were double stained at various timepoints for residual 007 antibody and either galactocerebroside (GC) or glial fibrillary acidic protein (GFAP). Shortly after plating, most 007+ cells were GC- and none expressed GFAP. These cells were round, although some had begun to extend very short processes. After 96 hr, greater than 95% of cells with residual 007 on their surfaces also expressed GC. By this time, all the 007+ cells had several processes of varying lengths extending from their cell bodies. Cells expressing both 007 and GFAP were never seen. The 007+/GC+ OLs were not induced to differentiate from 007+ bipotential progenitors since they were grown in fetal calf serum. These results show that under our culture conditions the 007 antibody is OL specific. Immunostaining for bromodeoxyuridine, a marker for dividing cells, revealed that some 007+ cells were proliferating. The majority of these proliferating cells had already extended three or more processes. We therefore conclude that immature, process-bearing cells can be committed to the OL lineage at times before they express detectable amounts of GC. Since these young 007+ OLs are actively proliferating, committed cells can serve as an important source of new OLs.

Astroglia in CNS injury

The astroglial response to CNS injury is considered in the context of neuron-glial relationships. Although previous models suggested that astroglial cells present in "scars" impede axon regrowth owing to irreversible changes in the glial cell following injury, recent in vivo and in vitro studies indicate that astroglial cells exhibit considerable plasticity, elevating expression of the glial filament protein and altering expression of properties which support axons, including extracellular matrix components and cell surface adhesion systems. Both in vivo and in vitro studies on neuron-glia interactions in different brain regions suggest that glia express region-specific properties, including ion channels, neurotransmitter uptake and receptor systems, and cell surface adhesion systems. Together these findings suggest that a more detailed analysis of glial response to injury in different brain regions will lead to an appreciation of the diversity of the astroglial response to injury, and its regulation by neuron-glia relationships.

Tracing glial cell lineages in the mammalian forebrain

Astrocytes and oligodendrocytes emerge in late gestational and early post-natal development in the mammalian CNS. The nature, and number, of progenitors for each glial type is a central question. This review will focus upon several unresolved issues relating to glial cell lineages and describe new methods to try to illuminate these issues further: 1) How can developmental patterns by which immature neuroectodermal cells give rise to classes of neurons and glia be understood in the context of lineage? 2) What are the lineage relationships among the various cell classes, how many glial lineages are there in the developing CNS, and how can recent methods of clonal analysis using stable markers be used to clarify lineage patterns? 3) Do patterns of gliogenesis vary in different regions of the CNS? 4) How do patterns of gliogenesis observed in vitro relate to those in vivo?