Glial domains and nerve fiber patterns in the fish retinotectal pathway

Optic nerve fibers run parallel from the retina as far as the optic tract in fish, then suddenly criss-cross into a new pattern matching the tectal map. This change coincides with a unique demarcation between two astroglial territories in the retinotectal pathway, located where the optic chiasm occurs in other vertebrates, which we defined using antibodies directed against intermediate filaments (IF). We found that astroglia in optic nerve territory express an Mr 56,000 IF polypeptide, band 3, which we identify as the fish equivalent of vimentin in mammals. These astrocytic cells lack glial fibrillary acidic protein (GFAP; cf. Dahl and Bignami, 1973). Conversely, glia in brain territory, that is, in the optic tract and elsewhere in the CNS, lack the fish vimentin, but express GFAP. By electron microscopy, we obtained evidence that new retinal axons extend swiftly through the growing optic nerve, where they are tightly shepherded into a narrow track by newly differentiating glial cells, positive for the fish vimentin. In the GFAP-positive glial territory of the optic tract, by contrast, growing axons are slowed down and probably branch. We suggest that this allows them to fasciculate accurately with older fibers and thereby propagate a tectotopic pattern established by pioneer axons in the embryo.     

The effect of activated microglia on astrogliosis parameters in astrocyte cultures

In the diseased central nervous system, astrogliosis is accompanied by microglial activation. Depending on the context of their activation, reactive astrocytes are involved in neuronal survival and regeneration in an either protective or impedimental way. Major reactive changes of astrocytes in vivo are the upregulation of the intermediate filaments GFAP (glial fibrillary acidic protein) and vimentin with accompanying cellular hypertrophy and/or hyperplasia. To examine the involvement of activated microglia in the onset and maintenance of astrogliosis, we used an in vitro model of purified cultures of astrocytes and assessed as parameters for astrogliosis GFAP, vimentin, astroglial hypertrophy and cell growth after treatment with medium conditioned by LPS (lipopolysaccarides)-stimulated microglia. Furthermore, IL-6 as a typically upregulated cytokine in proinflammatory processes in the brain was determined in treated astrocytes. GFAP, the classical marker for astrogliosis, was downregulated on its protein and in parallel with vimentin on its mRNA level. The expression of actin, another cytoskeleton protein used as control, remained unchanged. Ultrastructural studies of astroglial intermediate filaments supported these findings. No hypertrophy was found. Nevertheless, LPS-activated microglia stimulated astrocytes as demonstrated by an increased cell number and an enhanced mRNA expression of IL-6. Resting microglia did not change any of the determined parameters. Our results suggest that the role of activated microglia in astrogliotic processes following injury of the brain has to be reevaluated, as microglia in their activated state might support the onset of astrogliosis on the one hand, but might delay or reduce subsequent glial scar formation on the other hand.     

Localization of Nogo and its receptor in the optic pathway of mouse embryos

We have investigated the localization of Nogo, an inhibitory protein acting on regenerating axons in the adult central nervous system, in the embryonic mouse retinofugal pathway during the major period of axon growth into the optic chiasm. In the retina, Nogo protein was localized on the neuroepithelial cells at E12 and at later stages (E13-E17) on radial glial cells. Colocalization studies showed expression of Nogo on vimentin-positive glia in the retina and at the optic nerve head but not on most of the TuJ1- and islet-1-immunoreactive neurons. Only a few immature neurons in the ventricular and peripheral regions of the E13 retina were immunoreactive to Nogo. In the ventral diencephalon, Nogo was expressed on radial glia, most strongly on the dense radial glial midline raphe within the chiasm where uncrossed axons turn and in the initial segment of the optic tract. In vitro studies showed that the Nogo receptor (NgR) was expressed on the neurites and growth cones from both the ventral temporal and dorsal nasal quadrant of the retina. In the optic pathway, NgR staining was obvious in the vitreal regions of the retina and on axons in the optic stalk and the optic tract, but not in the chiasm. These expression patterns suggest an interaction of Nogo with its receptor in the mouse retinofugal pathway, which may be involved in guiding axons into the optic pathway and in governing the routing of axons in the optic chiasm.     

Behaviour of macroglial cells,as identified by their intermediate filament complement,during optic nerve regeneration of Xenopus tadpole

Assessment of glial cell behaviour during optic nerve (ON) regeneration in Xenopus tadpoles is hampered by the lack of classical cellular markers that distinguish different glial cells in mammals. We thus have characterized the intermediate filament (IF) complement of tadpole glial cells and used it to follow the fate of glial cell subsets during the first 10 days after ON crush. Glial cells synthesize a restricted number of cytokeratin (CK) species and vimentin. This pattern remains essentially unchanged during metamorphosis and regeneration. However, vimentin turnover is specifically enhanced after injury. The expression of CKs and vimentin has been followed immunocytochemically in situ and in isolated cells recovered from dissociated ON segments. In the normal nerve, 79% of ramified glial cells express both CK and vimentin, 1% CK and 4% vimentin only, whereas 16% express neither IF protein. We tentatively classified CK expressing cells as mature astrocytes and those without IF proteins as oligodendrocytes. In the regenerating ON, the relative number of oligodendrocytes is decreased, while the astrocytic subset becomes accordingly larger but is decreased by day 10 already in favour of cells expressing vimentin only. Astrocytes invade the lesion site soon after crush, arrange into a central core within the distal nerve segment and establish a peripheral scaffold that is readily crossed by axons. Unlike mammalian astrocytes that remain absent from the lesion site but form a scar at some distance to it, amphibian astrocytes appear to provide active guidance to axons growing through the lesion site. © 1995 Wiley-Liss, Inc.     

Transformation of cells of astrocyte lineage into macrophage-like cells in organotypic cultures of mouse spinal cord tissue

Phagocytic cells on the surface of the explants and their relationships to the surface were examined morphologically and immunocytochemically in organotypic cultures of mouse spinal cord tissue. Phagocytic cells were rounded, had smooth cytoplasmic surfaces and were occasionally closely apposed to underlying cells by junctional complexes. These cells contained dense bodies, vacuoles, smooth and coated vesicles, a few microtubules and bundles of intermediate filaments similar to astroglial filaments. The superficial layer of the explant which usually consisted of astroglial cell bodies and their processes, sometimes contained immature neuroepithelial cells with numerous free ribosomes, centrioles, Golgi apparatus, microtubules and infrequently, intermediate filaments. Overall, the cells resembled poorly differentiated astrocytes. Numerous dense bodies and coated vesicles were observed in some of these immature cells as well as in astrocytes in the surface layer of the explant. Cytoplasmic bridges between immature cells within the explant and phagocytic cells on the surface were observed. Immunocytochemistry revealed the presence of glial fibrillary acidic protein within these surface phagocytic cells. It thus appears that immature neuroepithelial cells of astrocytic lineage are capable of transforming into macrophage-like cells in organotypic culture.     

Spatiotemporal distribution of glia in and around the developing mouse optic tract

In the developing mouse optic tract, retinal ganglion cell (RGC) axon position is organized by topography and laterality (i.e., eye-specific or ipsi- and contralateral segregation). Our lab previously showed that ipsilaterally projecting RGCs are segregated to the lateral aspect of the developing optic tract and found that ipsilateral axons self-fasciculate to a greater extent than contralaterally projecting RGC axons in vitro. However, the full complement of axon-intrinsic and -extrinsic factors mediating eye-specific segregation in the tract remain poorly understood. Glia, which are known to express several guidance cues in the visual system and regulate the navigation of ipsilateral and contralateral RGC axons at the optic chiasm, are natural candidates for contributing to eye-specific pre-target axon organization. Here, we investigate the spatiotemporal expression patterns of both putative astrocytes (Aldh1l1+ cells) and microglia (Iba1+ cells) in the embryonic and neonatal optic tract. We quantified the localization of ipsilateral RGC axons to the lateral two-thirds of the optic tract and analyzed glia position and distribution relative to eye-specific axon organization. While our results indicate that glial segregation patterns do not strictly align with eye-specific RGC axon segregation in the tract, we identify distinct spatiotemporal organization of both Aldh1l1+ cells and microglia in and around the developing optic tract. These findings inform future research into molecular mechanisms of glial involvement in RGC axon growth and organization in the developing retinogeniculate pathway.     

E587 antigen is upregulated by goldfish oligodendrocytes after optic nerve lesion and supports retinal axon regeneration

The properties of glial cells in lesioned nerves contribute quite substantially to success or failure of axon regeneration in the CNS. Goldfish retinal axons regenerate after optic nerve lesion (ONS) and express the L1-like cell adhesion protein E587 antigen on their surfaces. Goldfish oligodendrocytes in vitro also produce E587 antigen and promote growth of both fish and rat retinal axons. To determine whether glial cells in vivo synthesize E587 antigen, in situ hybridizations with E587 antisense cRNA probes and light- and electron microscopic E587 immunostainings were carried out. After lesion, the goldfish optic nerve/tract contained glial cells expressing E587 mRNA, which were few in number at 6 days after ONS, increased over the following week and declined in number thereafter. Also, E587-immunopositive elongated cells with ultrastructural characteristics of oligodendrocytes were found. Thus, glial cells synthesize E587 antigen in spatiotemporal correlation with retinal axon regeneration. To determine the functional contribution of E587 antigen, axon-oligodendrocyte interactions were monitored in co-culture assays in the presence of Fab fragments of a polyclonal E587 antiserum. E587 Fabs in axon-glia co-cultures prevented the normal tight adhesion of goldfish retinal growth cones to oligodendrocytes and blocked the preferential growth of fish and rat retinal axons on the oligodendrocyte surfaces. The ability of glia in the goldfish visual pathway to upregulate the expression of E587 antigen and the growth supportive effect of oligodendrocyte-associated E587 antigen in vitro suggests that this L1-like adhesion protein promotes retinal axon regeneration in the goldfish CNS. GLIA 23:257–270, 1998. © 1998 Wiley-Liss, Inc.     

Differentiation of radial glia from radial precursor cells and transformation into astrocytes in the developing rat spinal cord

Radial glial cell origins and functions have been studied extensively in the brain; however, questions remain relating to their origin and fate in the spinal cord. In the present study, radial glia are investigated in vivo using the neuroepithelial markers nestin and vimentin and the gliogenic markers GLAST, BLBP, 3CB2, and glial fibrillary acidic protein (GFAP). This has revealed heterogeneity among nestin/vimentin-positive precursor cells and suggests a lineage progression from neuroepithelial cell through to astrocyte in the developing spinal cord. A population of self-renewing radial cells, distinct from an earlier pseudo-stratified neuroepithelium, that resemble radial glial cells in morphology but do not express GLAST, BLBP, or 3CB2, is revealed. These radial cells arise directly from the spinal cord neuroepithelium and are probably the progenitors of neurons and the earliest appearing radial glial cells. GLAST/BLBP-positive radial glia first appear in the ventral cord at E14, and these cells gradually transform through one or more intermediate stages into differentiated astrocytes. Few if any neurons appear to be derived from radial glial cells, which are instead the major sources of astrocytes in the spinal cord. Evidence for the nonradial glial cell origins of some white matter astrocytes is also presented.     

Development of astroglial elements in the suprachiasmatic nucleus of the rat: with special reference to the involvement of the optic nerve

The development of astroglial cells and the effect of the retinohypothalamic tract on it were studied by vimentin and glial fibrillary acidic protein (GFAP) immunocytochemistry in the suprachiasmatic nucleus (SCN) of the rat. At the embryonic stage, vimentin-immunoreactive (VIM-IR) radial glia, precursors of astrocytes, were dominant. However, their filaments vanished in the first few postnatal days. Instead of VIM-IR glial filaments, GFAP-immunoreactive (GFAP-IR) astrocytes appeared at E20 and grew rapidly from the P3 stage. GFAP immunoreactivity in the ventrolateral portion of the SCN (VLSCN) was measured using a computer-assisted image analyzing system. In normal rats, GFAP immunoreactivity showed a stepwise pattern with two slopes at P3-P4 and P20-P25. Bilaterally eye-enucleated rats operated on the day of birth showed lower GFAP immunoreactivity than normal rats and the GFAP immunoreactivity did not increase between P20 and P25 when GFAP-IR glial processes rapidly expand. Electron microscopic investigation at P50 (adult stage) revealed that neurons in the VLSCN had often direct apposition without astroglial processes and the frequency of this finding was significantly higher in eye-enucleated rats than in the control rats. These findings strongly suggest that the postnatal development of astroglial elements, particularly the expansion of GFAP-IR processes in the SCN, is regulated by retinohypothalamic projection.     

NG2 proteoglycan-expressing cells of the adult rat brain: possible involvement in the formation of glial scar astrocytes following stab wound

Stab wound lesion to the adult central nervous system induces strong proliferative response that is followed by the formation of a dense astroglial scar. In order to determine the origin of those astrocytes composing the glial scar, the cell proliferation marker bromodeoxyuridine (BrdU) was administered to lesioned rats that were fixed 3 h or 6 days later. At 3 h after the BrdU administration, labeled nuclei were frequently associated with either NG2(+) cells or microglia/macrophages, but rarely with astrocytes expressing glial fibrillary acidic protein (GFAP). Six days later, by contrast, numerous BrdU-labeled nuclei were associated with astrocytes located along the lesion borders. After the injection of a viral vector of the green fluorescent protein (GFP) into the lesional cavity, GFP was preferentially detected within NG2- or GFAP-labeled cells when lesioned animals were fixed 1 or 6 days after the injections, respectively. The combined detection of glial markers within cells present in the lesioned area indicated that, although they rarely express GFAP, the marker of mature astrocytes, NG2(+) cells located along the lesion borders frequently express nestin and vimentin, i.e., two markers of immature astrocytes. Lastly, chronic treatment of lesioned rats with dexamethasone was found to inhibit the proliferation of NG2(+) cells present within the lesioned area and to subsequently alter the formation of a dense astroglial scar. Taken together, these data strongly suggest that following a surgical lesion, at least a portion of the astrocytes that constitute the glial scar are issued from resident NG2(+) cells.     

Gliosis during optic fiber regeneration in the goldfish: an immunohistochemical study.

Antisera directed against the 48 kDa and 50 kDa cytoskeletal antigens were used to examine changes in the astroglial fabric of the goldfish visual pathways following optic nerve crush. Several major observations are described. First, an optic nerve crush lesion in these animals appears to be devoid of glial cells for at least the first month after surgery. As a corollary, regenerating axons that grow across the lesion may do so over an aglial substrate. Once the axons cross the lesion, their growth is confined to the astroglial domains of the proximal nerve stump. In the optic nerve, gliosis comprises hypertrophy of astrocytic processes such that the open framework characterizing the normal nerve is obscured. In addition, during regeneration, optic nerve glia express large amounts of the 50 kDa cytoskeletal protein, which they ordinarily express at only minimal levels. In the optic tract, gliosis is reflected in a markedly increased expression of the 50 kDa protein as well as an apparent increase in the number and complexity of glial processes. In addition, optic tract glia begin to express the 48 kDa antigen during regeneration. This protein is ordinarily confined for the most part to the optic nerve and is not seen in the tract glia. Finally, no obvious changes were seen in the glia of the optic tectum. These results demonstrate many points of similarity between gliosis in the goldfish and in mammals. However, in some particulars the two responses differ, and it is possible that these differences are related to the differing ability of central axons to regenerate in the two groups of organisms.     

Glial fibrillary acidic protein and vimentin immunohistochemistry in the developing and adult midbrain of the lizard Gallotia galloti

The distribution of glial fibrillary acidic protein (GFAP)- and vimentin-containing cells was studied by immunohistochemistry in the midbrain of the lizard Gallotia galloti. At embryonic stage 32 (E32), vimentin immunoreactivity appeared first in cell bodies located in the ventricular walls, in radial fibers, and subpial end-feet and increased in these structures until E34/E35. Faint GFAP immunoreactivity gradually appeared in the same structures between E34 and E37, and this increased until adulthood, whereas vimentin immunoreactivity decreased after E35, becoming limited to a few end-feet and fibers in the adult, mainly in the tegmentum. Thus, in developing Gallotia midbrain a shift from vimentin-containing to GFAP-containing intermediate filaments begins around E36 or E37. At E40, in addition to the cell bodies in the ependymal area, dispersed GFAP-positive cells, possibly immature astrocytes appeared. These cells showed the same shift. In the adult lizard, GFAP-positive radial glia are still present and coexist with GFAP-positive astrocytes, which are prefentially located in the marginal optic tract and the oculomotor nuclei, but are absent in the fasciculus longitudinalis medialis. Optic tectum, pretectum, tegmentum, and isthmic nuclei are the areas richest in GFAP-positive radial fibers: these were much less abundant in the deep mesencephalic nuclei. Thus, in this lizard, GFAP-positive astrocytes display a clear cut regional distribution: they are present in mesencephalon, whereas they are absent in telencephalon.     

Peripapillary glial cells in the chick retina: A special glial cell type expressing astrocyte, radial glia, neuron, and oligodendrocyte markers throughout development

Peripapillary glial cells of the chick are a special type of glia, not only because of their position, forming a boundary between the retina on one side and the optic nerve head (ONH) and the pecten on the other, but also because although they have the same orientation and similar shape as the retinal Müller cell (a type of radial glia) and express common markers for these cells and astrocytes, they do not express glutamine synthetase (GS) or carbonic anhydrase C (CA-C), enzymes intensely expressed by Müller cells and astrocytes. In this study, we present further molecular characterization of these cells, using immunohistochemistry techniques. We show that peripapillary glial cells express a novel neuron antigen, 3BA8, that in the adult retina is located only in one neuron type (the amacrine cell) and in the inner plexiform layer (IPL). They also express an antigen specific to myelin and oligodendrocytes, MOSP, and a glial antigen, 3CB2, expressed by radial glia and astrocytes throughout the CNS. The study of the developmental expression of these three antigens in the peripapillary glial cell territory shows different spatiotemporal labeling patterns: 3CB2 and 3BA8 are expressed much earlier (embryonic days E3 and E5, respectively) than MOSP (E12), and during a developmental window (E6-E10) 3BA8 labels the peripapillary glial cells intensely and does not label the ONH or the optic nerve (ON), which are labeled later. The expression of 3CB2 is much more intense in the peripapillary glial cells than in Müller cells from early stages of development up to E16, and the expression of MOSP starts earlier in the peripapillary glial cells than in the Müller cells and is maintained with much higher intensity in the peripapillary glial cells throughout development. These findings show that Müller and peripapillary glial cells follow independent courses of differentiation, which together with the fact that the peripapillary glial cells express molecules typical of neurons, oligodendrocytes, radial glia, and astrocytes are evidence that peripapillary glial cells are a unique type of glia in the CNS.     

GFAP-expressing cells in the postnatal subventricular zone display a unique glial phenotype intermediate between radial glia and astrocytes

Neural stem cells in the adult subventricular zone (SVZ) derive from radial glia and express the astroglial marker glial fibrillary acidic protein (GFAP). Thus, they have been termed astrocytes. However, it remains unknown whether these GFAP-expressing cells express the functional features common to astrocytes. Using immunostaining and patch clamp recordings in acute slices from transgenic mice expressing green fluorescent protein (GFP) driven by the promoter of human GFAP, we show that GFAP-expressing cells in the postnatal SVZ display typical glial properties shared by astrocytes and prenatal radial glia such as lack of action potentials, hyperpolarized resting potentials, gap junction coupling, connexin 43 expression, hemichannels, a passive current profile, and functional glutamate transporters. GFAP-expressing cells express both GLAST and GLT-1 glutamate transporters but lack AMPA-type glutamate receptors as reported for dye-coupled astrocytes. However, they lack 100 microM Ba2+-sensitive inwardly rectifying K+ (K(IR)) currents expressed by astrocytes, but display delayed rectifying K+ currents and 1 mM Ba2+-sensitive K+ currents. These currents contribute to K+ transport at rest and maintain hyperpolarized resting potentials. GFAP-expressing cells stained positive for both K(IR)2.1 and K(IR)4.1 channels, two major K(IR) channels in astrocytes. Ependymal cells, which also derive from radial glia and express GFAP, display typical glial properties and K(IR) currents consistent with their postmitotic nature. Our results suggest that GFAP-expressing cells in concert with ependymal cells can perform typical astrocytic functions such as K+ and glutamate buffering in the postnatal SVZ but display a unique set of functional characteristics intermediate between astrocytes and radial glia.     

Different response of astrocytes and Bergmann glial cells to portacaval shunt: an immunohistochemical study in the rat cerebellum.

The present study was performed in order to follow the response of rat cerebellum astroglial cells (Bergmann glial cells and astrocytes) to long-term portacaval shunt (PCS), by means of glial fibrillary acidic protein (GFAP) and vimentin immunoreactivities. Bergmann glia accumulated GFAP in response to PCS, whereas astrocytes decreased GFAP immunoreactivity when compared to control rats. The increase of GFAP occurs in cells located in the cerebellar layer where glutamate is mainly released. Since the vimentin content remained unaltered in response to PCS, when compared to control rats, it can be concluded that only the GFAP filaments are affected by PCS. Nevertheless, GFAP immunoreactivity presents regional differences in the cerebellar astroglial population, and the factors responsible for these variations are still unknown.     

Growth cone morphology varies with position in the developing mouse visual pathway from retina to first targets

We have labeled the growth cones of retinal ganglion cell axons with HRP in intact mouse embryos. This has allowed us to visualize growth cone morphology during outgrowth along an entire CNS pathway from origin to target; to ask whether growth cone forms, and thus behaviors, differ at various points along the pathway; and to study the relationships of growth cones with the cellular environment. During the major period of axon outgrowth between embryonic day (E) 12 and 15, growth cones in the optic nerve are highly elongated (up to 40 microns) and have lamellopodial expansions, but the majority lack the microspikes or filopodia characteristic of many growth cones. Within the optic chiasm (E13-15), most growth cones shorten and spread, and project several short filopodia. In the optic tract, growth cones become more slender and again lack filopodia, resembling sleeker versions of optic nerve growth cones. Near the first target region (lateral geniculate nucleus), growth cones with filopodia arise from individual axon lengths and turn medially toward the target. Within target regions, the branches of immature axon arbors are tipped by minute swellings rather than by the enlarged growth cones prevalent during outgrowth toward targets. Electron-microscopic analysis of identified labeled growth cones in the optic nerve reveal intimate interactions between growth cones and glia or other growth cones in the form of invaginating contacts. In the optic nerve, growth cones contact immature glial (neuroepithelial) cells somewhere along their length, and also envelop bundles of neurites. In the chiasm, single growth cones simultaneously relate to many different profiles. These results demonstrate that in this single pathway from origin to targets, growth cone morphology varies systematically with position along the visual pathway. During outgrowth, simple growth cones are prominent when axons follow well-defined common pathways, and more elaborate filopodial forms appear when growth cones diverge, as they turn or come to decision regions. Together with observations in vitro and in nonmammalian nervous systems in situ, these data serve as reference points for testing to what extent growth cone form reflects intrinsic factors and interactions with the environment.     

Immunohistochemical study of ethylnitrosourea-induced rat gliomas with vimentin and astroprotein (GFAP)

Expression of two different types of intermediate filaments, vimentin filaments and glial filaments, was studied immunohistochemically in experimental rat gliomas. Although vimentin filaments are most commonly seen in mesenchymal cells, recent immunocytochemical study demonstrated that this type of filaments can be recognized also in glial cells during early cell differentiation and in tumor cells of epithelial origin. In the present communication, distribution of vimentin filaments in rat glial tumors was investigated and compared with that of glial filaments by using specific antiserum to each protein subunit, vimentin and astroprotein (GFAP). Ethylnitrosourea (50 mg/kg) was injected subcutaneously into 3 day-old Wistar rats. After four to ten months, brains of animals were removed, fixed in 95% ethanol and embedded in paraffin. Peroxidase-antiperoxidase method was carried out on 6 micron-thick sections. In normal portion of the brain, immunoreaction for vimentin was noted in ependymal cells and in vascular endothelial cells but not in astrocytes. This distribution contrasted with that of astroprotein (GFAP), which distributed in astrocytes but not in normal ependymal cells. These findings confirmed that the two antisera used in the present study do not crossreact to each other. In contrast to the absence of vimentin immunoreaction in normal astrocytes, a number of tumor cells showed positive reaction to the antiserum to vimentin. Mixed glioma with astrocytoma and oligodendroglioma had both astroprotein (GFAP)-positive and negative cells. Well developed cellular processes were noted in astroprotein (GFAP)-positive cells (astrocytoma cells). Weak immunoreaction for vimentin was noted in those cells.(ABSTRACT TRUNCATED AT 250 WORDS)     

Origin of retinal astrocytes in the rat: evidence of migration from the optic nerve

To test recent ideas on the origin of retinal astrocytes, we have studied the spread of astrocytes in the developing retina of the albino rat. Astrocytes were identified with antibodies to their intermediate filaments, glial fibrillary acidic protein (GFAP). Astrocytes were first detected at E(embryonic day) 18, forming a corona of processes around the optic disc. Over subsequent days, astrocytes extended over the retina, covering approximately 35% of the retina at birth (typically E21-22) and reaching the edge of the retina by P(postnatal day)8. As they spread, astrocytes were closely associated with the developing vasculature, spreading ahead of patent vessels by a small but distinct margin. The most peripheral astrocytes assumed a bipolar morphology and extended processes towards the margin of the retina. Astrocytes nearer the optic disc showed the stellate shape characteristic of mature cells. The appearance of astrocytes at the optic disc at E18, 2 days after the appearance of type-1 astrocytes in the optic nerve (Miller et al.: Dev. Biol. 111:35-41, '85), suggests that retinal astrocytes may be type-1 astrocytes generated in the optic nerve. Watanabe and Raff (Nature 332:834-837, '88) have recently reported an independent study supporting the same conclusions.     

Cyclopia: histogenesis of the single optic nerve

We report a case of cyclopia with alobar holoprosencephaly in a fetus examined at 18 weeks of gestation. There was a single eyelid opening and two small eyeballs showing partial fusion at the posterior portion within a single orbit. A single optic nerve emerged from the midline of the posterior portion of the eyeballs. Histologically, within the nerve, a highly cellular central zone and a less cellular marginal zone were distinguishable. The former was composed mainly of glial cells, and only the latter contained argyrophilic axons. Immunohistochemical expression of tau, a microtubule-associated protein expressed in outgrowing axons, was demonstrated in the marginal zone. The central zone showed staining for glial fibrillary acidic protein. These findings indicate that the axon fibers from each eyeball coursed, respectively, along the unilateral marginal zone of the single nerve, and that most neuroepithelial cells in the central zone developed into astrocytes in the absence of axons. Received: 7 April 1997 / Accepted: 5 May 1997     

Immunocytochemical studies on astroglia of the cat retina under normal and pathological conditions

The cell density and morphology of astrocytes in whole-mounted adult cat retinae have been studied by immunocytochemical localization of glial fibrillary acidic protein (GFAP). There was a peak density at the optic nerve head of about 2,000 astrocytes/mm2 dropping to approximately 200 astrocytes/mm2 in the far periphery. In the central area there was a local minimum of astrocyte density. Whereas the processes of astroglia in the outermost retinal periphery were radially arranged, thus giving the cells a characteristic star-shaped appearance, the majority of astroglial processes in the central region of the retina were aligned in parallel with the ganglion cell axon bundles. To study the correlation between optic nerve fibres and astrocytes, ganglion cell axons were caused to degenerate following photocoagulation lesions close to the optic disc. Postlesion the processes of astrocytes in the central retina lost their directional preference and linearity, thus appearing to develop a star-shaped morphology typical of astroglia in the far periphery of normal retinae. The density of astrocytes decreased by approximately 50% on the optic disc side of the lesions and by up to 80% on the peripheral side of the lesions. The results show that the morphology and number of astrocytes vary as a function of their proximity to optic nerve fibres and to the density of these fibres.