Distribution of Müller cells in the turtle retina: an immunocytochemical study

Glia-Mediated Regenerative Response Following Acute Excitotoxic Damage in the Postnatal Squamate Retina

The retina is a complex tissue responsible for both detection and primary processing of visual stimuli. Although all vertebrate retinas share a similar, multi-layered organization, the ability to regenerate individual retinal cells varies tremendously, being extremely limited in mammals and birds when compared to anamniotes such as fish and amphibians. However, little is yet known about damage response and regeneration of retinal tissues in "non-classical" squamate reptiles (lizards, snakes), which occupy a key phylogenetic position within amniotes and exhibit unique regenerative features in many tissues. Here, we address this gap by establishing and characterizing a model of excitotoxic retinal damage in bearded dragon lizard (Pogona vitticeps). We particularly focus on identifying, at the cellular and molecular level, a putative endogenous cellular source for retinal regeneration, as diverse self-repair strategies have been characterized in vertebrates using a variety of retinal injury and transgenic models. Our findings reveal for the first time that squamates hold the potential for postnatal retinal regeneration following acute injury. Although no changes occur in the activity of physiologically active progenitors recently identified at the peripheral retinal margin of bearded dragon, two distinct successive populations of proliferating cells at central retina respond to neurotoxin treatment. Following an initial microglia response, a second source of proliferating cells exhibit common hallmarks of vertebrate Müller glia (MG) activation, including cell cycle re-entry, dedifferentiation into a progenitor-like phenotype, and re-expression of proneural markers. The observed lizard glial responses, although not as substantial as in anamniotes, appear more robust than the absent or neonatal-limited regeneration reported without exogenous stimulation in other amniotes. Altogether, these results help to complete our evolutionary understanding of regenerative potential of the vertebrate retina, and further highlight the major importance of glial cells in retinal regeneration. Furthermore, our work offers a new powerful vertebrate model to elucidate the developmental and evolutionary bases of retinal regeneration within amniotes. Such new understanding of self-repair mechanisms in non-classical species endowed with regenerative properties may help designing therapeutic strategies for vertebrate retinal diseases.

Comparative analysis of glucagonergic cells, glia, and the circumferential marginal zone in the reptilian retina

Retinal progenitors in the circumferential marginal zone (CMZ) and Müller glia-derived progenitors have been well described for the eyes of fish, amphibians, and birds. However, there is no information regarding a CMZ and the nature of retinal glia in species phylogenetically bridging amphibians and birds. The purpose of this study was to examine the retinal glia and investigate whether a CMZ is present in the eyes of reptilian species. We used immunohistochemical analyses to study retinal glia, neurons that could influence CMZ progenitors, the retinal margin, and the nonpigmented epithelium of ciliary body of garter snakes, queen snakes, anole lizards, snapping turtles, and painted turtles. We compare our observations on reptile eyes to the CMZ and glia of fish, amphibians, and birds. In all species, Sox9, Pax6, and the glucocorticoid receptor are expressed by Müller glia and cells at the retinal margin. However, proliferating cells were found only in the CMZ of turtles and not in the eyes of anoles and snakes. Similar to eyes of chickens, the retinal margin in turtles contains accumulations of GLP1/glucagonergic neurites. We find that filamentous proteins, vimentin and GFAP, are expressed by Müller glia, but have different patterns of subcellular localization in the different species of reptiles. We provide evidence that the reptile retina may contain nonastrocytic inner retinal glial cells, similar to those described in the avian retina. We conclude that the retinal glia, glucagonergic neurons, and CMZ of turtles appear to be most similar to those of fish, amphibians, and birds.

Glia cells of the monkey retina--II. Müller cells

In this paper, for the first time a quantitative description of the morphology and distribution of Müller cells in the macaque monkey retina using immunohistochemistry and high resolution confocal laser scanning microscopy is given. By their morphological features Müller cells are ideally adapted to their neuronal environment in the various retinal layers, with a dense network of horizontal processes, especially in the inner plexiform layer, and close contacts to neuronal somata especially in the outer nuclear layer and ganglion cell layer. Morphology varies with retinal eccentricity. The thickness of the inner trunk increases significantly with increasing retinal eccentricity. According to the overall thickness of the retina, Müller cells in central retina are longer than in peripheral regions. In the parafoveal region, the outer trunks of Müller cells in the outer plexiform layer are immensely elongated. These Müller fibres can reach lengths of several hundred micrometers as they travel through the outer plexiform layer from the foveal centre towards the foveal border where they enter the inner nuclear layer. Müller cell density varies between 6000 cells/mm2 in far peripheral and peak densities of > 30,000 cells/mm2 in the parafoveal retina. There is a close spatial relationship between Müller cells and blood vessels in the monkey retina, suggesting a role of Müller cells in the formation of the blood-retinal barrier, in the uptake of nutrients and the disposal of metabolites.

Vitread proliferation of filamentous processes in avian Müller cells and its putative functional correlates

In order to examine to what extent the neuronal and metabolic activities of avascular vertebrate retinae are reflected in the morphology of their Müller cells we have studied (by using several monoclonal antibodies) the morphology of Müller cells in two species of diurnal birds (chicken, Gallus domesticus, and pigeon, Columba livia) and one species of nocturnal saltwater crocodiles (Crocodylus porosi). In all three species, the outer nuclear layer is relatively thin and the Müller cell trunks divide into rootlets that wrap around the photoreceptors. In both diurnal birds studied, the trunks of Müller cells in the inner plexiform layers invariably divide into numerous fine filamentous processes that terminate in small expansions covering most of the vitreal surface of the retina. Furthermore, the networks of filamentous processes of birds' Müller cells exhibit conspicuous horizontal lamination in the inner plexiform layer. In contrast, the filamentous processes arising from the individual Müller cell trunks of the crocodile, if present, are much less numerous and less widely spread than those of diurnal birds. It is proposed that the splitting of the Müller cell trunks into numerous filamentous processes terminating in small vitreal expansions represents a morphological adaptation for: 1) effective spatial buffering of K+ ions in thick and presumably metabolically highly active, yet avascular, avian retinae, and 2) effective absorption and distribution of nutrients leaking from the vitreally located supplemental nutritive organ, the pecten.

Morphology and distribution of Müller cells in the retina of the toad Bufo marinus

We have previously shown that an antibody against neuron-specific enolase (NSE) selectively labels Müller cells (MCs) in the anuran retina (Wilhelm et al. 1992). In the present study the light- and electron-microscopic morphology of MCs and their distribution were described in the retina of the toad, Bufo marinus, using the above antibody. The somata of MCs were located in the proximal part of the inner nuclear layer and were interconnected with each other by their processes. The MCs were uniformly distributed across the retina with an average density of 1500 cells/mm2. Processes of MCs encircled the somata of photoreceptor cells isolating them from each other by glial sheath, except for those of the double cones. Some of the photoreceptor pedicles remained free of glial sheath. Electron-microscopic observations confirmed that MC processes provide an extensive scaffolding across the neural retina. At the outer border of the ganglion cell layer these processes formed a non-continuous sheath. The MC processes traversed through the ganglion cell layer and spread beneath it between the neuronal somata and the underlying optic axons. These processes formed a continuous inner limiting membrane separating the optic fibre layer from the vitreous tissue. Neither astrocytic nor oligodendrocytic elements were found in the optic fibre layer. The significance of the uniform MC distribution and the functional implications of the observed pattern of MC scaffolding are discussed.

Müller cells in vascular and avascular retinae: a survey of seven mammals

Eight monoclonal antibodies were used to label Müller cells in four mammals that have vascular retinae (cats, dogs, humans, and rats) and in three with avascular retinae (echidnas, guinea pigs, and rabbits). Müller cells were found to have a fairly uniform retinal distribution in seven species, with a mean density of 8,000-13,000 cells mm-2. Müller cells in avascular retinae differ from their vascular counterparts in four respects. First, they are shorter than those in vascular retinae. This difference is mainly due to a reduction in the thickness of the outer nuclear layer. Second, the trunks of Müller cells in avascular retinae tend to be thicker, although those in echidnas are an exception to this trend. Third, Müller cell rootlets in avascular retinae follow a more tortuous course than those in vascular retinae, reflecting the fact that photoreceptor nuclei in the two types of retina have different shapes and stacking patterns. Fourth, due to a reduction in the density of photoreceptors in avascular retinae, there are fewer neurones per Müller cell. Although these four features may enable Müller cells to assist the nutrition of neurones in the inner layers of avascular retinae, they are unlikely to be morphological specializations that have evolved for that purpose. Rather, these features appear to be a direct consequence of the fact that avascular retinae are thinner and have a differently organised outer nuclear layer. These features aside, Müller cells in avascular retinae closely resemble their counterparts in vascular retinae.

RPE conditioned medium stimulates photoreceptor cell survival, neurite outgrowth and differentiation in vitro

In the present study we have investigated retinal pigment epithelium-photoreceptor cell interactions in vitro, and their contributions to photoreceptor cell survival and differentiation. Preparations enriched for intact photoreceptor cells from neonatal rat retina were grown in either serum-free medium supplemented with RPE-conditioned medium (RPE-CM) or in serum-free medium alone. A variety of substrate conditions were tested for the best neurite outgrowth. Cultures were monitored for 7 days by light and electron microscopy, as well as by opsin, vimentin and carbonic anhydrase-C immunocytochemistry. RPE-CM was found to stimulate both proliferation of flat cells and photoreceptor differentiation. The number of photoreceptors bearing neurites and their neurite length measurements showed significant differences between the RPE-CM group and the control group within 20 hr in culture. Elimination of contaminating flat cells by the addition of an antimitotic drug prevented photoreceptor cell morphological maturation; however, these cells survived as round cell bodies without processes for at least 10 days in the presence of RPE-CM and expressed opsin during this period. Conditioned medium from the flat-cell monolayers did not support photoreceptor differentiation or their survival. However, the presence of flat cells was a requisite to achieve any neurite outgrowth even in the presence of RPE-CM. In the absence of RPE-CM, neither photoreceptors nor flat cells survived or proliferated. Heat and trypsin treatment of the RPE-CM abolished all its growth-supporting activities which indicates its proteinaceous nature. This represents the first time in vitro that an RPE-derived factor(s) has been shown to be responsible for photoreceptor cell survival and differentiation.

Developmental expression of the glial fibrillary acidic protein (GFAP) gene in the mouse retina

1. In the nervous system, Glial fibrillary acidic protein (GFAP) is a well-known, cell type-specific marker for astrocytes. 2. In the mammalian retina, Muller cells, the major class of retinal glia, do not express GFAP or contain only low amounts of this protein. In retinas with photoreceptor degeneration, however, high levels of GFAP are found. It is possible that GFAP synthesis in these retinas could result from "dedifferentiation" of Muller cells as a consequence of disruption of normal neuron-glia interactions. 3. We have carried out immunocytochemical and in situ hybridization studies to examine whether GFAP or its mRNA is expressed by retinal cells early in embryonic development. 4. Our results show that GFAP-containing cells, which are probably astrocytes, are found only in the ganglion cell and nerve fiber layers and that these cells appear after postnatal day-1 (P-1) and continue to form until P-10. 5. Astrocyte formation starts from the optic disc and moves toward the periphery of the retina at a rate of approximately 160-200 microns per day. 6. An unexpected result from these studies is that GFAP mRNA levels are high in the first week of birth and decline rapidly as the animal develops. 7. Finally, we did not find either GFAP or GFAP mRNA in retinal cells other than astrocytes during normal development.

Development of the rabbit retina: II. Müller cells

Müller (glial) cells of the rabbit retina were stained with antibodies against the intermediate filament protein vimentin in retinal wholemounts from various developmental stages. Both the density of stained profiles and the mean diameter of these profiles were measured, with the microscope focus in the inner plexiform layer of the retinae. Within this retinal layer, every Müller cell possesses one stout vitread process; thus counts of the stained profiles allow an estimation of their number. After postnatal day (P) 9, the total number of stained cells was slightly above 4 million per retina; for the adult rabbit retina, this agrees well with earlier data obtained by our group based on another method, as well as with published data from other groups. We suggest that after P 9, only Müller cells are stained, and this population is numerically stable. In contrast, neonatal retinae contained significantly more stained profiles. This indicates that either the total number of Müller cells is reduced by "physiological cell death" or that additional cells are stained neonatally. We discuss why we favour the second possibility. After P 9, two peculiarities occur in the Müller cell population: (1) their density decreases gradually, to a greater extent in the retinal periphery than in the center (i.e., in the "visual streak"), and (2) Müller cell diameters increase, again more in the periphery than in the center. We argue that differential retinal expansion leads to dispersion of the pre-existing cell population and allows for widening of the Müller cell processes. We conclude that Müller cells can be used postnatally in the rabbit retina as "landmarks" of expansion.

Localization of cellular retinoic acid-binding protein to amacrine cells of rat retina

Monoclonal antibodies to performic acid-oxidized cellular retinoic acid-binding protein (CRABP) from bovine retina were prepared by fusion of spleen cells from immunized mice with mouse myeloma cells. Five antibodies were studied in detail. It was established by ELISA that the antibodies react with CRABP and oxidized CRABP, but not with other oxidized or unmodified retinoid-binding proteins. Competitive ELISA demonstrated that the antibodies react with heat-denatured antigen but not with native protein. Western blotting and immunostaining, following sodium dodecyl sulfate gel electrophoresis, provided evidence for recognition of a single component in retinal supernatants whose staining is prevented by preabsorption of the antibody with heat-denatured CRABP. The insoluble fraction from a retinal homogenate contains residual CRABP and two weakly-reacting components, whose staining is not affected by preabsorption of the antibody with antigen. Each antibody produces the same staining pattern on cryostat sections of rat retina by indirect immunofluorescence. Amacrine somata on both sides of the inner plexiform layer are labeled, as well as processes forming laminae within this layer. These results suggest that retinoic acid may play a functional role in the inner retina.

Müller cells in adult rabbit retinae: morphology, distribution and implications for function and development

We describe the morphology and distribution of Müller cells in wholemounts of rabbit retinae labelled with either monoclonal antibodies (anti-Vimentin, 3H3, 4D6, and 4H11), or intracellular horseradish peroxidase. Several new features of Müller cell organization are noted. First, Müller cells appear to compose a single morphological class and their morphology varies systematically with retinal thickness. Second, in contrast to other retinal glia, Müller cells have a neuronlike distribution, with a peak density of 10,700-15,000 cells per mm2 at the visual streak and a minimum density of 4,400-6,000 per mm2 at both the superior and inferior retinal edges. There are 4.2 +/- 0.5 x 10(6) Müller cells per retina. Third, unlike in other species, rabbit Müller cells do not contact blood vessels, suggesting that they do not participate in the transfer of metabolites or in the blood:retinal barrier. Fourth, each Müller cell has a vitread endfoot about 20-40 microns in diameter composed of numerous fimbriae. The fimbriae from a single Müller cell generally contact several axon fascicles in the nerve fibre layer, and at each point along its length each fascicle is enclosed by the overlapping fimbriae from several Müller cells. Fifth, in the inner and outer plexiform layers, numerous filamentous branchlets extend 20 microns or more from the radial trunk, interweaving with branchlets from nearby Müller cells to form dense and continuous strata. In the ganglion cell layer and outer nuclear layer, Müller cell processes completely wrap neuronal somata, whereas in the inner nuclear layer they partially wrap somata. We discuss the functional and developmental implications of these observations.

A monoclonal antibody marker for the paraboloid region of cone photoreceptors in turtle retina

Monoclonal antibodies that specifically label one or more cell types in retina have been produced; however, only a few antibodies that, in addition, recognize distinct subcellular structures in these cells have been reported. During a search for monoclonal antibodies that bind to specific cell types in the turtle (Pseudemys scripta elegans) retina, we obtained an antibody (20 93; an IgG) that labels the inner segment of cone photoreceptors. Ultrastructural immunocytochemistry using immunogold and avidin/biotin-peroxidase techniques showed that 20 93 antigen is localized to the paraboloid, a region specifically involved in glycogen metabolism in cones. In addition, a few bipolar cells were found to be labeled. The monoclonal antibody showed limited species cross-reactivity and failed to stain mouse, rat, rabbit, dog, cow, Anolis, and human retinas. Immunoblotting showed that monoclonal antibody 20-93 binds to a 40 KDa protein that is present in the retinal membrane. The antibody should be useful in immunological studies of the cone paraboloid.