Mller Cells

As already mentioned, Müller cells are radially oriented cells that traverse the retina from its inner (vitreal) border to the distal end of the outer nuclear layer (Fig. 1.2). Along their course, Müller cells extend branches that interdigitate with every class of retinal neuron, with other types of glia, and with the blood vessels of vascularized retinas.

This general description belies the remarkable morphological variability among species, which is prominently seen in the pattern of branching processes that project to the inner limiting membrane (Fig. 1.3A).

Figure 1.1. The four types of glial cells in the rabbit retina. A. Muller cell filled by intracellular injection with horseradish peroxidase. B. Astrocyte labeled with a monoclonal (MAI) antibody. C. An oligodendrocyted filled with biocytin extends processes that ensheathe the bundles of medullated nerve fibers that enter the optic nerve. D. lectin-labeled microglial cell shows the multiple small branches that project from the cell body (Robinson, 1992). (Copyright 1992 Today's Life Science, reprinted with permission.)

Figure 1.1. The four types of glial cells in the rabbit retina. A. Muller cell filled by intracellular injection with horseradish peroxidase. B. Astrocyte labeled with a monoclonal (MAI) antibody. C. An oligodendrocyted filled with biocytin extends processes that ensheathe the bundles of medullated nerve fibers that enter the optic nerve. D. lectin-labeled microglial cell shows the multiple small branches that project from the cell body (Robinson, 1992). (Copyright 1992 Today's Life Science, reprinted with permission.)

Müller cells are ubiquitous, and save for the optic nerve head, are found in all retinal regions of every vertebrate studied, including the central fovea of primates (Yamada, 1969; Distler and Dreher, 1996). In species with avascular retinas (lizards, amphibians, birds, and some mammals), Müller cells are the only glial elements that can be detected in the neural retina (Pedler, 1963; Rasmussen, 1974; Stone and Dreher, 1987; Schnitzer, 1988a).

There are between 106 and 107 Müller cells in the mammalian retina (Robinson and Dreher, 1990; Dreher et al., 1992; Reichenbach and Robinson, 1995; Distler and Dreher, 1996), and they occupy between 6-10% of the total cytoplasmic volume (Rasmussen, 1975; Reichenbach and Wohlrab, 1986). The measurements ofJeon et al. (1998) on mouse, rabbit, and monkey retinas indicate that Müller cells account for 16-22% of the cell bodies in the inner nuclear layer (INL); these data are shown in Fig. 1.3B together with comparable values for some of the other retinal cell types. However, the population density and morphology of Müller cells vary in different parts of the retina. Cells located in regions close to the ora serrata are shorter, have stouter trunks and broader endfeet, and are present in lower density (Fig. 1.4) than in more central locations (Uga, 1974; Rasmussen, 1974,1975; Dreher et al., 1988; Gaur et al., 1988; Reichenbach et al., 1989; Robinson and Dreher, 1990). In the far periphery of monkey retina, for example, cell density is about 6,000/mm2, whereas in the parafoveal region, the Müller cells have long slender trunks and reach a peak density of more than 30,000/mm2 (Distler and Dreher, 1996). The extreme variability in these morphometric parameters undoubtedly reflects the functional requirements of different retinal regions, and the special properties of the microenvironment in which the cells develop (Reichenbach et al., 1989).

Regional differences aside, Müller cells of all species have many common features. At the ultrastructural level (Fig. 1.5), the Müller cell cytoplasm appears more electron dense than neighboring neurons, and contains a well-developed endoplasmic reticulum and varying amounts of glycogen granules (Hogan et al., 1971; Uga, 1974). The cell nuclei are typically oval or polygonal and are generally located in the middle of the INL.

Mitochondria are located throughout the Müller cell cytoplasm, and in some retinas they may be found concentrated toward one end of the cell (cf. Uga and Smelser, 1973b; Rasmussen, 1974). It has been suggested that the location of the mitochondria within the Müller cell relates to the energy requirements of its neuronal neighbors (Rasmussen, 1973), but this seems unlikely. For example, mitochondrial density is often highest near the external limiting membrane in a region bordering the photoreceptor inner segments (Uga and Smelser, 1973a; Rasmussen, 1975); this area of the visual cell contains one of the highest concentrations of mitochondria of any body tissue. Recently, studies of mitochondrial migration and localization in

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