MULLER GLIAL-CELLS OF THE TREE SHREW RETINA

The tree shrew is one of the few mammalian species whose retinae are strongly cone dominated, which is usually the case in reptilian and avian retinae. Muller cells of the tree shrew (Tupaia belangeri) retina were studied by transmission electron microscopy of tissue sections and freeze-fracture replicas, by immunolabeling of the intermediate filament protein vimentin in radial paraffin sections and in whole retinae, as well as by intracellular dye injection in slices of retinae. In addition, enzymatically isolated cells were stained by Pappenheim's panoptic staining method. The cells showed an ultrastructure that is similar to other mammalian Muller cells with two exceptions: Due to the extensive lateral fins of cone inner segments, the apical microvilli of Muller cells are arranged in peculiar palisades, and the basket-like Muller cell sheaths around neuronal somata in both nuclear layers consist of unusual multilayered membrane lamellae. Unlike Muller cells in other mammalian species studied thus far, but similar to reptilian and avian Muller cells, those of tree shrews commonly have two or more vitread processes rather than one main trunk. Muller cell densities range between some 13,000 mm(-2) in the periphery and about 20,000 mm(-2) in the retinal center. Neuron: (Muller) glial cell ratios were estimated to be 7.9:1 in the center and 6.2:1 in the periphery. For each Muller cell, about 1.5 (cone) photoreceptor cells, four or five interneurons of the inner nuclear layer, and about one cell of the ganglion cell layer were counted. This is a much lower number of neurons per Muller cell than in most other mammals studied. (C) 1995 Wiley-Liss, Inc.