Retinal structure in Latimeria chalumnae.

Abstract

The retina of Latimeria chalumnae contains four types of visual cells; most are rods, and there are three types of cones. Rod outer segments are cylindrical and appearances at their bases suggest that they may be renewed discontinuously from the inner segment. The rods have simple synaptic spherules, each bearing a single basal filament ending in a club-shaped expansion. Type 1 cones contain an oil droplet, and have a complex synaptic pedicle bearing about 12 basal filaments. Type 2 cones have no droplet, and a pedicle bearing about six basal filaments and of complexity between that of rods and type 1 cones. Type 3 cones resemble type 2, except that they have a clear vacuole, but not an oil droplet, in the inner segment. The pigment epithelium contains abundant phagosomes, but pigment granules are absent where the epithelium overlies the choroidal tapetum lucidum. Regular arrays of tubules occur in the cytoplasm, some of which appear to be formed from three interlacing hexagonal nets. Two types of bipolar cell are present. Most are displaced bipolars, with nuclei in the outer nuclear layer. The rest are large, with nuclei in the horizontal cell layer. Both types bear Landolt’s clubs, which penetrate the outer limiting membrane. Their endings contain a cilium complex, and a single large mitochondrion. Some contain 60 nm vesicles, which are also found near disrupted club endings. Two types of horizontal cell are present. A few dark-staining cells with extensive web-like processes occur next to the outer plexiform layer. The expansions of rod basal filaments make contact with these cells. More voluminous pale staining cells with long cylindrical processes occur vitread to the dark cells. Presumed amacrine cells form a layer vitread to the horizontal cells; they and the inner plexiform layer were not well fixed. Sparse ganglion cells occur at the same level as the nerve fibre bundles. Radial fibres penetrate the horizontal cell layer as compact columns. They do not contribute to the outer plexiform or horizontal cell layers, but elsewhere spread amongst the other retinal elements. Their expansions determine the inner contour of the retina. Cells, probably microglial, which contain lysosomes are scattered amongst the other elements. 143000 myelinated fibres are present in the optic nerve, which also contains non-myelinated fibres. Retinal cell counts are given.