The visual cells of the skate retina: Structure, histochemistry, and disc-shedding properties


  • R. Bruce Szamier,

    1. Berman-Gund Laboratory for the Study of Retinal Degenerations, Harvard Medical School, Boston, Massachusetts 02114 Marine Biological Laboratory, Woods Hole, Massachusetts 02543
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  • Dr. Harris Ripps

    Corresponding author
    1. Departments of Ophthalmology, Physiology, and Biophysics, New York University School of Medicine, New York, New York 10016, Marine Biological Laboratory, Woods Hole, Massachusetts 02543
    • Department of Ophthalmology, New York University School of Medicine, 550 First Avenue, New York, NY 10016
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Earlier studies have shown that visual function in skate is subserved solely by the rod mechanism and that the retina of this elasmobranch contains only rod photoreceptors. Nevertheless, the skate retina is capable of responding to levels of illumination that extend well into the photopic range, and we have detected in histological sections (usually from younger animals) small, proximally displaced, conelike photoreceptors which possibly represent another class of visual cell. However, ultrastructural and histochem-ical studies showed that the membranous discs of the outer segments of these cells were isolated from the plasma membrane, and that their synaptic terminals appeared immature and unlike those usually associated with cone receptors. In addition, the pattern of incorporation of 3H-fucose, as revealed by radioautography, was similar for both the rods and the smaller visual cells; i.e., the label was concentrated along the basal discs of the outer segment. When we examined the disc-shedding behavior of the visual cells in skates entrained for 2 weeks or longer to a 12-hour light: 12-hour dark cycle, enhanced phagocytic activity was seen only following light onset; there was no significant increase following light offset. On the available evidence, it seems reasonable to conclude that the small visual cells are rods that have recently differentiated, and are growing and being incorporated into the photoreceptor layer of the retina.