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Abstract
Homozygous 020/A mutant mice bearing the rds gene for slow inherited retinal degeneration have been observed to develop normal photoreceptor inner segments connecting cilia and synaptic contacts but fail to form outer segments. Their retinas are responsive to light, however. In order to assess the sources of these physiological responses we investigated the distribution of opsin in photoreceptors by means of immunoelectron microscopy. Opsin was detected in the inner segment plasma membrane and the distal ciliary plasma membrane. Antibody also bound to lamellar and vesicular membranes in the interphotoreceptor space and, in a small fraction of the photoreceptors, to membranes projecting from the distal cilium. These membranes may represent abortive formation of rod discs in this form of retinal degeneration. Failure to form an organized outer segment may contribute to the persistence of opsin in the inner segment plasma membranes of adult mutant mice.