Microtubules in a rod-specific cytoskeleton associated with outer segment incisures

In many vertebrate retinas the outer segments of rod photoreceptors have multiple incisures, that is, there are numerous indentations in the highly curved membrane forming the edge of their disks and in the plasma membrane enclosing the entire stack of disks. Immunofluorescent localization of tubulin in amphibian photoreceptors yielded a novel series of thin, parallel, fluorescent lines in rod outer segments that extended their full length and coincided with their multiple incisures. Electron-microscopic examination of amphibian retinas revealed the structures responsible for this fluorescence: longitudinally oriented microtubules were associated with incisures at heights throughout rod outer segments. These microtubules were located between the disk rims and the overlying plasma membrane, in the small cytoplasmic compartment at the mouth of incisures; the microtubules and membranes were separated from each other by distances that were uniform, as though interconnected by filaments described in other studies. Thus, in amphibian rod outer segments the incisures mark the site of a cytoskeletal system containing longitudinal microtubules distinct from those of the ciliary axoneme, linked by filaments to the adjacent membranes. This cytoskeleton is expected to be important for the normal structure, function, and renewal of rod outer segments. In amphibian cone outer segments, which do not have incisures, the only anti-tubulin immunofluorescence and the only microtubules were at the axoneme. These findings may help elucidate the diverse properties of rods and cones in many vertebrate retinas and could prove relevant for human retinal degenerations.