INTERSEGMENTAL FUSION IN VERTEBRATE ROD PHOTORECEPTORS - ROD CELL STRUCTURE REVISITED

Purpose. To determine the cause of intersegmental fusion-a fusion betw een the plasma membranes of the inner and outer segments-in vertebrate rod cells. Methods. Rod cells from adult salamander, frog, and rat re tinas were examined with electron microscopy. The fine structure of fu sion and the frequency of occurrence were assessed in three types of p reparations (intact eyeballs, eyecup preparations, and neural retinas detached from the retinal pigmented epithelium). The effects of temper ature, depolarization, and calcium were examined in isolated neural re tinas by maintaining retinas in culture under defined conditions. Reti nas from anesthetized animals also were examined. Results. Intersegmen tal fusion in rod cells was consistently observed in all species after retinal detachment and maintenance in a standard Ringer's solution fo r 20 to 30 minutes. The amount of fusion seen in isolated amphibian re tinas was sensitive to a number of factors: Elevated temperature incre ased the number of fused rod cells, whereas high KCl, low CaCl2, and T ricaine, which has calcium-related effects, reduced or prevented fusio n. In attached retinas, fusion was seen infrequently; the incidence of fused cells in amphibian eyecup preparations, however, could be incre ased by incubation in the cold. The extensive continuity between the i nner and outer segments created by their fusion did not disrupt intrac ellular structure. Moreover, the fusion seemed to be reversible with t ime. Conclusions. Intersegmental fusion is a feature of Vertebrate rod cells. Although seen predominantly after retinal detachment, it is in fluenced by factors that affect other well-known exoplasmic membrane f usions. Thus, it may be one of the many regulated membrane fusion-fiss ion events within the rod photoreceptor.