The structure and distribution of the cross-striated fibril and associated membranes in guinea pig photoreceptors.

Examination of longitudinally and transversely sectioned photoreceptor cells of the guinea pig retina revealed an aggregate of thin filaments forming a single cross-striated fibril coursing through the full length of the non-receptor portion of the cell. The fibril begins as the ciliary rootlet from the region of the basal body of the connecting cilium. From the basal body it passes between the mitochondria of the ellipsoid and along the Golgi zone of the myoid region of the inner segment, narrowing from an irregularly shaped bundle to a ribbon-shaped aggregate. The fibril separates into discrete strands, each curving along the nucleus, reuniting into a single bundle to pass down the cell's axon, and terminating deep within the synaptic terminal. The fibril is flanked by two separate membranous saccules, each continuous along nearly its full length. The fibril's extensive course in the guinea pig and its association with continuous membranes necessitates a reexamination of earlier proposals for the function of cross-striated filamentous structures in photoreceptor cells.