The accessory outer segment of rods and cones in the retina of the guppy, Poecilia reticulata P. (Teleostei). An electron microscopical study.

The ultrastructure of the accessory outer segment (AOS)--a cilium-like structure emanating from the inner segment and running alongside the outer segment of photoreceptors--is described. The AOS occurs in both rods and cones of Poecilia reticulata. Its ultrastructure, including the arrangement of microtubules, which originate from the ciliary stalk, is the same in rods and cones. The cone-AOS is connected with the outer segment by a thin plasmabridge, whereaa the rod-AOS lies embedded within the outer segment. The outer segment of the cone, in contrast to that of the rod, is separated from the pigment epithelium by a large extracellular space. An intimate contact, however, is secured by the AOS; its membrane is closely appositioned to the pigment epithelium membrane. The functional significance of the AOS and its possible occurrence in other vertebrate classes, are discussed.