Immunogold localization of myosin in the photoreceptor cilium

The photoreceptor outer segment (OS) develops from the distal end of a nonmotile cilium and exhibits a continuous renewal of its disc structures throughout life. Immunocytochemical studies have localized an actin-rich domain to the ciliary axoneme at the base of the OS, and recent ultrastructural studies of detergent-extracted retinas have demonstrated actin filaments at this site. These filaments are believed to participate in the formation of new outer segment discs. In this study, rat neural retinas were treated with the detergent saponin, fixed with aldehydes, and embedded in London resin white resin for immunoelectron microscopy. In thin tissue sections, actin filaments were observed within the appropriate ciliary domain and these filaments could be positively stained with anti-actin antibodies. When this tissue was immunogold labeled with anti-myosin antibodies, myosin also was found in this locale. The results of this study demonstrate that saponin-extracted retinas can be used for immunoelectron microscopy studies of the photoreceptor cytoskeleton. In addition, the presence of myosin within the actin-filament-containing portion of the cilium suggests that an actomyosin contractile mechanism may play a role in outer segment disc morphogenesis.