Immunoferritin localization of actin in retinal photoreceptors

The contractile protein actin was recently localized to the distal portion of the connecting cilium in frog photoreceptors (Chaitin et al J Cell Biol 99:239-247, 1984). This is the site where the ciliary plasma membrane evaginates to form new outer segment disks (Steinberg et al J Comp Neurol 190: 501-518, 1980). In the present study, aldehyde fixed mammalian retinas were embedded in Lowicryl K4M, and thin tissue sections were reacted with antiactin antibodies using indirect immunoferritin labeling. Utilizing this technique, actin has been localized to the distal cilium in rat, cow, monkey, and human photoreceptors. These results provide additional evidence that an actin mediated contractile mechanism may regulate outer segment disk morphogenesis in vertebrate photoreceptors. As previously noted in frog retina, antiactin also labeled the bundle of filaments within photoreceptor calycal processes, and this label extended into the inner segment, subjacent to the plasma membrane. Within the inner segment, however, the striated rootlet was unlabeled.