Actin filaments in apical projections of the primate pigmented epithelial cell.

A highly-ordered array of filaments is found within the apical processes of retinal pigmented epithelial cells in monkeys and humans. These filaments, approximately 100 A in diameter and 250 A apart, line the cytoplasmic face of the plasma membrane, in parallel with the long axis of the apical processes. Since these filaments bind rabbit myosin subfrafment-1 to form arrowhead complexes, we conclude that they contain actin. Such membrane-bound actin filaments could have any of several different functions: they could stabilize the apical projections and by so doing play a cytoskeletal role, and/or they could take part in the phagocytosis of shed outer segment discs.