Pigment epithelial ensheathment and phagocytosis of extrafoveal cones in human retina

Abstract

The association between extrafoveal cone outer segments and pigment epithelial cells was studied by transmission electron microscopy in three human retinas; ages 5, 45 and 60. The pigment epithelial apical surface from a fourth human retina, age 38, was viewed in the scanning electron microscope. Multiple villous-like apical processes protrude from the pigment epithelium into the space above each cone. Sometimes one or more of these processes is sheet-like in form and contains a wealth of intracellular organelles, including mitochondria. One or more of the villous-like procesess reaches the cone and expands to ensheath the upper one-third of the outer segment. Like vertebrate rods, extrafoveal human cones shed their terminal disks in packets and these packets are phagocytosed by the ensheathing apical processes. The phagosomes then ascend in the processes toward the pigment epithelia soma. Digestion of phagosomes appears to begin in the apical processes.