Phagocytosis and surface morphology in cultured retinal pigment epithelial cells

Phagocytosis of latex spheres was studied in explant cultures of retinal pigment epithelium from rabbit, calf, and rat (normal and dystrophic). Calf and rabbit pigment epithelial cells showed a "latent period" of about 12 and 17 hr, respectively, during which latex spheres were not ingested even when the cultures were pre-incubated for up to 24 hr prior to exposure. Scanning electron microscopy demonstrated a profusion of apical processes on the surface of cells from both species that was present throughout the latent period. In contrast, rat pigment epithelial cells showed no latent period and phagocytosed latex spheres after a pulse of 1 hr. It was concluded that the latent period is not common to all species and, when present, is not due to an absence of apical processes. The surfaces of normal and dystrophic pigment epithelial cells showed abundant apical processes that did not differ from each other in morphology, number, or distribution. In cells from pigmented species (calf and rat), frequent fusions were observed between latex phagosomes and melanosomes or melanin granules.