The microvilli on the apical surface of the RPE are involved in phagocytosis of which structure?

• A. Photoreceptor outer segments
• B. Photoreceptor inner segments
• C. Retinal nerve fibers
• D. Choroidal vessels

Answer: (A)

The apical microvilli of the retinal pigment epithelium engage in phagocytosis of photoreceptor outer segments as part of the daily renewal of these cells. Photoreceptors continuously shed the distal portions of their outer segments; the RPE's apical microvilli reach into the subretinal space to engulf and digest this shed material in lysosomes. This process keeps photoreceptors healthy by removing damaged discs and supplying components for the visual cycle.

Inner segments are not shed and phagocytosed by the RPE, retinal nerve fibers reside in the nerve fiber layer of the retina rather than in the subretinal space, and choroidal vessels are blood vessels in the choroid, not targets of this phagocytic activity.