Which of the following is a main action of the retinal pigment epithelium?

• A. Phagocytosis of photoreceptor outer segments
• B. Transfer of ions, water, and metabolites
• C. Vitamin A storage
• D. All of the above

Answer: (D)

The retinal pigment epithelium supports photoreceptors through several key roles. It routinely phagocytoses the shed tips of photoreceptor outer segments, renewing the outer retina and recycling disc components so photoreceptors remain functional. It also actively transfers ions, water, and various metabolites between the subretinal space, the RPE, and the choroid, maintaining the proper ionic balance and hydration needed for optimal photoreceptor activity. In addition, the RPE stores vitamin A as retinyl esters and participates in the visual cycle by converting retinol to 11-cis-retinal, supplying the chromophore needed to regenerate visual pigment. Because the RPE performs all of these actions, the best answer is all of the above.