In which retinal layer is the visual cycle (retinoid recycling) primarily completed?

• A. Inner Nuclear Layer
• B. Retinal Pigment Epithelium
• C. Outer Nuclear Layer
• D. Ganglion Cell Layer

Answer: (B)

Regenerating the photopigment relies on recycling retinaldehyde, and this process happens mainly in the retinal pigment epithelium. After light bleaches the visual pigment, all-trans-retinal is released and converted first to all-trans-retinol in the photoreceptors, then transported to the RPE. There, enzymes such as RPE65 isomerize it to 11-cis-retinal, which is then re-esterified and finally released back to the photoreceptors where it recombines with opsin to reform the pigment. The RPE sits just behind the photoreceptor outer segments, providing the necessary enzymatic machinery and retinoid transport between photoreceptors and the choroidal blood supply. The other retinal layers—the inner nuclear, outer nuclear, and ganglion cell layers—are neural tissue and do not carry out this regenerative chemistry.