Does the opsin portion of photoreceptors come from the RPE or the inner segment of the photoreceptors?

The opsin portion of photoreceptors is synthesized in the inner segment of the photoreceptors themselves. Photoreceptor cells, including both rods and cones, have a structure that consists of an outer segment, inner segment, and synaptic terminal. The inner segment houses the cell's organelles, including the nucleus and the machinery necessary for protein synthesis. It is here that the opsins, which are light-sensitive proteins that play a crucial role in the phototransduction cascade, are produced.

Once synthesized, the opsin proteins are transported to the outer segment of the photoreceptors, where they combine with the chromophore (typically retinal) to form the functional photopigments that allow the photoreceptors to absorb light. The retinal itself is derived from the pigment epithelium, but the opsin portion is specifically generated in the inner segment.

Thus, the correct understanding of where opsin comes from in the context of photoreceptor function focuses on its synthesis in the inner segment.