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

The chromophore portion of photoreceptors, specifically 11-cis-retinal, originates from the retinal pigment epithelium (RPE) rather than the inner segment of the photoreceptors. In the process of phototransduction, light photons are absorbed by the chromophore, which undergoes a conformational change from 11-cis-retinal to all-trans-retinal.

The RPE plays a crucial role in the retinoid cycle, a process where vitamin A (in the form of retinol) is converted into 11-cis-retinal within the RPE. This chromophore is then transported to the photoreceptor cells (rods and cones) to replenish the visual pigment. Therefore, understanding this relationship highlights the importance of the RPE in maintaining the functionality of photoreceptors and is essential in the context of visual physiology.

The inner segment of photoreceptors contains cellular components responsible for metabolism and phototransduction but is not directly involved in the synthesis of the chromophore. Thus, attributing the origin of the chromophore to the RPE is distinctly accurate in this context.