= Retinal pigment epithelium
= RPE
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The <retinal pigment epithelium>, or <RPE>, is a single layer of specialized <epithelial cells> at the back of the <retina>, closely apposed to the light-sensitive <photoreceptors>. It supports photoreceptor metabolism and participates in the <visual cycle>, the chemical regeneration needed to sustain light detection. Support of a receptor and detection by that receptor are distinct cellular tasks.<Molecular Mechanisms Section by the National Eye Institute>[About our work: RPE monolayer, photoreceptor dependence and vitamin-A metabolism]{ref}
A 1998 genetic experiment made that dependence concrete. Mice lacking <RPE65> showed altered vitamin-A-related chemistry and lacked normal <rod cell> responses measured by <electroretinography>, although cone responses remained. They also lacked <rhodopsin> despite retaining its <opsin> protein component. The comparison connected defective regeneration chemistry with loss of functional visual pigment rather than simply absence of all photoreceptor protein.<Rpe65 is necessary for production of 11-cis-vitamin A in the retinal visual cycle by T. M. Redmond and others>[Abstract: retinoid composition,electroretinography,rhodopsin and opsin]{ref}
An illuminating experimental comparison therefore measures both sides: a chemical assay tests the available pigment-related molecules, while an electrical recording tests the <retina>'s response to <light>. Either observation alone leaves more possible explanations. The result also shows why a layer that does not itself provide the main light-triggered signal can be essential to vision.
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