Retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, such as cones and rods, and is firmly attached to the underlying choroid (via the Bruch's membrane) and overlying retinal visual cells. The retinal pigment epithelium is composed of a single layer of hexagonal cells that are densely packed with pigment granules. At the ora serrata, the retinal pigment epithelium continues as a membrane passing over the ciliary body and continuing as the back surface of the iris. This generates the fibers of the dilator. The front end continuation of the retina is the posterior iris epithelium, which takes on pigemnt when it enter the iris.
The retinal pigment epithelium is involved in the phagocytosis of the outer segment of photoreceptor cells and it is also involved in the vitamin A cycle where it isomerizes all trans retinol to 11-cis retinal. The retinal pigment epithelium also serves as the limiting transport factor that maintains the retinal environment by supplying small molecules such as amino acid, ascorbic acid and D-glucose while remaining a tight barrier to choroidal blood borne substances. Homeostasis of the ionic environment is maintained by a delicate transport exchange system. In the eyes of albinos, the cells of this layer contain no pigment. Dysfunction of the RPE is found in Age-Related Macular Degeneration and Retinitis Pigmentosa.