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Retinal pigment epithelium, located between the choroid and photoreceptor extracellular segments, is a transport channel for ions, water, nutrients and metabolic end products between the lower retinal cavity and choroid blood vessels. Retinal pigment epithelium is involved in retinol circulation, phagocytosis of exfoliated photoreceptor extracellular segments to maintain photoreceptor cell excitability, and secretes a variety of growth factors to help maintain the structural integrity of choroidal vascular endothelial cells and photoreceptor cells.