The retinal pigment epithelium (RPE) contributes to the blood-retinal barrier in conjunction with the endothelium of the retinal vessels and has many functions including ion and water transport and secretion of growth factors and cytokines. Retinal Pigment Epithelium – the outermost retinal layer that spans a width of a single cell located between the neural retina and the Bruch membrane, adjacent to the highly-vascularized choroid layer. The retina, more specifically, subdivides into ten distinct layers that are described in order from the innermost layers closer to the pupil to the layers further towards the posterior and periphery of the eyeball: Vitamin A deficiency is a significant risk factor for blindness in young children and remains prominent in under-developed regions, including South Asia and sub-Saharan Africa. Retinal is vitamin A aldehyde, making vitamin A an essential dietary component for the facilitation of the phototransduction pathway. Rod cells contain rhodopsin, which is a light-sensitive pigment made of retinal that allows for the absorption of photons. The overlap of detectable wavelength spectrums between the three types of cones results in the visible light spectrum perceived by humans. In terms of sensing colored light, each type of cone cell can respectively characterize as detecting blue, green, and red wavelengths. There exist three types of cones including tritans, deutrans, and protans, named for detecting short, medium, and long wavelengths, respectively. There are approximately 6 million cones and often more than 100 million rods within the retina. Cones localize in the center of the retina at the fovea. Cones are more sensitive in daylight (photopic vision) and capture wavelengths of colored light. Rods are more sensitive in dim light (scotopic vision) and reside in the periphery of the retina. Photoreceptor cells include rods and cones and are uniquely located towards the posterior aspect of the retinal sublayers, further away from the pupil where light enters the eye.
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