Retinal pigment epithelium

The pigmented layer of retina or retinal pigment epithelium (RPE) is the pigmented cell layer just outside the neurosensory retina that nourishes retinal visual cells, and is firmly attached to the underlying choroid and overlying retinal visual cells. The RPE was known in the 18th and 19th centuries as the pigmentum nigrum, referring to the observation that the RPE is dark (black in many animals, brown in humans); and as the tapetum nigrum, referring to the observation that in animals with a tapetum lucidum, in the region of the tapetum lucidum the RPE is not pigmented. The RPE is composed of a single layer of hexagonal cells that are densely packed with pigment granules. When viewed from the outer surface, these cells are smooth and hexagonal in shape. When seen in section, each cell consists of an outer non-pigmented part containing a large oval nucleus and an inner pigmented portion which extends as a series of straight thread-like processes between the rods, this being especially the case when the eye is exposed to light. The RPE has several functions, namely, light absorption, epithelial transport, spatial ion buffering, visual cycle, phagocytosis, secretion and immune modulation. Light absorption: RPE are responsible for absorbing scattered light. This role is very important for two main reasons, first, to improve the quality of the optical system, second, light is radiation, and it is concentrated by a lens onto the cells of the macula, resulting in a strong concentration of photo-oxidative energy. Melanosomes absorb the scattered light and thus diminish the photo-oxidative stress. The high perfusion of retina brings a high oxygen tension environment. The combination of light and oxygen brings oxidative stress, and RPE has many mechanisms to cope with it. Epithelial transport: As mentioned above, RPE compose the outer blood–retinal barrier, the epithelia has tight junctions between the lateral surfaces and implies an isolation of the inner retina from the systemic influences.