The cells of the RPE are postmitotic cells with a hexagonal shape that form a single layer of cuboidal epithelial cells that separate the external portion of the photoreceptors from the choroid. The RPE provides metabolic and functional support for the external portion of the photoreceptors. Each human eye contains between 4 and 6 million RPE cells. In the central part of the retina, the shape and dimensions of the RPE cells are uniform. They are circa 14 mm in diameter and 12 mm in height. At the equator the cells are taller and larger and at the extreme periphery lose the uniformity of size and shape. Some cells may contain more than one nucleus, and at the ora serrata the RPE cells may measure up to 60 mm in diameter (Panda Jonas et al., 1996).
These cells are formed of a basal portion, an apical portion, and six lateral faces. The basal portion has a cell membrane with numerous invaginations, which can sink up to 1 mm into the cytoplasm, in order to increase the absorbent surface. The basal membrane of these cells is adjacent to the basal lamina, which forms the proximal layer of the Bruch's membrane. The basal invaginations increase the surface area of the cellular membrane, as this is involved in transport functions. The apical portion of the RPE cells, which sits in front of the acromere of the photoreceptors, is folded to form microvilli of 5 to 7 mm length, that surround the third terminal of the acromere of the photoreceptors. As the acromeres of the rods and the cones are of different dimensions, the villi that surround the external portion of the rods are smaller (3 mm) than those that surround the cones. From a functional point of view there are two different types of microvilla—one softer, which is dedicated to transepithelial transport, and the other connected to the distal lamina of the photoreceptors.
The lateral portions of the RPE are linked (zonula occludens and adherens) to create the external haemato-retinal barrier and at the same time are interconnected through intercellular junctions. The cells have a round basal nucleus, and the cytoplasm is rich in lysosomes, smooth endoplasmic reticulum, mitochondria at the basal level, round pigmented granules, and oval ones containing melanin. The majority of the melanin granules are found in the apical portion or in the villi. The granules measure up to 1 mm in diameter and from 2 to 3 mm in length. The pigment granules adsorb light, preventing diffusion and also act as free radical ''scavengers.'' The RPE cells in the macular and equatorial regions contain the major quantity of pigment (Katz and Robison, 1984).
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