Retinal Pigment Epithelium
The RPE is a monolayer of pigmented cells derived from the outer layer of the optic cup. This layer is continuous with the pigment epithelium of the ciliary body and iris. RPE cells are hexagonal, cuboidal cells approximately 16 μm in diameter. In the macula, however, the cells are taller and denser than in the periphery. The lateral surfaces of adjacent cells are closely apposed and joined by tight junctional complexes (zonulae occludentes) near the apices, forming apical girdles and the outer blood–ocular barrier. Each RPE cell has an apex and base; the apical portion envelops the outer segments of the photoreceptor cells with villous processes (Fig 1-9). The basal surface of the cells shows a rich infolding of the plasma membrane. The basement membrane does not follow these infoldings. The typical RPE cell has several melanosomes, each designed to absorb light. Melanosomes are spheroidal; their melanin is distributed on protein fibers.
The RPE contributes to retinal function in several ways; it
phagocytoses rod and cone outer segments
participates in retinal and polyunsaturated fatty acid metabolism
forms the outer blood–ocular barrier
maintains the subretinal space
heals and forms scar tissue
regenerates and recycles visual pigment
RPE cells serve a phagocytic function, continually ingesting the disc membranes shed by the outer segments of the photoreceptor cells. Over the course of a lifetime, each RPE cell is thought to phagocytose billions of outer segments. This process of shedding, phagocytosis, and photoreceptor renewal follows a daily (circadian) rhythm. Rods shed discs at dawn, and cones shed them at dusk. The ingested outer segments are digested gradually, broken down by enzymes from lysosomes.
Visual pigments contain 11-cis-retinaldehyde that is converted to 11-trans-retinal-dehyde in the outer segments of the retina. Most of regeneration of the 11-cis to the 11-trans configuration occurs in the RPE and requires a highly efficient transfer of metabolites from the outer segments to the RPE cells and back. The interdigitation of the RPE and outer segments help facilitate the regeneration by increasing the surface area of contact and allowing for close proximity. The visual pigments’ biochemical cycle is discussed in more detail in BCSC Section 2, Fundamentals and Principles of Ophthalmology.
A variety of pathologic changes—caused by such factors as genetic defects, drugs, dietary insufficiency (of vitamin A), or senescence—can impair the process of phagocytosis and renewal. Physical separation of the retina from the RPE, which occurs when subretinal fluid (ie, retinal detachment) or blood is present, also disrupts the important exchange of metabolites.
Figure 1-9 Illustrations of the Bruch membrane. A, The RPE and its relationship to the photoreceptors and Bruch membrane. Note the interdigitations and villi from the RPE that make contact with the outer segments of both rods and cones. B, The folded plasmalemma of the RPE rests on its smooth basement membrane (0.3 μm thick) bordering the inner collagenous layer (1.5 μm thick). The outer collagenous layer (0.7 μm thick) borders the elastic layer (0.8 μm thick) and is continuous with intercapillary bridges and the subcapillary fibrous tissue.
(Part A courtesy of the University of Rochester; part B by Daniel Casper, MD, PhD.)
The RPE functions as a barrier to prevent diffusion of metabolites between the choroid and the subretinal space. Because of this, the environment of the photoreceptors is largely regulated by the selective transport properties of the RPE. The RPE has a high capacity for water transport, so in a healthy eye, fluid does not accumulate in the subretinal space. This RPE-mediated dehydration of the subretinal space also modulates the bonding properties of the interphotoreceptor matrix, which acts as a bridge between the RPE and photoreceptors and helps bond the neurosensory retina to the RPE. With deterioration or loss of the RPE, there is corresponding atrophy of the overlying photoreceptors and underlying choriocapillaris.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.