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The Function of the Retinal Pigment Epithelium
Introduction
The retinal pigment epithelium (RPE) is a monolayer of pigmented cells located between the light-sensitive photoreceptor outer segments and the fenestrated endothelium of the choriocapillaris. On both sides, specialized extracellular matrices enable a close interaction of the RPE with its adjacent tissues. On the basolateral side the multilayered Bruch's membrane combines barrier function and selective transport matrix as part of the blood–retina barrier. On the apical side of the RPE, the interphotoreceptor matrix provides an interface for interaction between the photoreceptor outer segments and the RPE. In this interaction, the RPE forms a functional unit with the photoreceptors ( Fig. 13.1 ). In humans, every RPE cell is in interaction with a mean of 23 photoreceptors. The formation of that functional unit is already of importance during the embryonic development. The RPE and the neuronal retina depend on each other in the process of differentiation and maturation. Mutations in genes expressed in the RPE can lead to primary photoreceptor degeneration and mutations in genes expressed in photoreceptors can lead to primary RPE degenerations. In this functional unit the RPE fulfills a multitude of tasks which are essential for visual function. The failure of one of these functions leads to retinal degeneration.
Absorption of light
As a pigmented layer of cells the RPE covers the inner wall of the bulbus and absorbs scattered light to improve the optical quality. In the human eye light is focused by a lens onto the macula. This represents a high density of energy which is absorbed by the melanin granula of the RPE leading to an increase in the temperature of the RPE choroid complex. The heat is transported away by the bloodstream in the choriocapillaris. However, this functional arrangement bears the danger of photo-oxidative damage. , The relative blood perfusion of the choriocapillaris is higher than that of the kidney. , However, only small amounts of oxygen are extracted by the adjacent tissues and venous blood from the choroid still shows an oxygen saturation of more than 90 percent. Thus, there is an overflow of oxygen in combination with a large density of light energy. This leads to a large production of reactive oxygen species. The RPE is protected by various lines of defense against this intoxication: melanin of the melanosomes and the carotenoids lutein and zeaxanthin absorb light energy, ascorbate, α-tocopherol and β-carotene, and glutathione are non-enzymatic antioxidants and melanin itself can function as an anti-oxidant. This is supplemented by the cell's natural ability to repair damaged DNA, lipids and proteins.
Transepithelial transport
The RPE cells form a part of the blood–retina barrier. The RPE is by its electrophysiological parameters a tight epithelium: the paracellular resistance is at least ten times larger than that of the transcellular resistance. , This barrier function is of great importance for the immune privilege of the eye (see below). Due to this tight barrier function, the complete exchange of molecules and ions between the bloodstream and the photoreceptor side relies on transepithelial transport through the RPE.
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