The Retinal Pigment Epithelium

The simplest light detecting organs are composed of two cell types: the light sensitive photoreceptor and the pigmented cell. Both cell types appear in conjunction in every eye of the animal kingdom from insects, mollusca to higher vertebrates (1). Functional interaction of both the pigmented cell and the photoreceptor cells is essential for visual function (2-5). Already beginning with the embryonic development the functional differentiation of the photoreceptor layer and the retinal pigment epithelium (RPE) depend on each other (6, 7). When the communication between the developing RPE and the developing neuronal retina is interrupted the RPE is able to form a multilayered retina-like structure by itself (8). The RPE and the neural retina co-differentiate during development of the eye. Parts of the RPE/photoreceptor interaction such as importing the vitamin A derivitive, retinal, its metabolism and recycling are initiated before vision starts, and serve as differentiation signals for both tissues (9, 10). Furthermore, analyses of hereditary types of retinal degeneration reveal a strong dependence of the RPE on the photoreceptors and vice versa. For example, mutations in genes which are expressed in the photoreceptor cells can lead to a primary RPE disease with loss of photoreceptors occurring secondarily. Or, on the other hand, mutations in genes which are expressed in the RPE can lead to primary photoreceptor degeneration. Thus the ability of the photoreceptor cell to detect light depends on its interaction with the pigmented cell.