RPE Histopathology and Morphometry

Normal human retinal pigmented epithelium (RPE) is a monolayer of cuboidal cells in which the cytoplasm is filled with melanin pigment granules. The basal surface of an RPE cell faces the Bruch’s membrane, a five-layer structure lying between the retina and choroid. The apical surface borders the subretinal space, the apical microvilli interdigitate with the photoreceptor outer segments. The special apical-basal polarized nature of the RPE is required for the proper development of the photoreceptors and choroid. This polarization allows for multiple functions of the RPE cells. RPE cells also contain lipofuscin granules, which increase in number and size throughout life due to outer segment phagocytosis. The total number of RPE cells has been reported to vary between 3.6 and 6.1 million in a healthy human eye, and cells are mainly regular hexagons. Both aging and certain retinal disorders are associated with not only a decline in the relative number of hexagonal RPE cells and an increased variability in sidedness (pleomorphism), but also with an increase in the variability of cell area (polymegethism). Fundamental differences have been shown in RPE cell density and morphometry across topographical location. RPE dysfunction can play either a primary role or a secondary role through interactions with the neurosensory retina, Bruch’s membrane, and choriocapillary network in different diseases such as age-related macular degeneration, proliferative retinopathies, central and peripheral retinal dystrophies, mitochondrial diseases, and tumors of the RPE.