Retinal Pigment Epithelium
The retinal pigment epithelium (RPE) develops from the outer layer of the optic cup and consists of a monolayer of hexagonal cells that extends anteriorly from the optic nerve head to the ora serrata, where it merges with the pigmented epithelium of the ciliary body (Fig 2-45). Its structure is deceptively simple considering its many functions:
vitamin A metabolism
maintenance of the outer blood–ocular barrier
phagocytosis of the photoreceptor outer segments
absorption of light (reduction of scatter)
formation of the basal lamina of the Bruch membrane
production of the mucopolysaccharide matrix surrounding the outer segments
maintenance of retinal adhesion
active transport of materials into and out of the RPE
Like other epithelial and endothelial cells, RPE cells are polarized. The basal aspect is intricately folded and provides a large surface of attachment to the thin basal lamina that forms the inner layer of the Bruch membrane. The apices have multiple villous processes that envelop and engage with the photoreceptor outer segments (see Fig 2-45). Separation of the RPE from the neurosensory retina is called retinal detachment.
Figure 2-45 RPE. Left: Rod and cone outer segments interact with apical processes of the RPE. Note the pigment granules in the apical aspect of the RPE. Center: Flat preparation of RPE cells. Note the hexagonal shape and melanin granules. Right: Tight junctions between RPE cells act as a barrier to diffusion of solutes from the choriocapillaris and constitute the outer blood–retina barrier. Note the phagosome containing digested photoreceptor discs. Numerous mitochondria, which are required for this highly metabolic tissue, are also depicted.
(Modified with permission from Spalton D, Hitchings R, Hunter P. Atlas of Clinical Ophthalmology. 3rd ed. Oxford: Elsevier/Mosby; 2005:401.)
Contiguous RPE cells are firmly attached by a series of lateral junctional complexes. The zonulae occludentes and zonulae adherentes not only provide structural stability but also play an important role in maintaining the outer blood–ocular barrier (see Fig 2-45). The zonula occludens is the junction at which adjacent plasma membranes are fused, forming a circular band or belt around the surface of adjacent cells. A small intercellular space is present between zonulae adherentes.
RPE cell diameter varies from 10–14 μm in the macula to 60 μm in the periphery. In addition, compared with RPE cells in the periphery, RPE cells in the fovea are taller and thinner, contain more melanosomes, and have larger melanosomes. These characteristics account in part for the decreased transmission of choroidal fluorescence observed during fundus fluorescein angiography. The eye of a fetus or infant contains between 4 and 6 million RPE cells. Although the surface area of the eye increases appreciably with age, the increase in the number of RPE cells is relatively small. No mitotic figures are apparent within the RPE of the normal adult eye.
The cytoplasm of the RPE cells contains multiple round and ovoid pigment granules (melanosomes) (see Fig 2-45). These organelles develop in situ during formation of the optic cup and first appear as nonmelanized premelanosomes. Their development contrasts sharply with that of the pigment granules in uveal melanocytes, which are derived from the neural crest and later migrate into the uvea.
Lipofuscin granules probably arise from the discs of photoreceptor outer segments and represent residual bodies from phagosomal activity. This so-called wear-and-tear pigment is less electron dense than are the melanosomes, and its concentration increases gradually with age. Clinically, these lipofuscin granules are responsible for the signal observed with fundus autofluorescence.
RPE cells also possess phagocytic function; they continually ingest the disc membranes shed by the outer segments of photoreceptor cells, enclosing them within phagosomes. Several stages of disintegration are evident at any given time. In some species, shedding and degradation of the membranes of rod and cone outer segments follow a diurnal rhythm synchronized with daily fluctuations of environmental light.
The cytoplasm of the RPE cell contains numerous mitochondria (which are involved in aerobic metabolism), rough-surfaced endoplasmic reticulum, a Golgi apparatus, and a large round nucleus (see Fig 2-45).
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.