The RPE consists of a monolayer of hexagonal cells with apical microvilli and a basement membrane at the base of the cells. This monolayer has the following specialized functions:
vitamin A metabolism
maintenance of the outer blood–retina barrier phagocytosis of the photoreceptor outer segments absorption of light
heat exchange
formation of the basal lamina of the inner portion of the Bruch membrane
production of the mucopolysaccharide matrix that surrounds the photoreceptor outer segments active transport of materials into and out of the subretinal space
Compared with that of the retina, the topographic variation of the RPE is subtle. In the macula, the RPE is taller, narrower, and more heavily pigmented, and it forms a regular hexagonal array. In the equatorial and midperipheral area, the RPE cells are larger in diameter and thinner. Variability in the diameter of the RPE cells increases in the peripheral retina. The amount of cytoplasmic pigment, primarily lipofuscin, increases with age, particularly within the RPE in the macular region.
Congenital Anomalies
Albinism
Albinism is a general term that refers to a congenital dilution of the pigment of the skin, the eyes and the skin, or just the eyes. The condition results from genetic mutations that cause abnormalities in the biosynthesis of melanin pigment. True albinism has been subdivided into oculocutaneous and ocular albinism. This distinction is somewhat helpful clinically, but in reality all cases of ocular albinism have some degree of mild cutaneous involvement. There is a pathophysiologic difference between the 2 types of albinism. In oculocutaneous albinism, transmission is commonly autosomal recessive, and the amount of melanin in each melanosome is reduced, whereas in ocular albinism, transmission is commonly X-linked recessive, and the number of melanosomes is reduced (Fig 11-3). See BCSC Section 6, Pediatric Ophthalmology and Strabismus, and Section 12, Retina and Vitreous, for further discussion.
Myelinated Nerve Fibers
Generally, myelination of the nerve fibers in the optic pathways terminates at the lamina cribrosa. However, myelination produced by oligodendroglial cells in the NFL can occur (see Fig 26-7 in BCSC Section 6, Pediatric Ophthalmology and Strabismus). Though usually contiguous with the optic nerve head, myelination may also occur in isolation away from the optic nerve head and, if large, can produce a clinically significant scotoma. Myelinated nerve fibers have been associated with myopia, amblyopia, strabismus, and nystagmus.
Figure 11-3 Albinisim. A, Iris transillumination. B, Fundus hypopigmentation. C, Photomicrograph illustrates decreased pigmentation in the iris pigment epithelium (smaller melanosomes) allowing visualization of the nuclei (arrow). No appreciable pigmentation is present in the iris stroma (asterisk). D, Photomicrograph shows RPE and choroid in an albino eye. Note the apical distribution of melanin granules and overall decreased pigmentation in the RPE (arrow), rare giant melanosomes (arrowhead) in the RPE, and lack of appreciable pigmentation in the choroidal stroma (asterisk). (Parts A and B courtesy of Robert
H. Rosa, Jr, MD; parts C and D courtesy of Tatyana Milman, MD, and Ralph C. Eagle, Jr, MD.)
Vascular Anomalies
There are numerous congenital anomalies of the retinal vasculature, including capillary hemangioma and cavernous hemangioma, parafoveal telangiectasia, and Coats disease. Many of these anomalies and their clinical features are covered in BCSC Section 12, Retina and Vitreous. In Coats disease, exudative retinal detachment occurs as a result of leakage from abnormalities in the peripheral retina, including telangiectatic vessels, microaneurysms, and saccular dilations of retinal vessels (Fig 11-4). Histologically, retinal detachments secondary to Coats disease are characterized by the presence of “foamy” macrophages and cholesterol crystals in the subretinal space.
Figure 11-4 A, Leukocoria as a result of Coats disease. B, Total exudative retinal detachment in Coats disease. Note the dense subretinal proteinaceous fluid (asterisk). C, Telangiectatic vessels (asterisks) and “foamy” macrophages (arrowhead) typical of Coats disease. D, High magnification of subretinal exudate showing lipidand pigment-laden macrophages
(arrows) and cholesterol clefts (arrowheads). (Parts A–C courtesy of Hans E. Grossniklaus, MD; part D courtesy of George J. Harocopos, MD.)
Congenital Hypertrophy of the RPE
Congenital hypertrophy of the RPE (CHRPE), a relatively common congenital lesion, is characterized clinically by a flat, dark black lesion varying in size from a few to 10 mm in diameter (see Chapter 17, Fig 17-10). Frequently, central lacunae and a peripheral zone of less dense pigmentation appear within the lesion. This lesion is histologically characterized by enlarged RPE cells with densely packed and larger-than-normal, spherical melanin granules (Fig 11-5). This benign congenital condition can generally be distinguished from choroidal nevi and melanoma on the basis of ophthalmoscopic features. Adenoma and adenocarcinoma of the RPE may develop, in rare instances, in an area of CHRPE. RPE lesions mimicking CHRPE may be present in Gardner syndrome, or familial adenomatous polyposis. Histologic study of the RPE changes in Gardner syndrome reveals that they are more consistent with hyperplasia of the RPE than with hypertrophy. The RPE changes in Gardner syndrome are probably more appropriately termed hamartomas, consistent with the loss of regulatory control of cell growth that gives rise to the other soft-tissue changes in this syndrome.
Traboulsi EI. Ocular manifestations of familial adenomatous polyposis (Gardner syndrome). Ophthalmol Clin North Am. 2005;18(1):163–166.