Pigmentation

A characteristic feature of the RPE is the presence of melanin pigment. Pigment granules are abundant in the cytoplasm of adult RPE cells, predominantly in the apical and midportions of the cell (see Fig 13-5B). During development, activation of the tyrosinase promoter triggers the onset of melanogenesis in this cell and marks the commitment of the neuroectoderm to become RPE. Although most melanogenesis occurs before birth, melanin production in the RPE occurs throughout life, albeit at a slow rate. As humans age, the melanin granules fuse with lysosomes; thus, the fundus of an older person is less pigmented than that of a young person. Clinically, this is most evident in the peripheral fundus.

The exact role of melanin within cells remains speculative. One universally recognized function of melanin is to act as a neutral-density filter in scattering light. In so doing, melanin may have a protective role. But even in the minimally pigmented fundus, visual acuity can be 20/20. Visual problems in persons with albinism are attributable to foveal hypoplasia, not optical scatter. When melanin levels are below a critical level, as in oculocutaneous albinism, there is aberrant neuronal migration in the visual pathway (more contralateral projections of ganglion cells), incomplete foveal development, low vision, nystagmus, and strabismus.

Melanin is also a free-radical stabilizer and can bind many toxins and drugs (such as chloroquine and hydroxychloroquine). Some regard this feature as protective; others think that it contributes to tissue toxicity.

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.