Evidence for melanogenesis in the retinal pigment epithelium of adult cattle and golden hamster.

1. The ultrastructure of the retinal pigment epithelium (RPE) of adult Syrian golden hamsters and cattle was examined with respect to pigment granules and phagosomes involved in degradation of disk membranes from rod outer segments. 2. In the RPE of cattle, phagosomes were found that contained an electron-dense melanin-like material that was not autofluorescent and therefore not lipofuscin. 3. Disk membranes of rods are about 4 nm thick and become enlarged (7-20 nm) and electron-dense during degradation in the RPE. 4. Additionally electron-dense vesiculo-globular bodies (10-100 nm) were found in phagosomes during disk membrane degradation and in mature melanin granules. 5. In the RPE of adult hamsters that had been exposed to intense light, premelanosomes containing unmelanised filaments with a striated periodicity were found in the cytoplasm or in association with mature melanin granules. Early and late stage melanosomes were also present. Phagosomes in the RPE contained degraded disk membranes, melanin-like material and melanofilaments. 6. Dopa oxidase was detected ultrastructurally within shed disk membranes that were in close contact with the microvilli of the RPE. 7. The possibility of melanogenesis within phagosomes during disk membrane degradation is discussed.