A newly discovered pathway of melanin formation in cultured retinal pigment epithelium of cattle.

According to a recent hypothesis the melanin granules in the retinal pigment epithelium of mammals originate from photosensory membrane degradation. To test this hypothesis the retinal pigment epithelium of cattle was kept in tissue culture and exposed to gold-labelled rod outer segments. Gold granules were later detected inside phagosomes, melanosomes and mature melanin granules. Tyrosinase, the key enzyme in melanogenesis, was additionally localized inside phagosomes. These results indicate that in cultured retinal pigment epithelium the matrix of the melanosome can originate from phagosomes. Therefore, the melanosome is a specialized lysosome.