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 p igment epithelium of mammals originate from photosensory membrane degr adation. To test this hypothesis the retinal pigment epithelium of cat tle was kept in tissue culture and exposed to gold-labelled rod outer segments. Gold granules were later detected inside phagosomes, melanos omes and mature melanin granules. Tyrosinase, the key enzyme in melano genesis, was additionally localized inside phagosomes. These results i ndicate that in cultured retinal pigment epithelium the matrix of the melanosome can originate from phagosomes. Therefore, the melanosome is a specialized lysosome.