Melanosome formation in the goldfish: the role of multivesicular bodies

Melanosome formation in differentiating melanophores of embryonic and adult xanthic goldfish was studied by cytochemical methods at the ultrastructural level. Large vesicles (0.4–0.5 μm) are blebbed from rough endoplasmic reticulum. Golgi-derived small vesicles, containing the enzyme tyrosinase, fuse with the membrane of the large vesicle, invert and reform within the large vesicle exposing the tyrosinase. This multivesicular body represents the premelanosome. Melanin synthesis begins around the periphery of the inverted vesicles and eventually fills the intervesicular spaces and finally the inverted vesicles. Inasmuch as an internal lamellar matrix is not characteristic of goldfish premelanosomes, the incorporation of Golgi-derived vesicles not only provides tyrosinase for melanin synthesis, but serves as the structural foundation for melanin synthesis. Melanization of pterinosomes as suggested by other workers ( 29, 30, 37, 38) was not observed. A structural control mechanism in the activation of tyrosinase is discussed.