Ultrastructural differences between forming eumelanin and pheomelanin as revealed by the pink-eye mutation in the fowl

Abstract

The ultrastructural events occurring during melanin formation in the fowl were studied. Melanin precursors are apparently “packaged” in Golgi vesicles. Several of these small vesicles seem to coalesce, forming a large vesicle in which globular bodies become visible. The internal contents of these “multivesiculated bodies” probably contain melanin granule precursors. As longitudinal strands form in the vesicles, they become folded in a zigzag fashion. Melanin is then deposited upon this premelanosome matrix. When eumelanin is synthesized, crosslinks form between the longitudinal strands during the early stages of melanin deposition. Melanin is deposited until a melanosome is formed which shows no internal structure. The pink-eye ( pk) mutation retards melanin deposition so that the matrix structure is visible. Premelanosome formation in retinal melanocytes from both normal and pink-eyed embryos was very similar, suggesting that the mutation does not alter matrix structure. The longitudinal strands of the rod-shaped eumelanin premelanosome are coupled by crosslinks that occur every 200 Å. The resulting network is a matting of connected components showing a repeating hexagonal configuration. Each side of a hexagon is 115 Å long. This matting appears rolled up inside the premelanosome membrane when viewed in cross section. In the ovoid or spherical pheomelanin premelanosomes the zigzag longitudinal strands form but crosslinks are few or absent, resulting in a disorganized matrix. Lighter-than-normal pheomelanin granules show even shorter longitudinal strands and less organization.