Comparison of macromelanosomes and autophagic giant melanosome complexes in nevocellular nevi, lentigo simplex and malignant melanoma

This study compared the fine structure of macromelanosomes with that of giant melanosome complexes formed through melanosomal autophagocytosis in nevocytes and melanocytes of nevocellular nevi, lentigo simplex and malignant melanoma. While macromelanosomes were found only on rare occasions in these pigmentary disorders [2 of 79 nevocellular nevi (2 junctional nevi), 3 of 12 lentigo simplex and 2 of 93 malignant melanoma], the giant autophagic melanosome complexes were always present, indicating the macromelanosomes are not synthesized simply through melanosomal autophagocytosis. Although both macromelanosomes and giant melanosome complexes exhibited acid phosphatase activity similar to melanosomes, they showed many different ultrastructural features. Characteristically, macromelanosomes contained numerous vesiculoglobular bodies, whereas these bodies were absent in giant melanosome complexes. In those tissues where the presence of macromelanosomes had been ruled out by light microscopy, none of the giant melanosome complexes revealed ultrastructural features indicative of macromelanosomes. Various phases of melanosomal degradation were seen, indicating that they were not simply end-products of lysosomal degradation of melanosomes. It was thought that the key process in the development of macromelanosomes was the accumulation of vesiculoglobular bodies.