Fine structural characterization of melanosomes in dysplastic nevi.

This study characterized the fine structure of melanosomes in melanocytes of the dysplastic nevi in six cases with lentiginous melanocytic dysplasia and one case with epithelioid-cell melanocytic dysplasia, and compared that structure with those of common melanocytic nevi and malignant melanoma. It was found that: fine structural features of melanosomes in dysplastic melanocytes were similar in these two histologic variants, and they were markedly aberrant and quite different from those of common melanocytic nevi; the aberrant melanosomes could be grouped into four subtypes, type 1 melanosomes corresponding to ellipsoidal-lamellar melanosomes, type 2 to spherical-incompletely lamellar melanosomes, type 3 to spherical-granular melanosomes, and type 4 to spherical-vacuolated melanosomes; type 2, 3, and 4 melanosomes were seen in all seven cases although their substructures and numbers varied in individual cases; and these type 2, 3, and 4 melanosomes were not seen in epidermal melanocytes of common melanocytic nevi, but were characteristically seen in superficial spreading melanoma and nodular melanoma. The findings indicated that the fine structural changes in synthesis and melanization of melanosomes are unique to the dysplastic nevi and that they may be helpful in diagnosis of the nevi and may fill the gap of abnormal melanogenesis that exists between common melanocytic nevi and malignant melanoma.