Malignant melanoma, which is histologically classified into melanotic and amelanotic types, develops mainly in the skin. However, it rarely occurs in the oral and maxillofacial region. Only a few reports have examined histopathological differences between melanotic and amelanotic types. We examined biopsy specimens from three cases of malignant melanoma (melanotic: 2 cases, and amelanotic: 1 case) to determine histopathological differences between amelanotic tumor cells (AMTCs) and melanotic tumor cells (MTCs). Studies included histopathological, immunohistochemical, and electron microscopical. analyses. Pathological examination showed that AMTCs lacking melanin granules proliferated mainly at the front of tumor invasion, whereas many MTCs were located at the center of tumor nests. Immunohistochemically, AMTCs had a higher PCNA-positive-cell rate (71.6%) than MTCs (48.8%). On electron microscopical examination, a melanosome-like structure without melanin production was observed in AMTCs, whereas melanosomes with melanin production were found in MTCs. These findings suggest that AMTCs are more immature and have more aggressive potential than MTCs. Moreover, AMTCs might invade the surrounding area before MTCs and change into MTCs.
