The presence of stage II melanosomes (premelanosomes) in neoplasms other than melanomas.

Aside from melanomas, other nonmelanocytic pigmented tumors synthesize melanin or contain benign passenger melanocytes. While Stage IV melanosomes (mature melanosomes) occur in neoplasms which synthesize melanin as well as in those with benign companion melanocytes, Stage II melanosomes (premelanosomes), which are found in melanocytes and cells of pigmented nonmelanocytic tumors of neural crest origin, are considered the morphologic hallmark of in vivo melanin synthesis. To test this widely held concept, we studied the ultrastructure of representative malignant melanomas and other pigmented tumors (pigmented variants of the nevocellular nevus, squamous cell carcinoma, schwannoma, basal cell carcinoma, and seborrheic keratosis). Discrete intracytoplasmic Stage II melanosomes were noted in neoplastic cells of tumors of neural crest origin (melanoma, schwannoma, and nevocellular nevus), which are widely believed to synthesize melanin. In addition, they were also detected in neoplastic epithelial cells of a squamous cell carcinoma, basal cell carcinoma, and seborrheic keratosis. In these epithelial tumors, a spectrum of melanosomes from Stage II through Stage IV were presumably acquired from nonneoplastic companion melanocytes, which were an integral part of the tumor. Because squamous epithelium has not been shown to synthesize melanin, this study suggests that the finding of intracytoplasmic Stage II melanosomes does not necessarily imply melanin synthesis. When accompanied by melanocytes, epithelial and perhaps other tumors may contain ingested Stage II melanosomes.