Squamous carcinoma in situ of the skin containing premelanosomes, with melanocytic colonization of the tumor

Abstract

Premelanosomes in nonmelanocytic epithelial neoplasms and “colonization” of nonmelanocytic tumors by melanocytes are two phenomena rarely documented in the literature. We report a squamous carcinoma-in-situ of skin displaying both phenomena. Light microscopy showed clusters of tumor cells in the epidermis, some of which contained melanin. Dendritic melanocytes were admixed with the tumor cells. No ulceration was present. Immunoperoxidase stains for keratin showed no staining of tumor cells. Some nondendritic cells stained for HMB-45, consistent with a melanocytic lesion. Electron microscopy showed two cell types, one with desmosomes, tonofilaments, and premelanosomes and a second dendritic type with only premelanosomes. Premelanosomes were also present free in the intercellular space. These findings suggest that premelanosomes may first be discharged by melanocytes into the intercellular space and are then phagocytosed by the neoplastic cells. Thus the presence of premelanosomes in a tumor cell is not pathognomonic for melanoma or other neural crest tumors.