Squamomelanocytic tumor: a case report and further insights into its possible histogenesis.

We report a case of combined squamomelanocytic tumor of the skin. Clinically, the lesion was felt to be a squamous cell carcinoma. Histologically, it was characterized by large epithelioid cells admixed with basaloid cells with central squamous differentiation. Immunohistochemical staining showed both cell populations to be reactive with Melan A, BEREP4, and Pan Keratins. Ultrastructural studies revealed simultaneous features of squamous differentiation (dense cytoplasmic tonofilaments with well-developed desmosomes) and melanocytic differentiation (mature/pigmented melanosomes) in the same cell population. This is the second reported case in the English literature with documented biphenotypic or divergent differentiation at the ultrastructural level. The behavior of squamomelanocytic tumor is uncertain given the rarity of reported cases.