Melanotic adenocarcinoma of the anorectum.

Epithelial cells of an otherwise typical anorectal mucinous adenocarcinoma contained melanin pigment. Ultrastructurally, three cell populations were present. The largest population contained membrane-bound compound melanosomes in cells having glandular epithelial characteristics including surface specializations, microvilli, and mucin. Additionally, occasional melanocytes were intermixed with the tumor cells and finally very rare cells having both mucin and isolated melanosomes were present. We believe that this phenomenon is better explained by phagocytosis or transfer of melanin from melanocytes to neoplastic cells, rather than by melanin synthesis in adenocarcinoma.