Melanotic adenocarcinoma of the anorectum

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Abstract
Epithelial cells of an otherwise typical [human] anorectal mucinous adenocarcinoma contained melanin pigment. Ultrastructurally, 3 cell populations were present. The largest population contained membrane-bound compound melanosomes in cells having glandular epithelial characteristics including surface specializations, microvilli and mucin. Occasional melanocytes were intermixed with the tumor cells, and rare cells having both mucin and isolated melanosomes were present. This phenomenon is better explained by phagocytosis or transfer of melanin from melanocytes to neoplastic cells than by melanin synthesis in adenocarcinoma cells.