Primary malignant melanoma of the rectum. Evidence for origination from rectal mucosal melanocytes.

Reports of rectal melanoma often attribute the lesion to tumor extension from anal melanocytes which have undergone malignant transformation while the existence of true primary melanoma of the rectum has been disputed. This dispute primarily relates to past inability to demonstrate normal melanocytes in rectal mucosa. In this case report, two polypoid melanomas clearly located in the rectum were discovered at sigmoidoscopy. Thorough histologic examination failed to demonstrate atypical anal melanocytes, and the presence of normal melanocytes in the rectal mucosa was confirmed by electron microscopy and immunohistochemistry. Although rare, it appears that primary malignant melanoma may arise from melanocytes among columnar rectal epithelium.