Blue nevus and melanosis of the prostate. Electron-microscopic and immunohistochemical studies.

Two cases of blue nevus and one case of melanosis of the prostate were studied with ultrastructural and immunohistochemical methods. All patients complained of urinary obstruction, and the clinical impression in all was benign prostatic hyperplasia. Melanin was present in the stroma of the prostate in all cases. In one, pigment was also demonstrated both in benign and malignant epithelial cells. Electron microscopically, melanosomes in different stages were present in the two white patients, but only mature stage IV melanosomes were demonstrated in the black patient. The melanin in epithelial cells consisted only of mature melanosomes. Immunohistochemically, the stromal cells that contained melanin stained with S-100 protein. The evidence suggests that the pigmented cells in the prostatic stroma are melanocytes and the melanin in the glandular epithelium is a result of the transfer of pigment from the stromal melanocytes.