Distinguishing Cowper's glands from neoplastic and pseudoneoplastic lesions of prostate: immunohistochemical and ultrastructural studies.

Mucin-producing Cowper's glands, which are situated in the urogenital diaphragm, can be sampled inadvertently by transurethral resection of the prostate and rarely by needle biopsy. Because they are small, closely packed glandular units, Cowper's glands can be misinterpreted as prostatic adenocarcinoma. A panel of immunoperoxidase and mucin stains performed on 10 Cowper's glands showed negative immunoreactivity for prostatic-specific antigen, prostatic alkaline phosphatase, S-100 protein, and carcinoembryonic antigen. Acini in nine of the 10 Cowper's glands were negative for high-molecular-weight cytokeratin K-903 (34beta E12). One case showed faint focal staining of cells around the periphery of acinar units. Smooth muscle actin consistently stained the periphery of acini in all cases. Ultrastructural examination of one Cowper's gland showed the presence of myoepithelial cells at the periphery of the acini. Contrary to previous reports, the acini were lined by a prominent secretory cell layer underlain by an attenuated myoepithelial cell layer. A negative stain for K-903. without additional immunohistochemical study on Cowper's glands taken during transurethral resection or needle biopsy, may substantiate an erroneous diagnosis of prostatic adenocarcinoma. This potential misdiagnosis of carcinoma can be averted if samples stain positive for mucin and smooth muscle actin and negative for prostate-specific antigen and prostatic alkaline phosphatase.