NEUROENDOCRINE CARCINOMA OF THE SKIN (MERKEL CELL-CARCINOMA) - AN IMMUNOELECTRONMICROSCOPIC CASE-STUDY

An unusual tumor with a controversial name as well as histogenesis, th e neuroendocrine carcinoma of the skin (also known as ''Merkel cell ca rcinoma,'' ''trabecular carcinoma of the skin'') has previously been e xtensively studied by immunohistochemical methods at the light-microsc opic level. Ultrastructural descriptions of this tumor have also been extensive, although immunocytochemical study of this neoplasm at the e lectron-microscopic level has been limited. In this report, we have us ed postembedding protein A-gold immunocytochemistry on thin sections f rom tumor embedded in Lowicryl K4M to investigate the expression and u ltrastructural localization of a panel of commercially available, diag nostically useful antibodies. Antibodies associated with epithelial de rivation included anti-keratin monoclonal antibody AE1/AE3, polyclonal anti-keratin, and monoclonal anti-cytokeratin cocktail (MAK-6), as we ll as a monoclonal antibody against epithelial membrane antigen (EMA). Antibodies associated with neuroendocrine derivation included monoclo nal anti-chromogranin A and monoclonal anti-synaptophysin. Although st aining with a polyclonal antibody directed against neuron-specific eno lase (NSE) was equivocal, there was no labeling with a monoclonal anti -neurofilament antibody. The finding of positive keratin labeling of f ilaments arranged in paranuclear aggregates correlates well with the p reviously described immunohistochemical staining pattern at the light- microscopic level. Moreover, the presence of cytoplasmic synaptophysin and chromogranin positivity over dense-core granules exemplifies the neuroendocrine differentiation present in this fascinating tumor of th e skin.