Neuroendocrine carcinoma of the skin (Merkel cell carcinoma). An immunoelectron-microscopic case study

An unusual tumor with a controversial name as well as histogenesis, the neuroendocrine carcinoma of the skin (also known as "Merkel cell carcinoma," "trabecular carcinoma of the skin") has previously been extensively studied by immunohistochemical methods at the light-microscopic level. Ultrastructural descriptions of this tumor have also been extensive, although immunocytochemical study of this neoplasm at the electron-microscopic level has been limited. In this report, we have used postembedding protein A-gold immunocytochemistry on thin sections from tumor embedded in Lowicryl K4M to investigate the expression and ultrastructural localization of a panel of commercially available, diagnostically useful antibodies. Antibodies associated with epithelial derivation included anti-keratin monoclonal antibody AE1/AE3, polyclonal anti-keratin, and monoclonal anti-cytokeratin cocktail (MAK-6), as well as a monoclonal antibody against epithelial membrane antigen (EMA). Antibodies associated with neuroendocrine derivation included monoclonal anti-chromogranin A and monoclonal anti-synaptophysin. Although staining with a polyclonal antibody directed against neuron-specific enolase (NSE) was equivocal, there was no labeling with a monoclonal anti-neurofilament antibody. The finding of positive keratin labeling of filaments arranged in paranuclear aggregates correlates well with the previously 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 the skin.