Intermediate filaments in Merkel cell tumors.

A series of ten Merkel cell tumors is described, with special emphasis on intermediate filament expression. The presence of cytoskeletal proteins was studied with a polyclonal antiserum directed against cytokeratin and with monoclonal antibodies against cytokeratin, neurofilament, and vimentin by the immunoperoxidase technique. Cytokeratin was demonstrated in nine of ten tumors. Neurofilament was observed in the two snap-frozen tissues tested and in three of the eight formalin-fixed, paraffin-embedded tissues. No reactivity for vimentin was found. By electron microscopy desmosomes were found to be present in all cases, while tonofilaments were found in only a few cases. neurosecretory granules, although seen in all tumors, were generally present in low numbers. The results of this study indicate that the Merkel cell tumor is a poorly differentiated small cell carcinoma that has the ability to express some neuroendocrine features.