Neuroendocrine (Merkel-cell) carcinoma of the skin. Cytology, intermediate filament typing and ultrastructure of tumor cells in fine needle aspirates.

The light and transmission electron microscopic findings and the intermediate filament typing of tumor cells from fine needle aspirates of primary, recurrent and metastatic neuroendocrine (Merkel-cell) carcinoma of the skin are described. The tumor cells in the smears coexpressed keratins and neurofilaments and were characterized by "intermediate filament buttons," i.e., buttonlike fragments of cytoplasm of tumor cells, which could be observed either by staining with intermediate filament-specific antibodies using immunofluorescence or by their appearance in hematoxylin-and-eosin-stained smears. These features may help in the differential diagnosis of fine needle aspirates of neuroendocrine carcinomas of the skin.