Neuroendocrine (Merkel cell) tumor of the skin: fine-needle aspiration cytology, histology, electron microscopy and immunohistochemistry of 12 cases.

A series of 12 cases of neuroendocrine (Merkel cell) tumor of the skin, observed in the last 4 years, is reported. The mean age of the patients, 10 females and 2 males, was 62.4 years. The head, buttocks and extremities were the primary locations. The recurrence rate was 33.3\% and 2 patients died of their disease. The histology is characterized by small round cells growing in solid sheets, nests or trabeculae. Ultrastructural study showed cytoplasmic membrane-bound granules of neurosecretory type and collections of perinuclear intermediate filaments as constant findings. The presence of cytoskeletal proteins was studied with monoclonal antibodies against cytokeratin and neurofilaments, as well as neuron-specific enolase reactivity, by the immunoperoxidase technique in 7 cases. Cytokeratin was demonstrated in 5 tumors and neurofilaments in 3, whereas the neuron-specific enolase was positive in all tested cases. In 4 patients a fine-needle aspiration biopsy of the primary, skin recurrences or lymph node metastasis was performed, showing the reliability of this technique in the diagnosis and staging of the disease.