Immunohistochemical diagnosis of a Merkel cell tumor in a dog.

We examined the clinical and immunohistochemical features of a Merkel (neuroendocrine) cell tumor on the skin between the eyes of a 12-year-old male Yorkshire Terrier dog. The tumor was characterized by locally expansive dermal nodules composed of solid nests or clusters of epithelioid cells surrounded by fine fibrous stroma. Basal cell epithelioma, Merkel cell tumor, and extramedullary plasmacytoma were also considered as diagnoses. Because the cytoplasm of the tumor cells stained positively for cytokeratin and chromogranin A but not for immunoglobulins, the tumor was diagnosed as a Merkel cell tumor. An electron-microscopic study of a tissue specimen revealed electron-dense granules approximately 200 nm in diameter. These granules were irregularly dispersed throughout the cytoplasm of the tumor cells, which would be expected in neuroendocrine cells. Twelve months after resection, a 0.8-cm-diameter tumor recurred at the original site. However, further follow-up of 22 months revealed no evidence of additional tumor growth, invasion, or metastasis, so we concluded that this tumor was benign.