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Merkel cell carcinoma (neuroendocrine carcinoma of the skin).

We review the literature on Merkel cell carcinoma (MCC; primary neuroendocrine carcinoma of the skin) and add 36 cases to the over 800 already described in the literature. Though generally considered a tumor of the elderly, MCC can also occur in young patients. Microscopically, there are dermal proliferations of small cells possessing nuclei with finely dispersed chromatin. The occasional presence of squamous or eccrine differentiation suggests a pluripotential stem cell origin. Immunohistochemistry is of great help in distinguishing these tumors from potential mimics such as malignant melanoma, lymphoma, or metastatic small cell (neuroendocrine) carcinomas. Recent chromosomal studies have enhanced our understanding of the biology of these tumors. Despite the high metastatic rate associated with Merkel cell carcinoma, spontaneous regression has been reported, and there have been some therapeutic successes. The high survival rate for stage I lesions indicates the importance of early recognition and treatment of these tumors.