Merkel cell carcinoma.

Merkel cell carcinoma is a rare, highly aggressive neuroendocrine cutaneous neoplasm with a variable clinical presentation. Histologically, it is a predominantly dermal-based lesion composed of monotonous small round cells with scanty cytoplasm, often difficult to differentiate from small round cell tumors, metastatic small cell carcinoma, blastic hematologic malignancies, and melanoma. The malignant cells express both epithelial and neuroendocrine immunohistochemical markers, a unique feature that helps differentiate this neoplasm from other entities. The pathogenesis of Merkel cell carcinoma has remained a mystery despite its association with various chromosomal abnormalities and with growth signaling and apoptotic pathways. The discovery of the Merkel cell polyomavirus suggests another clue to its pathogenesis. This virus integrates into band 3p14 and promotes carcinogenesis by altering the activity of tumor suppressor and cell cycle regulatory proteins. This discovery of the Merkel cell polyomavirus may greatly enhance our understanding of this rare aggressive neoplasm and allow further advancements in treatment.