Eccrine and squamous differentiation in Merkel cell carcinoma. An immunohistochemical study.

Of the 42 Merkel cell carcinomas that we studied, two showed numerous tubular structures within sheets and nests of small cells. The small cells stained for both neuron-specific enolase and keratin. The keratin decorated a dot-like paranuclear structure. The ducts stained positively for carcinoembryonic antigen (CEA) and CF-1 (cystic fibrosis-1, a monoclonal antibody that only stains eccrine duct and acrosyringium). Electron microscopy performed on one case showed cytoplasmic dense-core neurosecretory granules and intercellular lumina lined by cells containing microvilli. These ultrastructural and immunohistochemical features support the concept of eccrine differentiation in these tumors. A third case contained foci of typical keratinizing squamous cell carcinoma admixed with sheets of small cells. The immunohistochemical and ultrastructural characteristics of this tumor were essentially similar to those of a conventional Merkel cell carcinoma. Our findings suggest that Merkel cell carcinomas, similar to neuroendocrine tumors from other anatomic sites arise from a primitive totipotential stem cell that has the capacity to differentiate along different cell lines.