Merkel cell (small cell) carcinoma of the skin: immunohistochemical and ultrastructural demonstration of distinctive perinuclear cytokeratin aggregates and a possible association with B cell neoplasms.

Three cases of Merkel cell (small cell) carcinoma of the skin are presented with immunohistochemistry for epithelial and neuroendocrine antigens. All three cases showed distinctive punctate perinuclear cytoplasmic positivity for cytokeratin which corresponded to aggregates of intermediate filaments, seen ultrastructurally in two cases. Epithelial membrane antigen was also identified in two cases. Only one case showed cytoplasmic positivity for neuron specific enolase, and immunostaining for a battery of polypeptide hormones was negative. The demonstration of cytokeratin perinuclear inclusions provides a distinctive immunohistochemical feature to aid in their diagnosis. Two of the three patients had chronic lymphocytic leukaemia years before the diagnosis of Merkel cell carcinoma. The possible association of lymphoproliferative disorders, particularly B cell tumours, and Merkel cell carcinoma is discussed.