Merkel cell carcinoma with glandular differentiation admixed with sweat gland carcinoma and spindle cell carcinoma: histogenesis of merkel cell carcinoma from hair follicle stem cells.

We describe a unique case of Merkel cell carcinoma (MCC) with a heterogeneous differentiation exhibiting distinct triphasic phenotypic differentiation features: small cells typical of MCC, sweat gland carcinoma (sweat gland Ca.) with possible decapitation secretion, and spindle cell carcinoma (spindle cell Ca.). The patient was an 84-year-old Japanese woman. We evaluated the present case immunohistochemically with various antibodies. The histological features showed a gradual transition from MCC to sweat gland Ca. and spindle cell Ca. For clarifying the histogenesis, immunophenotypic analysis of the 3 different components of the carcinoma was performed using hair follicle stem cell markers (eg, CK15, CK19, and CD200) that have been identified as biomarkers of human bulge cells. The triphasic components immunohistochemically shared the characteristic feature of CK19 and CD200 expression. We posit that the MCC arose from hair follicle stem cells residing within the bulge area where Merkel cells are preferentially situated. Based on our findings, we recommend adding this rare neoplasm to the expanding morphological spectrum of MCC.