Cytokeratin 15 expression in apocrine mixed tumors of the skin and other benign neoplasms with apocrine differentiation

To clarify the features of apocrine mixed tumors (AMT) of the skin among benign neoplasms with apocrine differentiation in their relationship to follicular stem cells, we investigated the immunohistochemical expression of CK15 (LHK15 and C8/144B), which is a relatively specific marker of hair follicle stem cells in the bulge, in 35 cases of eight different benign neoplasms with presumed apocrine differentiation. All eight cases of AMT of the skin showed CK15 immunostaining of the neoplastic cells, and all four cases of syringocystadenoma papilliferum, all five cases of spiradenoma, and both cases of cylindroma also showed a focally positive reaction to CK15. None of the other benign neoplasms with presumed apocrine differentiation showed CK15 expression. In AMT of the skin, the proportion of CK15-positive cells in the follicular or sebaceous differentiation group (78.8%, average of four cases) was significantly higher than the group without this differentiation (8.8%, average of four cases). AMT of the skin are unique among benign neoplasms with apocrine differentiation in their substantial and constant CK15 expression, suggesting that they derive from multipotent epithelial stem cells in the bulge. AMT of the skin with follicular or sebaceous differentiation are considered to show an immature stage of apocrine differentiation still rich in stem cells or to originate from stem cells with an incompletely established apocrine fate. The partially positive reaction for CK15 in syringocystadenomas papilliferum and spiradenoma/cylindroma may depend on the ability to express CK15 in stem cells with an apocrine fate or result from the follicular and apocrine nature of this neoplasm.