An immunohistochemical study of the apocrine type of cutaneous mixed tumors with special reference to their follicular and sebaceous differentiation.

An immunohistochemical analysis of 8 cases of the apocrine type of cutaneous mixed tumor is reported. Histologically, 7 cases of the tumor were suggested to have follicular and/or sebaceous differentiation. Carcinoembryonic antigen (CEA), epithelial membrane antigen and gross cystic disease fluid protein-15 were present in the inner surface and/or in the luminal cell bodies of the tubular structures. CEA and non-specific cross-reacting antigen were also detected in the keratinous cysts. Positive reactions for S-100 protein and vimentin were observed in the solid nests, the outer layer of the tubular structures and some stromal cells. However, smooth muscle actin was only focally expressed in the outer cells of the tubular nests in one case. Involucrin was positive in the inner layers of the keratinous cysts. Various staining patterns were observed in the keratin immunohistochemistry. The solid nests showed heterogeneous expressions of CK1/5/10/14, CK7, CK10/11, CK14, CK8/18 and CK19. The inner cells of the tubular structures were constantly positive for CK7, CK8/18 and CK19, and heterogeneously for CK1/5/10/14, CK10/11 and CK14. The outer cells were heterogeneously positive for CK1/5/10/14 and CK14. The keratinous cysts showed positive reactions for CK1/5/10/14 throughout the whole cell layers, and for CK14 in the basal layer. The inner layers in some keratinous cysts expressed CK6, CK10/11 and CK17, but only CK10 in others. CK13 was positively stained in the transitional portion between the matricial structure and the column of shadow cells in the cyst wall in only one case. When compared with the immunohistochemistry of the normal skin, the apocrine type of cutaneous mixed tumor can show various immunophenotypical patterns similar to those of entire structures of hair follicles, sebaceous glands and all components of apocrine glands, that is to say, the folliculosebaceous-apocrine unit.