Apocrine type of cutaneous mixed tumor with follicular and sebaceous differentiation.

Eight cases of apocrine (tubular branching lumina) type cutaneous mixed tumors with follicular and sebaceous differentiation are presented. All eight tumors arose on facial skin; six patients were male and two were female. The lesions showed a cystic or nodular clinical appearance and were surgically excised. Histopathological examination confirmed the diagnosis of apocrine type of cutaneous mixed tumor in each case. Follicular differentiation consisted of (a) keratinous cysts with infundibular keratinization (infundibular differentiation); (b) hair bulbs with papillary mesenchyma, matricial differentiation with basophilic, transitional, and shadow cells, trichohyaline granules, vellous hair shafts, and clear cells of the outer root sheath (anagen differentiation); and (c) epithelial columns composed of inner cells with plump oval nuclei and scant cytoplasm, and similar cells at the periphery that were arranged in a palisade, resembling the inferior segment of a normal hair follicle in telogen. Sebaceous differentiation was represented by mature sebaceous cells, either as single cells or as small islands, within epithelial tracts of the tumor. The proportion of the areas showing these different types of differentiation varied among lesions, but some follicular differentiation was always present, whereas three cases lacked sebaceous differentiation. Immunohistochemical analysis in three cases with respect to their eccrine or apocrine differentiation showed contradictory results as in a previously reported series of cutaneous mixed tumors. The presence of follicular and sebaceous differentiation in the apocrine (tubular branching lumina) type of cutaneous mixed tumor is a confirmation of the apocrine nature of this neoplasm as well as an expression of the common embryologic derivation of all elements of the folliculosebaceous-apocrine unit.