Phenotypic heterogeneity of nodular hidradenoma. Immunohistochemical analysis with emphasis on cytokeratin expression.

We studied the immunohistochemical phenotype in 13 cases of the nodular hidradenoma (NH), with special emphasis on the expression of different types of keratins (cytokeratins, 7, 10, 6/18, 8/18, and 10/17/18 and their distribution in normal sweat glands. Variable reactions with keratins, alpha-smooth muscle actin, and epithelial membrane antigen (EMA) were found, as these markers were present in different cellular components of the tumors. The most constant finding was almost complete absence of cytokeratins (all but keratin 10/17/18, which was positive in two of 13 cases) in clear cells, which yet were positive for EMA. The tumors expressed mostly cytokeratin 6/18, 7, 8/18, and 10/17/18, which were found in 11, 13, 11, and 12 cases, respectively. The cellular distribution and quantity of stained cells differed, as keratins 6/18, 8/18, and 7 produced the most abundant staining and were predominantly localized in small squamoid cells and the cells lining the tubular and cystic spaces. Cytokeratin 10/17/18 was expressed in smaller or larger clusters of squamoid cells and rarely in clear cells. Cytokeratins 10, 19, and 20 were found sporadically in single cells or small cellular clusters. alpha-Smooth muscle actin was expressed in four cases, whereas we did not find reactivity of S-100 protein. Comparing these results with the pattern of keratin distribution and antigenic reactivity in eccrine sweat glands, we conclude that NH presents cellular heterogeneity of its elements and differentiation toward different parts of the sweat gland.