Myoepithelial differentiation in eccrine spiradenoma : an analysis of benign and malignant cases

Nine cases of benign eccrine spiradenoma (ES) and one case of malignant ES, which all fulfilled the histologic description of the tumor, were analyzed clinicopathologically and immunohistochemically. Histologically, two types of benign ES could be distinguished including 5 cases of ES with numerous tubular structures and 4 cases of ES with poor tubular differentiation. Immunohistochemical studies revealed that positive immunolabelling for α-smooth muscle actin (α-SMA), which is a marker of myoepithelial differentiation, was clearly seen on numerous small dark cells in the first type of ES, while it was not observed on most such cells in the second type. These results indicated that the myoepithelial cells were one of the cellular components of ES and the degree of myoepithelial differentiation corresponded to that of tubular differentiation ; this suggested that ES is a distinct histopathological entity, however the cellular differentiation of tumor cells varies considerably among the different specimens of ES. These findings lead to the speculation that the variation in the cellular differentiation of ES may be one the causes of the controversial results regarding the histogenesis of this tumor observed by several investigators. Since positive immunoreactions for α-SMA, PKK-1 and S-100 protein were seen in tumor cells, it is strongly suggested that ES differentiates towards the secretory portion of the eccrine gland.