Immunohistochemical demonstration of myoepithelial cells in sweat gland carcinomas.

Although myoepithelial cells are detectable in many benign sweat gland tumours, little is known about their role in sweat gland carcinomas. To specifically demonstrate myoepithelial cells, paraffin sections from 46 sweat gland carcinomas were stained, using a standard avidin-biotin-peroxidase complex method, with the monoclonal alpha-smooth muscle actin antibody 1A4. Myoepithelial cells were not found in adenoid cystic eccrine carcinoma (n = 2), malignant nodular hidradenoma (n = 2), porocarcinoma (n = 4), extramammary Paget's disease (n = 12), sclerosing sweat duct carcinoma (n = 4) or in adenosquamous-mucoepidermoid carcinoma (n = 1). In contrast, myoepithelial cells were demonstrated in two of eight apocrine adenocarcinomas, one of six mucinous eccrine carcinomas and two of seven eccrine adenocarcinomas. In all these tumours myoepithelial differentiation was found in peripheral cells of solid tumour islands, or in basal cells of tubular structures. However, in most areas of the tumours, myoepithelial layers were discontinuous. Cells in the centre of solid tumour nodules, and luminal cells of tubular structures, were negative for alpha-smooth muscle actin. In analogy to breast tumours, in which malignancy and invasiveness correlate with scattered or absent myoepithelial cells, we suggest that disrupted myoepithelial layers in sweat gland carcinomas may be interpreted as a loss of the invasion barrier.