Apocrine differentiation in human mammary carcinoma.

Six invasive carcinomas that contained apocrine differentiation as the primary morphologic pattern were selected from a series of 1500 prospectively examined breast carcinomas (0.4%). While apocrine features were seen in many breast tumors, these six cases were identified by uniformly fine granular, pale, eosinophilic cytoplasm with apical cytoplasmic projections similar to that seen in apocrine metaplasia. In each example, ultrastructural analysis revealed the presence of numerous 400-600 nm membrane bound vesicles with dense homogeneous osmophilic cores. These granules clustered toward the apex of the cytoplasm in the majority of the epithelial cells. All six tumors were deficient in high-affinity, low-capacity 8S estrogen and progesterone proteins, while a high-capacity, low-affinity, nonsaturable 4S progesterone-estrogen binding protein was observed. Cortisol did not bind to this protein. These observations characterize the ultrastructure of apocrine carcinoma as a variant of human mammary carcinoma.