Smooth‐muscle differentiation in stromal cells of malignant and non‐malignant breast tissues

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Abstract
A mouse monoclonal antibody (MAb) recognizing α-smooth-muscle actin has been used to study smooth-muscle differentiation features in the stromal cells of desmoplastic reactions accompanying mammary tumors. We have studied, by the same immunohistochemical technique, a series of malignant and non-malignant human breast tissues. Cells composing the desmoplastic reaction were found to express α-smooth-muscle actin in all the 11 breast carcinomas examined, whereas no immunostain was demonstrated in the stromal cells of 7 breast tissue samples histologically defined as normal. Three of 9 cases of fibrocystic disease showed a minority of positively stained stromal cells, generally in association with epithelial hyperplasia. All the 7 cases of sclerosing adenosis, 3 of 4 cases of diffuse papillomatosis and all 3 intraductal papillomas exhibited a majority of immunoreactive stromal cells. Numerous stromal cells in 3 of 11 circumscribed fibroadenomas analyzed expressed low amounts of α-smooth-muscle actin. The factor(s) responsible for smooth-muscle differentiation in stromal cells are presently unknown, but the detection of this previously unsuspected stromal cell phenotype in non-malignant mammary tissues might help in characterizing the variant morphological aspects designated under the label “fibrocystic disease” and in understanding the biology of pre-malignant or early malignant lesions of the breast.