Effects of relaxin on the mouse mammary gland. I. The myoepithelial cells.

Myoepithelial cells (MEC) were studied in the mammary gland of ovariectomized virgin mice without any hormonal treatment and following the administration of either estrogen or estrogen plus relaxin (either purified porcine relaxin standard or partially purified human relaxin extracted from decidua). Changes in MEC lineage were observed at both the light and electron microscopic levels in the hormonally treated animals as compared with the untreated controls. After 7 days of estrogen administration, there was a moderate elongation and branching of ducts. At the growing tips and buds undifferentiated precursors and promyoepithelial cells at various stages of their differentiation were seen with some frequency. After relaxin administration (18-20 hrs) to mice pre-treated with estrogen, branching and elongation of ducts were greatly increased as compared to mice treated with estrogen alone. At the growing points the ducts showed multi-layered walls and undifferentiated precursors were quite frequently found with some mitotic figures. Moreover several promyoepithelial cells at various stages of differentiation were observed. These findings, which were similar after administration of the two different relaxin preparations, indicate a high rate of proliferation and differentiation of MEC in these experimental conditions and hence suggest that relaxin greatly enhances the de novo formation of MEC. A stimulatory effect of relaxin on MEC growth is discussed in terms of a possible role of this hormone in the genesis of benign and malignant neoplasms of the mammary gland, in which MEC proliferation can be recognized.