The effects of relaxin on the mouse mammary gland. II. The epithelium.

The study deals with the effects of relaxin (RLX) on epithelial cells of the mammary ducts in ovariectomized virgin mice, evaluated by light and electron microscopy. One microgram (3 GPU) pure porcine RLX (NIH-RXN-P1) was administered either alone or after pretreatment with estrogen. The possibility of a non specific binding of RLX to insulin receptors on the mammary epithelial cells was excluded by comparing the effects of the administration of the two hormones at the same doses. Our findings indicate that RLX greatly enhances the growth of the mammary duct epithelial cells, as demonstrated by the extensive branching and elongation of the ducts, and by the rather frequent occurrence of mitoses in the epithelial cell precursors and of the intermediate stages of their differentiation into mature duct cells. Three GPU RLX, administered alone, had only an inconstant and slight effect on the epithelial cell growth, a priming dose with estrogen being necessary to obtain the highest levels of duct growth. Moreover the treatment with 1 microgram of insulin for the same time showed a very inconstant and lesser effect in comparison to RLX. We did not find any evidence of alveolar maturation and milk secretion in the RLX-treated mice. The recognition of a stimulatory role of RLX on the proliferation and growth of the mammary duct epithelium may be of a certain importance in the understanding of hormonal control of the mammary development in physiological conditions and the genesis and growth of mammary neoplasms.