Effects of relaxin on the mouse mammary gland. III. The fat pad

The effects of relaxin on the growth of the mammary fat cells of ovariectomized virgin mice have been studied histologically and morphometrically. To characterize the effects of relaxin and investigate a possible synergism in promoting growth of the mammary fat pad, some animals were treated with other mammotrophic hormones, namely estrogen and insulin. The data obtained after 18-20 h of relaxin treatment suggest that this hormone induces hypertrophy and hyperplasia of adipose cells. The degree of hypertrophy is the same if relaxin is given either alone or after estrogen priming. The action of estrogen seems to be obligatory to obtain fat cell hyperplasia. Indeed, a de novo formation of fat cells occurs in all animals treated with estrogen, either alone or in association with relaxin or insulin. However, the maximum degree of adipose cell hyperplasia was attained only when relaxin followed a pretreatment with estrogen, thus suggesting a synergistic action of these two hormones in promoting the overall growth of the mammary fat pad. These findings follow the observation that relaxin stimulates the proliferation and differentiation of epithelial and myoepithelial cells of the duct system and strongly support the idea that relaxin may be regarded as a trophic hormone for the parenchymal and stromal components of the mammary gland.