Individual and combined effects of relaxin, estrogen, and progesterone in ovariectomized gilts. II. Effects on mammary development.

Marked development of the mammary parenchyma (ducts, lobules, and alveoli) occurs during the last third of the 115-day gestation period in the gilt. These changes in the mammary gland are temporally correlated with elevated blood levels of relaxin, estrogen, and progesterone. We recently demonstrated that relaxin plays a major role in promoting the development of the mammary parenchyma that occurs in pregnant gilts. The roles of estrogen and progesterone in the mammary gland in gilts remain poorly understood. Accordingly, this study determined the influence of relaxin, estrogen, and progesterone, individually and in combination, on mammary development. Fifteen days after ovariectomy, six to eight nonpregnant sexually mature gilts were assigned to one of the following eight treatment groups: ovariectomized controls, relaxin treated, estrogen treated, progesterone-treated, estrogen plus relaxin treated, progesterone plus relaxin treated, estrogen plus progesterone treated and progesterone plus estrogen plus relaxin treated. Treatment was given for 10 days, with doses of relaxin (0.5 mg, four times daily), estradiol benzoate (1 mg, twice daily), and progesterone (50 mg, twice daily) selected to provide blood levels similar to those occurring between days 100-110 of gestation. The fourth mammary gland from the anterior end was collected. The cross-sectional area of the parenchymal tissue was used to measure the extent of parenchymal development. To examine mammary parenchymal development in more detail, the effect of hormone treatment on lobulo-alveolar development was conducted using morphometric analysis. Relaxin alone had little effect on mammary development, and estrogen alone stimulated mammary development only modestly. In contrast, combined treatment with estrogen and relaxin stimulated mammary development markedly. Progesterone alone had no effect on lobulo-alveolar development, but, surprisingly, when administered in combination with relaxin, progesterone reduced the organization of the collagen fibrils of the mammary stroma. In conclusion, this study is consistent with recent reports that relaxin plays an important role in the development of the mammary gland in gilts. Moreover, it demonstrates that relaxin acts in concert with estrogen to stimulate development of the mammary parenchyma. Finally, this study demonstrates that relaxin in concert with progesterone decreases the organization of the collagen matrix surrounding the growing mammary gland. This study is consistent with the view that relaxin, estrogen, and progesterone all very likely play roles in the development of the mammary gland in gilts.