INDIVIDUAL AND COMBINED EFFECTS OF RELAXIN, ESTROGEN, AND PROGESTERONE IN OVARIECTOMIZED GILTS .2. EFFECTS ON MAMMARY DEVELOPMENT

Marked development of the mammary parenchyma (ducts, lobules, and alve oli) occurs during the last third of the 115-day gestation period in t he 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 r emain poorly understood. Accordingly, this study determined the influe nce of relaxin, estrogen, and progesterone, individually and in combin ation, on mammary development. Fifteen days after ovariectomy, six to eight nonpregnant sexually mature gilts were assigned to one of the fo llowing eight treatment groups: ovariectomized controls, relaxin treat ed, estrogen treated, progesterone-treated, estrogen plus relaxin trea ted, progesterone plus relaxin treated, estrogen plus progesterone tre ated and progesterone plus estrogen plus relaxin treated. Treatment wa s given for 10 days, with doses of relaxin (0.5 mg, four times daily), estradiol benzoate (1 mg, twice daily), and progesterone (50 mg, twic e daily) selected to provide blood levels similar to those occurring b etween days 100-110 of gestation. The fourth mammary gland from the an terior end was collected. The cross-sectional area of the parenchymal tissue was used to measure the extent of parenchymal development. To e xamine mammary parenchymal development in more detail, the effect of h ormone treatment on lobuloalveolar development was conducted using mor phometric analysis. Relaxin alone had little effect on mammary develop ment, 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 lobu loalveolar development, but, surprisingly, when administered in combin ation with relaxin, progesterone reduced the organization of the colla gen fibrils of the mammary stroma. In conclusion, this study is consis tent with recent reports that relaxin plays an important role in the d evelopment of the mammary gland in gilts. Moreover, it demonstrates th at relaxin acts in concert with estrogen to stimulate development of t he mammary parenchyma. Finally, this study demonstrates that relaxin i n concert with progesterone decreases the organization of the collagen matrix surrounding the growing mammary gland. This study is consisten t with the view that relaxin, estrogen, and progesterone all very like ly play roles in the development of the mammary gland in gilts.