Effect of Relaxin on Mammary Development in Ovariectomized Pregnant Gilts*

Abstract

The role of relaxin in mammary development was studied between days 80-110 of pregnancy in ovariectomized gilts given progesterone to maintain pregnancy. To obtain an objective measurement of lobulo-alveolar (parenchymal) composition, mammary glands were cut in cross-section through the teat, and the area of parenchymal tissue on the exposed face of the gland was determined. Ovariectomy on day 80 or 100 followed by progesterone replacement therapy resulted in a dramatic reduction in the rate of growth of mammary parenchymal cross-section area on days 100 and 110 of gestation, respectively, compared to that in controls. In contrast, progesterone plus relaxin therapy, with highly purified porcine relaxin, restored the mammary parenchymal cross-section area to control values in ovariectomized gilts. Morphometric analysis of mammary tissue on day 110 of pregnancy indicated that both the absence of relaxin after ovariectomy and replacement therapy with porcine relaxin in ovariectomized gilts had little if any effect on the percentages of the lumen, stroma, or epithelial that comprised the mammary parenchyma. It is concluded that relaxin has a stimulatory effect on the growth of mammary parenchymal tissue during late gestation in the pig.

Regulation of Mammary Development as It Relates to Changes in Milk Production Efficiency

“…Although not classically considered to be involved in mammary development, research has shown that it is critical for normal mammary development in rodents (Bani et al, 1986), ruminants (Cowie et al, 1965), and pigs (Hurley et al, 1991;Bagnell et al, 1993). Relaxin is also thought to be involved in the inhibition of lactation prior to parturition (Harness & Anderson, 1975).…”

Effects of relaxin administration in early gestation or prior to mating on uterine length and fetal survival in gilts

“…Relaxin is secreted by the CL, and levels are elevated in the peripheral blood during the last third of the approximately 114-day gestation period [12,13]. Relaxin may contribute to the maintenance of fetal viability as well as to the development of both the cervix [14,15] and mammary glands [16] during this period. Although blood levels of relaxin are relatively low during the first 2 mo of gestation [13], they may contribute to the initiation of uterine growth during early pregnancy [17].…”

New Insights into Biological Roles of Relaxin and Relaxin-related Peptides

“…Studies of ovariectomized primiparous pigs demonstrated that endogenous relaxin not only is required for softening of the cervix, but also promotes development of the mammary gland parenchyma [23,44]. In contrast, it appears in rats that the major effects of relaxin on mammary growth and development target the nipple.…”