Effect of estradiol-17beta administration during the time of conceptus elongation on placental size at term in Meishan pigs

Meishan embryos transferred to recipient females on d 2.5 are larger, contain greater numbers of trophectoderm cells, and secrete greater amounts of estradiol-17beta (E2beta) when gestated in a Yorkshire as compared with Meishan uterus to d 12. Additionally, placentas of Meishan conceptuses are larger when gestated in a Yorkshire as compared with Meishan uterus throughout gestation. Embryonic E2beta secretion during elongation on d 12 to 13 of gestation is temporally associated with endometrial secretion of growth factors, including IGF-I, which has been shown to increase mitotic rate in the trophectoderm of pig embryos. This experiment was conducted to determine whether E2beta administration to Meishan gilts at the time of conceptus elongation would increase placental size at term. Meishan gilts (n = 12) were checked twice daily for estrus (0700 and 1900), and each was bred to a Meishan boar at 0 and 24 h after the onset of estrus (d 0). Gilts were randomly assigned in equal numbers to receive injections of sesame oil (VEH) starting on d 12 (control), 1 mg of E2beta in VEH starting on d 12 (E212), or 1 mg of E2beta in VEH starting d 13 (E(2)13). The injections were initiated at 0700 or 1900 (corresponding to the time of day they first exhibited estrus) and continued at 6-h intervals for 48 h, resulting in 8 mg of E2beta given in eight injections. Pregnant gilts were killed on d 112 of gestation, and ovulation rate, litter size, implantation site length, fetal weight, crown-rump length, placental weight, and placental surface area were quantified. There were no differences among E(2)12, E(2)13, and control females in ovulation rate or litter size, which averaged 16.3 +/- .7 and 11.8 +/- .7, respectively. Fetal weight and crown-rump length were not different (P > .10) among E(2)12, E(2)13, and control females, averaging 802 +/- 26 g and 24.3 +/- .3 cm. Placentas were markedly heavier (176 +/- 14 and 174 +/- 16 vs 134 +/- 10 g, P < .05) and larger (1,337 +/- 97 and 1,520 +/- 70 vs 978 +/- 29 cm2, P < .001) for E(2)12 and E(2)13 vs control gilts, respectively. Placental efficiency (estimated as fetal weight:placental weight) was greater (P < .05) in the control than in the E(2)12 and E(2)13 gilts (5.8 +/- .2 vs 4.8 +/- .2 and 5.1 +/- .4). These data demonstrate that the amount of E2beta exposure around the time of elongation affects placental size at term. Additionally, the difference in placental efficiency between control and E2beta groups indicate that E2beta-induced increases in placental size led to a reduced placental efficiency.