Effect of estradiol-17 beta 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, conta in greater numbers of trophectoderm cells, and secrete greater amounts of e stradiol-17 beta (E(2)beta) when gestated in a Yorkshire as compared with M eishan uterus to d 12. Additionally, placentas of Meishan conceptuses are l arger when gestated in a Yorkshire as compared with Meishan uterus througho ut gestation. Embryonic E(2)beta 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 determin e whether E(2)beta administration to Meishan gilts at the time of conceptus elongation would increase placental size at term. Meishan gilts (n = 12) w ere 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 rand omly assigned in equal numbers to receive injections of sesame oil (VEH) st arting on d 12 (control), 1 mg of E(2)beta in VEH starting on d 12 (E(2)12) , or 1 mg of E(2)beta in VEH starting d 13 (E(2)13). The injections were in itiated at 0700 or 1900 (corresponding to the time of day they first exhibi ted estrus) and continued at 6-h intervals for 48 h, resulting in 8 mg of E (2)beta given in eight injections. Pregnant gilts were killed on d 112 of g estation, and ovulation rate, litter size, implantation site length, fetal weight, crown-rump length, placental weight, and placental surface area wer e quantified. There were no differences among E(2)12, E(2)13, and control f emales in ovulation rate or litter size, which averaged 16.3 +/- .7 and 11. 8 +/- .7, respectively. Fetal weight and crown-rump length were not differe nt (P > .10) among E(2)12, E(2)13, and control females, averaging 802 +/- 2 6 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 cm(2), P < .001) for E212 and E213 vs control gilts, respect ively. Placental efficiency (estimated as fetal weight:placental weight) wa s 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 amou nt of E(2)beta exposure around the time of elongation affects placental siz e at term. Additionally, the difference in placental efficiency between con trol and E(2)beta groups indicate that E(2)beta-induced increases in placen tal size led to a reduced placental efficiency.