The impacts of uterine environment and fetal genotype on conceptus size and placental vascularity during late gestation in pigs

Meishan and Yorkshire gilts were bred exclusively to Yorkshire and Meishan boars, respectively, resulting in similar Meishan x Yorkshire fetuses gesta ting in the uteri of both maternal breeds. Gilts of both breeds were slaugh tered on d 90 and 110 of gestation, and the weight and volume of each uteri ne horn was determined. Fetal weights and crown-rump lengths were recorded. A section of the chorioallantoic-endometrial attachment site was collected for each conceptus and evaluated for placental and endometrial vascular de nsity. An intact placenta was recovered for each conceptus and weighed, and its surface area was determined. Fetal weight and crown-rump length increa sed (P < .001) markedly between d 90 and 110 of gestation in Meishan and in Yorkshire uteri, but they were markedly less (P < .001) in Meishan than in Yorkshire uteri. Placental weight and placental surface area were reduced by 40% in Meishan, compared with Yorkshire, uteri; however, placental size did not increase between d 90 and 110 in either uterine type. Placental vas cular density and associated endometrial vascular density were similar (P > .20) for conceptuses in Meishan and Yorkshire uteri on d 90 and 110 of ges tation. Additionally, even though the ratio of fetal weight:placental weigh t (placental efficiency) increased between d 90 and 110, placental efficien cy was similar for conceptuses in Meishan and Yorkshire uteri. In a previou s study using straightbred Meishan or Yorkshire conceptuses gestated in eit her a Meishan or Yorkshire uterus, we found Meishan conceptuses had a marke dly greater placental efficiency than did Yorkshire conceptuses, regardless of the type of uterus in which they were gestated. These data indicate tha t uterine type determines conceptus size and conceptus genotype controls pl acental efficiency.