The impact of either a Meishan or Yorkshire uterus on Meishan or Yorkshire fetal and placental development to days 70, 90, and 110 of gestation.J Anim Sci. 1998 Aug; 76(8):2169-76.JA
Straightbred Yorkshire (Y) conceptuses are larger than straightbred Meishan (M) conceptuses throughout gestation and at farrowing. In contrast, when Y and M conceptuses were gestated together in Y recipient females, the birth weight of M pigs was similar to that of their Y littermates. Even though placentae of M pigs remained markedly smaller than placentae of Y littermates, they were significantly more vascular. The objective of this study was to compare and contrast the changes in Y and M conceptus growth and placental-endometrial vascularity throughout late gestation in Y or M uteri. Gravid uteri were recovered at slaughter from M and Y females that were gestating either M or Y conceptuses on d 70, 90, or 110 of gestation. Uterine and conceptus measurements were recorded, and a section of the intact endometrial-placental attachment site for each conceptus was fixed, embedded, and later evaluated for placental and endometrial vascular density. Placental surface area and weight were greater (P < .001) when M or Y conceptuses were recovered from Y uteri compared with M uteri on each day of gestation examined. Further, by d 110, the surface area of Y placentae was greater (P < .001) than that of M placentae, regardless of uterine type in which they were gestated. The vascular density of M placentae and adjacent endometrium doubled (P < .05) between d 70 and 110 of gestation (3.0 and 2.5 vs 6.0 and 5.1%, respectively), with no significant increase in placental surface area. In contrast, the surface area of Y placentae doubled in size (P < .001) between d 90 and 110 of gestation, but placental and adjacent endometrial vascular density remained relatively constant, averaging 3.2 and 3.8%, respectively. These data are consistent with the premise that placental size is largely determined by the uterus in which a conceptus is gestated until approximately d 90. After d 90, fetal breed-specific mechanisms maintain optimal fetal growth. Between d 90 and term, M fetal growth depends on progressive increases in placental blood vessel density and requires no increase in placental size. In contrast, Y conceptuses seem to rely exclusively on placental growth to increase placental-endometrial surface area for nutrient exchange.