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

Straightbred Yorkshire (Y) conceptuses are larger than straightbred Me ishan (M) conceptuses throughout gestation and at farrowing. In contra st, when Y and M conceptuses were gestated together in Y recipient fem ales, the birth weight of M pigs was similar to that of their Y litter mates. Even though placentae of ICI: pigs remained markedly smaller th an 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 throug hout late gestation in Y or M uteri. Gravid uteri were recovered at sl aughter from M and Y females that were gestating either M or Y concept uses on d 70, 90, or 110 of gestation. Uterine and conceptus measureme nts were recorded, and a section of the intact endometrial-placental a ttachment site for each conceptus was fixed, embedded, and later evalu ated 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 great er (P < .001) than that of M placentae, regardless of uterine type in which they were gestated. The vascular density of M placentae and adja cent 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 increa se in placental surface area. In contrast, the surface area of Y place ntae doubled in size (P < .001) between d 90 and 110 of gestation, but placental and adjacent endometrial vascular density remained relative ly constant, averaging 3.2 and 3.8%, respectively. These data are cons istent with the premise that placental size is largely determined by t he uterus in which a conceptus is gestated until approximately d 90. A fter d 90, fetal breed-specific mechanisms maintain optimal fetal grow th. Between d 90 and term, M fetal growth depends on progressive incre ases in placental blood vessel density and requires no increase in pla cental size. In contrast, Y conceptuses seem to rely exclusively on pl acental growth to increase placental-endometrial surface area for nutr ient exchange.