Conceptus Competition for Nutrients in the Porcine Uterus: Different Strategies Exhibited by the Meishan and Yorkshire Pig Breeds

Previous research from our laboratory demonstrated that Meishan conceptuses develop more slowly and synchronously to day 30 of gestation than conceptuses of U.S. pig breeds. Furthermore, the reduced size of the Meishan conceptus on day 30 allows more Meishan than Yorkshire conceptuses to occupy the same amount of limited uterine space. As a result, Meishan litter size is significantly larger than that of U.S. pig breeds (13−14 vs. 9-10 piglets/litter). An additional consistent, but unexpected, finding in the Meishan pig was the observation that there was significantly greater amounts of unoccupied uterine space in the Meishan than the Yorkshire female at term. We previously demonstrated that an additional strategy of the Meishan female to increase fecundity was to super-vascularize its placental membranes so that oxygen and nutrient transfer from the sow could be accomplished over a reduced surface area, negating the necessity of further placental growth. These data suggested that when a Meishan conceptus dies, the placenta of its neighbors need not grow into this newly vacated space, whereas the Yorkshire conceptus might increase the size of its less vascular placenta to use the opportunity. Therefore, it was our objective to confirm that Yorkshire conceptuses, but not Meishan conceptuses increase their placental size when adjacent conceptuses are experimentally destroyed on day 40 of gestation. To accomplish this objective, pregnant Meishan and Yorkshire females were laporatomized on day 40. One uterine horn was randomly chosen to be receive alternative fetal crushing (i.e., every other fetus in the horn was crushed by mechanical pressure), whereas the other uterine horn served as the control horn. At slaughter on day 111 of gestation (term = 114 days), we found no differences in fetal weight between the control and treated horns regardless of breed. Similarly, there was no difference in placental weight or surface area or implantation site length (the length of placental attachment in the uterine horn) between the control and treated horns in the Meishan. In contrast, however, there was a marked increase in placental weight and surface area, as well as implantation site length for conceptuses in the treated horn of the Yorkshire gilts versus the control horn. Furthermore, the unoccupied spaces between Meishan conceptuses in the treated horn were 2-fold greater than for conceptuses in the control horn, whereas there were no differences in the length of unoccupied spaces between conceptuses in the Yorkshire’s control or treated horns. These data suggest that in the Meishan treated horn, conceptuses do not use this extra space as effectively as conceptuses in the Yorkshire treated horn. The inability of Meishan placenta to grow into adjacent unoccupied spaces may not be detrimental to conceptus survival due to its greater ability to increase vascular density in response to increasing fetal demands. If U.S. pig breeds have the potential to increase placental vascularity, rather than increase in placental size to nourish the growing fetuses, the potential exists for increasing litter size due to a decrease in uterine competition throughout gestation.