A Decreased Placental Size and Increased Vascular Density Results in Increased Prolificacy in the Meishan Pig
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The Chinese Meishan pig farrows 3 to 5 more pigs per litter than U.S. pig breeds. When Meishan and Yorkshire embryos are co-transferred to a Yorkshire recipient, Meishan fetuses and placentae are smaller than Yorkshire fetuses and placentae through day 90 of gestation. At farrowing, Meishan placentae are still smaller than Yorkshire placentae; however, Meishan fetuses are born at the same weight as their Yorkshire littermates. This tremendous growth of the Meishan fetus between day 90 and term is the result of an increased vascularization of the Meishan placenta. In this experiment we investigated the time course of placental vascular development of Meishan and Yorkshire conceptuses during late gestation. Our results demonstrate that Yorkshire fetuses increase the potential for nutrient extraction from the maternal blood by increasing the size of their placentae; however, Meishan fetuses achieve the same end by increasing the density of placental blood vessels while maintaining a constant placental size. The increased vascular density of the Meishan placenta then allows the conceptus to occupy less space in the uterus, allowing more fetuses to survive to term.