Embryonic and fetal development in different genotypes in pigs.

It is widely accepted that uterine capacity, not ovulation rate, is the greatest restraint on litter size in pigs. Recently, the reproductive strategy(s) of the Chinese Meishan pig, a breed which farrows three to five more piglets per litter than US or European pig breeds, has come under intense scrutiny. It was initially determined that the Meishan female could farrow more viable piglets per litter than US or European pig breeds, with a uterine size and ovulation rate equivalent to those of less prolific breeds. It has become apparent that the Meishan conceptus exhibits a reduced trophectoderm mitotic rate during the preimplantation period, elongates from fewer cells and remains smaller throughout gestation compared with conceptuses from less prolific US or European pig breeds. This strategy by the Meishan conceptus for a lower growth rate results in a marked reduction in conceptus loss through day 18 of gestation compared with less prolific breeds. An additional strategy is required in the Meishan to allow the larger number of viable fetuses to survive after day 30 of gestation when uterine capacity becomes limiting. Our research has demonstrated that the rapid growth of the fetus in US pig breeds appears to require continued placental growth to increase the surface area for nutrient exchange. In contrast, the increased number of smaller Meishan fetuses achieve the same increase in placental efficiency by markedly increasing the density of placental blood vessels at the fetal-maternal interface. This proliferation of placental blood vessels obviates the need for marked increase in placental size.