Submission Number: STE-4-417-194

Abstract Number: 3

UTERINE AND FETAL FACTORS IN PLACENTAL SIZE AND VASCULARITY.

Stephen P Ford

Department of Animal Science, Iowa State University, Ames, Iowa ¹

Abstract:
It is widely accepted that uterine capacity, not ovulation rate is the major factor limiting litter size in the pig. We evaluated the reproductive strategy(s) of the prolific Chinese Meishan pig which farrows 3-5 more piglets per litter than the Yorkshire breed but has a similar uterine size. Using reciprocal embryo transfer between the Meishan and Yorkshire, it was determined that both fetal genotype and uterine breed have an impact on placental size, vascularity and efficiency. Placental efficiency, defined in our lab as the ratio of a fetus's weight to that of its placenta was much greater in the Meishan breed which exhibited a smaller more vascular placenta. Because the pig placenta is epitheliochorial and diffuse, nutrients from maternal blood must diffuse from maternal to fetal blood across the endometrial-placental interface. The placenta can respond to increasing fetal nutrient demands by either increasing its size, or by increasing its vascularity. At farrowing, Meishan and Yorkshire conceptuses gestated in Meishan versus Yorkshire uteri had placental efficiencies averaging 8.7 and 4.1 versus 6.3 and 3.4. The difference in these placental efficiencies were due to marked differences in placental size (Yorkshire > Meishan) and vascularity (Meishan > Yorkshire), not differences in piglet weight. Further, there was a marked positive correlation between placental and adjacent endometrial vascular densities. This association implies that communication between the sow and conceptus occurs to optimize nutrient transfer. It is known that expression of one of the most potent placental angiogenic factors, vascular endothelial growth factor (VEGF), is stimulated by hypoxia and estrogen. Both the conceptus demand for oxygen and placental secretion of estrogen increase dramatically throughout late gestation in the pig. We have recently determined that placental VEGF mRNA expression increases throughout gestation in association with increases in placental vascularity. Further, within individual conceptuses of either breed, VEGF protein concentration in umbilical blood was variable and highly correlated with placental vascularity and efficiency. Studies are underway to determine the physiologic and genetic basis of the observed differences in placental efficiency.

Keywords: Placental angiogenesis, litter size, pig

This abstract is being presented at: 11:30 AM in session:
Minisymposium XII: ANGIOGENESIS IN THE UTERUS AND PLACENTA