The ratio of a fetus’s weight to that of its placenta has been used in our laboratory as an estimate of placental efficiency, which defines the number of grams of placenta required to support a gram of fetus. Because the pig placenta is noninvasive, nutrients from the mother must diffuse from uterine blood vessels to placental blood vessels at the placental-endometrial interface. A pig placenta can respond to increasing fetal nutrient demands by either increasing in size, and thus surface area in contact with the endometrium, or by increasing the number of blood vessels per unit area at the fetal-maternal interface. Previous studies from our laboratory have shown Meishan and Yorkshire conceptuses gestated in a Meishan uterus had markedly smaller placentae than Meishan or Yorkshire conceptuses gestated in a Yorkshire uterus whereas fetal weights across the two uterine environments were much more similar. These data suggested that conceptuses in a Meishan uterine environment have a greater vascular density at the placentalendometrial interface. Greater densities of blood vessels can be achieved by either vasodilation (increasing the diameter of existing blood vessels) or angiogenesis (growth of new vessels from preexisting ones). Hypoxia, or inadequate oxygen transport, has been shown to increase angiogenic factors, such as vascular endothelial growth factor (VEGF), which stimulate blood vessel development. It is possible that VEGF may be important in stimulating increased blood vessel density in the pig placenta, as it has been to shown to do in the ovine placenta. Our objective was to determine if VEGF mRNA expression was associated with placental and/or endometrial vascular density, placental efficiency, and litter size during late gestation. We observed a positive association of both placental vascular density (r=0.37, p<0.05) and VEGF mRNA levels (r=0.35; P<.05) with placental efficiency . There was also a positive correlation between VEGF mRNA levels and the number of conceptuses in a litter (r=0.42; P<.05) on day 70, 90, and 110 of gestation. Although Yorkshire uteri exhibited greater endometrial vascular density than Meishan uteri on day 70 of gestation, placentae of conceptuses gestated in a Meishan uterus had greater amounts of VEGF mRNA than placentae of conceptuses gestated in a Yorkshire uterus. The greater amounts of placental VEGF mRNA of conceptuses gestated in a Meishan uterus on day 70 may have resulted in the increased placental vascular density observed on day 90. We conclude that the increased litter size of the Meishan female may stem from her having smaller, more vascular placentae than placentae of conceptuses gestated in the uterus of a Yorkshire female, to allow for efficient nutrient delivery to the fetus. The increased placental vascular density of a conceptus gestated in the uterus of a Meishan female may result from increased placental VEGF mRNA production induced from an endometrial induced hypoxia on day 70 of gestation.
Iowa State University
Vonnahme, Kimberly A.; Wilson, Matthew E.; and Ford, Stephen P., "Role of Vascular Endothelial Growth Factor in Placental Vascularization" (2000). Swine Research Report, 1999. 7.