Pregnancy

Differential Expression of the Vascular Endothelial Growth Factor-Receptor System in the Gravid Uterus of Yorkshire and Meishan Pigs

Abstract
Litter size in the pig is limited by uterine capacity, which is dependent on uterine size, placental size and vascularity. Placentae of U.S. pig breeds such as the Yorkshire, exhibit marked growth from mid to late gestation, increasing their surface area of endometrial attachment. In contrast, placentae of the prolific Chinese Meishan pig exhibit little growth from mid to late gestation, but instead exhibit a marked and progressive increase in the density of placental blood vessels. Vascular endothelial growth factor (VEGF) is a potent angiogenic and permeability enhancing factor produced and secreted by placentae of several species including the pig. VEGF activity is mediated through two specific receptors (VEGF-R1 and VEGF-R2), both of which are expressed by placental and endometrial tissues in pigs, and are thought to play a role in mediating increased vascularization and/or permeability at the fetal:maternal interface. The objectives of this study were to determine concentrations of VEGF in fetal blood and placental fluids, as well as placental and adjacent endometrial mRNA expression of VEGF, VEGF-R1, and VEGF-R2 on days 30, 50, 70, 90, and 110 of gestation in Yorkshire and Meishan pigs. Day 90 Meishan conceptuses exhibited marked increases (P<0.05) in placental VEGF mRNA expression, as well as fetal blood and allantoic fluid concentrations of VEGF, which remained elevated through day 110. In contrast, Yorkshire conceptuses failed to exhibit increases in placental VEGF mRNA expression, or concentrations of VEGF in fetal blood or allantoic fluid until day 110. While receptor mRNA expression patterns differed between Meishan and Yorkshire conceptuses, there was no difference in their expression levels. Placental efficiency (fetal weight/ placental weight) was higher (P<0.05) on day 90 and 110 in Meishan than in Yorkshire conceptuses. The earlier increase in VEGF protein and mRNA expression in the Meishan vs the Yorkshire conceptus may explain the previously reported increased vascularity and increased placental efficiency of this breed compared the Yorkshire breed.

Key words: Pregnancy • Conceptus • Developmental biology • Placenta • Uterus

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