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Department of Animal Science, University of Wyoming, Laramie, WY 82071, USA
Selecting Yorkshire breeding stock for increased placental efficiency (PE; piglet weight divided by its placental weight) results in larger litters (i.e. 
3 more piglets litter-1) and reduced placental sizes. Placental vessel density increases progressively after day 50 of gestation in the pig, and is positively correlated with PE and placental expression of vascular endothelial growth factor (VEGF), a potent angiogenic and permeability factor. To elicit its vascular effects, VEGF must bind to its receptors (R), VEGF-R1 and VEGF-R2. The objective of this study was to compare placental and endometrial blood vessel density and placental VEGF, VEGF-R1 and VEGF-R2 mRNA expression in day 70 and 90 conceptuses from Yorkshire females selected for high PE, low PE, and from unselected controls. A greater (P < 0.05) PE was observed for conceptuses in the high PE selection group when compared to the low PE selection group. Placental blood vessel density increased (P < 0.05) from day 70 to 90 (1.8 ± 0.1 versus 2.8 ± 0.2) in association with increases (P < 0.05) in placental VEGF mRNA expression. No selection group differences were observed in expression of VEGF, VEGF-R1, or VEGF-R2 on day 70. By day 90, however, placentae of conceptuses from the high PE group expressed greater (P < 0.05) amounts of VEGF and VEGF-R1 mRNA than the unselected controls and the low PE group. These data demonstrate that increased placental expression of the VEGF receptor system is associated with increased placental vascular density observed with the advancement of gestation in the pig. Although placental blood vessel density was not increased in the high PE selection group, elevated levels of the VEGF receptor system suggest an effect on increasing placental and endometrial blood vessel permeability and/or proximity in the high PE group.
(Received 15 September 2003;
accepted after revision 22 October 2003;
first published online 24 October 2003)
Corresponding author S. P. Ford: 113 Animal Science/Molecular Biology Complex, Department of Animal Science, University of Wyoming, Laramie, WY 82071, USA. Email: spford{at}uwyo.edu
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