Developmental Expression of Vascular Endothelial Growth Factor and Its Receptors in Ovine Placenta and Fetal Membranes

Objectives: Vascularity of the surface of the placenta in humans and of the placenta and fetal membranes in several species including sheep is an important determinant of intramembranous absorption of amniotic fluid. Our previous studies have shown that the total blood vessel surface area in ovine amnion and chorion increases with advancing gestation. Vascular endothelial growth factor (VEGF) is a potent angiogenic and pemeability factor and is found to be expressed in the ovine placenta and fetal membranes. To investigate the role of VEGF in maintaining the absorptive function of the intramembranous microvessels, the present study was undertaken to determine the gestational change in gene expression of VEGF and its receptors, kinase insert domain-containing receptor (KDR) and fms-like tyrosine kinase (Flt-1), in ovine placenta, chorion, and amnion.

Methods: Total RNA was extracted from placental cotyledon, chorion, and amnion of ovine fetuses at 60-140 days of gestation. The relative abundance of VEGF, KDR, and Flt-1 mRNA was determined by Northern blot analysis, and VEGF molecular forms expressed were identified by reverse transcriptase polymerase chain reaction. The gestational changes in mRNA levels of VEGF and its receptors were analyzed by regression analysis.

Results: In ovine placenta, chorion, and amnion, VEGF mRNA levels increased significantly from 60 to 140 days. The major VEGF molecular form expressed in these tissues was VEGF₁₆₄, whereas VEGF₁₂₀, VEGF₁₄₄, and VEGF₁₈₈ were present at lower levels. In the placenta, KDR was the primary VEGF receptor expressed, although Flt-1 was also detected at very low levels. In the amnion and chorion, KDR was the only receptor expressed. A gestational-dependent change in VEGF receptor expression was not observed in the placenta and membranes.

Conclusions: The increase in VEGF gene expression with advancing gestation in the amnion and chorion where KDR is expressed suggests that VEGF and its receptor are important determinants of vascularity and permeability, and thus exchange capacity, of the intramembranous pathway.