A complete VEGF system consisting of the ligand and two of its receptors has been detected for the first time in the bovine cumulus-oocyte-complex (COC). In the course of a 24 hr in vitro maturation procedure (IVM), expression of the smaller VEGF transcripts and their specific receptors flt and flk changed remarkably in a time-dependent manner as observed by RT-PCR. The transcript concentrations of VEGF declined within 24 hr of culture, whereas both receptor mRNAs were found enriched between 6 and 12 hr of IVM. In the follicular fluid of growing ovarian follicles, immunoreactive VEGF, measured by RIA, increased significantly, reaching highest concentrations immediately before ovulation of the oocyte. The immunohistochemical localization of VEGF in bovine COCs revealed strong signals within the cumulus cell complex clearly extending beyond the oocyte cytoplasm at the beginning of in vitro maturation. After 24 hr, IVM immunoreactive VEGF disappeared remarkably from cumulus cells and the oocyte cytoplasm. An exogenous application of VEGF at the beginning of a 24 hr IVM significantly improved cleavage rates of zygotes and their development into bovine embryos showing obvious synergistic effects in combination with FSH, when compared with untreated control embryos. In addition, the number of blastomeres in deriving blastocysts increased after VEGF supplementation. These results indicate a functional VEGF system controlling important events beside the known angiogenetic effect during in vivo and in vitro maturation of the bovine COC, possibly affecting the early embryonic viability.
Copyright 2002 Wiley-Liss, Inc.