Angiogenic factors (VEGF, FGF and IGF) in the bovine corpus luteum

Angiogenesis, the development of the new capillaries by endothelial cell proliferation and outgrowth from pre-existing vessels, is one of the prominent features of early corpus luteum (CL). The process of angiogenesis is an important component of normal development and function of CL. Of the numerous promoters of angiogenesis and maintenance of new established capillaries that have been identified, the most important factors appear to be vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (FGF). The biological effects of VEGF and FGF factor families are mediated by signaling through specific tyrosine kinase receptors. The VEGF, FGF and IGF family members in bovine CL are discussed with the literature available for large domestic animals. The highest expression for VEGF, VEGFR-2, FGF-2, FGFR, IGF-1, IGF-2 and IGFR1 were found during the early luteal phase (Ld 1-4) followed by a significant decrease afterwards. The VEGF and IGF-1 protein was localized predominantly in luteal cells. FGF-1 mRNA expression peaked (P<0.05) during mid-luteal stage and FGF-1 protein was localized in cytoplasm of luteal cells, but endothelial cells are always negative. FGF-2 protein during angiogenesis (d 1-5) was found in cytoplasm of endothelial cells and changed thereafter exclusively to the cytoplasm of luteal cells. In contrast, the IGF-2 protein is exclusively localized in pericytes and few endothelial cells. The results obtained (mainly in ruminant) suggest an important role of these growth factors for angiogenesis and furthermore for maintenance and function of the bovine corpus luteum.