Angiogenesis is a process of vascular growth that is mainly limited to the reproductive system in healthy adult animals. The development of new blood vessels in the ovary is essential to guarantee the necessary supply of nutrients and hormones to promote follicular growth and corpus luteum formation.
Preantral follicles (PFs) have no vascular supply of their own, but rather, they depend on vessels in the surrounding stroma.
However, during antrum development, the thecal layer acquires a vascular sheath consisting of two capillary networks located in the theca interna and externa. These newly formed ovarian blood vessels guarantee an increasing supply of gonadotropins, growth factors, oxygen, and steroid precursors, as well as other substances to the growing follicle. The acquisition of an adequate vascular supply is possibly a rate-limiting step in the selection and maturation of the dominant follicle destined to ovulate. On the other hand, degeneration of the capillary bed in follicles that fail to develop is a relevant factor causing follicular atresia. Both the ovarian follicle and corpus luteum have been shown to produce several angiogenic factors; however, vascular endothelial growth factor A (VEGFA) is thought to play a pivotal role in the regulation of angiogenesis in the ovary.