Role of vascular endothelial growth factor in ovarian physiology - an overview.

In the female reproductive system, as in a few adult tissues, angiogenesis occurs as a normal process and is essential for normal tissue growth and development. In the ovary, new blood vessel formation facilitates oxygen, nutrients, and hormone substrate delivery, and also secures transfer of different hormones to targeted cells. Ovarian follicle and the corpus luteum (CL) have been shown to produce several angiogenic factors, however, vascular endothelial growth factor (VEGF) is thought to play a paramount role in the regulation of normal and abnormal angiogenesis in the ovary. Expression of VEGF in ovarian follicles depends on follicular size. Inhibition of VEGF expression results in decreased follicle angiogenesis and the lack of the development of mature antral follicles. The permeabilizing activity of VEGF is thought to be involved in follicle antrum formation and in the ovulatory process. In the CL, VEGF expression corresponds to different patterns of angiogenesis during its lifespan. In most the species, higher VEGF expression in the early luteal phase is essential for the development of a high-density capillary network in the CL. However, high VEGF expression may be still maintained in the mid-luteal phase to increase vascular permeability that results in enhancement of luteal function. During gestation, VEGF is thought to be important for the persistence of the CL function for a longer than in the nonfertile cycle period of time. Further elucidation of specific roles of VEGF in ovarian physiology may help to understand the phenomenon of luteal insufficiency and reveal novel strategies of ovarian angiogenesis manipulation to alleviate infertility or to control fertility.