Regulation of corpus luteum development and maintenance: specific roles of angiogenesis and action of prostaglandin F2alpha.

Development of the corpus luteum (CL) in ruminants occurs in a rapid and time-dependent manner within 1 week after ovulation, with morphologic and biochemical changes in the cells of the theca interna and granulosa cells of the preovulatory follicle. These changes involve luteinisation of steroidogenic cells and angiogenesis to establish normal luteal function (progesterone secretion). The CL is composed of a large number of vascular endothelial cells, large and small steroidogenic luteal cells, smooth muscle cells, pericytes, fibrocytes and immune cells, indicating that the CL is a heterogeneous tissue. Moreover, the CL produces and secretes growth factors (fibroblast growth factor, vascular endothelial growth factor and insulin-like growth factor), vasoactive factors (nitric oxide, angiotensin II and endothelin-1), steroids (progesterone is important for its own production), oxytocin and prostaglandins (PGF2alpha and PGE2) to regulate luteal formation and development. Clearly, the main function of the CL is to produce progesterone, which is a prerequisite for survival of the embryo, implantation and maintenance of pregnancy. Inadequate luteinisation and angiogenesis during the early luteal phase results in poor progesterone secretion and causes compromised embryo development and reduced fertility. Secretion of adequate amounts of progesterone during luteal development requires "precise luteinisation" of theca and granulosa cells to form luteal cells, neovascularization, and the establishment of a blood supply (angiogenesis). PGF2alpha in the developing CL acts as a local regulator to enhance progesterone secretion directly and indirectly by stimulating angiogenic factors, VEGF and FGF2. The preceding role of PGF2alpha may explain why the developing CL does not acquire luteolytic capacity until several days following ovulation. The balance between luteotrophic and luteolytic factors as well as stimulation and inhibition of angiogenic factors during luteal formation, development and maintenance can have a profound effect on the fate of the CL.