[The corpus luteum]

The primary function of the corpus luteum is secretion of progesterone for maintenance of pregnancy. The development and function of the corpus luteum from residual follicular granulosa and theca cells after ovulation is induced by the midcyclic peak of LH secretion followed by further pulsatile LH release. Due to this stimulation the follicular granulosa and theca cells are converted to large and small luteinized cells with high proliferation rate. During this process Vascular-Endothelial-Growth Factor (VEGF) plays a major role as a potent stimulator of neo-angiogenesis. Formation of new blood vessels is essential to ensure supply of LDL-Cholesterol as substrate for steroidogenesis. If pregnancy does not occurs, the corpus luteum must regress to initiate another cycle. Luteal regression seems to be initiated by PGF2 alpha which is secreted from the uterus. PGF2 alpha, reduces luteal blood flow and progesterone synthesis. Furthermore it is a potent inducer of apoptosis. If pregnancy occurs, sustained secretion of progesterone and other substances like estradiol and relaxin are required to provide an appropriate uterine environment for maintenance of pregnancy. In that case the corpus luteum is further stimulated by hCG secreted by the blastocyst and the trophoblast-cells until 8/9 weeks of gestational age, when synthesis and secretion of steroids is taken over by the placenta.