Corpus luteum
The corpus luteum (Latin for "yellow body") is a small, temporary endocrine structure in mammals that develops from an ovarian follicle after it has released a mature egg.The corpus luteum secretes the hormone progesterone, which thickens the uterine lining in preparation for the fertilized egg. If the egg is not fertilized, the corpus luteum stops secreting progesterone and decays after approximately 12 days. It then degenerates into a corpus albicans, which is mainly scar tissue. The uterine lining sloughs off without progesterone and is expelled through the vagina. This is called menstruation.
If fertilized, however, the egg secretes the hormone human chorionic gonadotropin (hCG). hCG signals the corpus luteum to continue progesterone secretion, thereby maintaining the thick lining of the uterus, and providing an area rich in blood vessels in which the zygote can develop. From this point on, the corpus luteum is called the corpus luteum graviditatis.