Progesterone, hormone formed by the granulosa cells of the corpus luteum of the ovary. The corpus luteum is a structure in the ovary that develops at the site where a mature egg was released at ovulation. Therefore the level of progesterone rises in the second half of the menstrual cycle (see Menstruation). If the released ovum is not fertilized, production of progesterone falls just before the onset of the next menstrual cycle and the corpus luteum degenerates. Progesterone was isolated and crystallized by three independent groups of investigators in 1934. It is a steroid hormone, a compound possessing the same chemical nucleus as the female estrogenic hormones and the male androgenic hormones, as well as cholesterol and adrenal steroid hormones. The principal function of progesterone is the preparation of the mucous membrane of the uterus for the reception of the ovum. The hormone also stimulates the formation of saclike structures in the lacteal glands in preparation for their function of producing milk, and inhibits the release in the pituitary of prolactin hormone.