Luteolysis is the structural and functional degradation of the corpus luteum, which occurs at the end of the luteal phase of both the estrous and menstrual cycles in the absence of pregnancy. In domestic animals, luteolysis is initiated by the hormones prostaglandin F2alpha and oxytocin. In sheep, communication between the pars nervosa, corpus luteum, and the uterus endometriumvia the circulatory system is required for luteolysis. Studies with sheep have found that, if the uterine horn is ipsalateral to the ovary possessing the CL is surgically removed, the lifespan of the corpus luteum will increase drastically. However, removal of the uterus in primates does not prolong the life of the corpus luteum. Primates do respond to PGF2a, and it is believed that PGF2a is made locally within the ovary. Asthmatics should take great care when handling this hormone as PGF2a is bronchoconstrictor. Estrogen, secreted by Granulosa Cells and primarily progesterone, secreted by the corpus luteum, inhibit the release of luteinizing hormone and Follicle Stimulating Hormone by the adenohypophysis via classicalnegative feedback mechanisms. This removes the luteotrophic support provided by the gonadotropin luteinizing hormone. During a pregnancy, the corpus luteum remains on the ovary releasing progesterone which will maintain a state of uterine quiescence and close the cervix until the delivery of the fetus. Alternatively if no implantation of a blastocyst occurs, the corpus luteum is degraded to a corpus albicans by PGF2alpha released by uterine endometrial cells. Degradation of the corpus luteum will result in reduced levels of progesterone, promoting an increase in follicle-stimulating hormone secretion by the adenohypophysis, which will trigger the development of a new follicle on the ovary. In humans, the placental hormone human chorionic gonadotropin continues to maintain the corpus luteum. This is the hormone tested for by the pregnancy test. If pregnancy occurs in equine, the placental hormone equine chorionic gonadotropin released by endometrial cup acts like LH and FSH and stimulates additional ovulations, although the oocytes released during these ovulations are not fertilized. The resulting accessory corpora luteaproduce progesterone and keep levels of progesterone high during pregnancy.
