Luteolysis (also known as luteal regression) is the structural and functional degradation of the corpus luteum (CL), 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 (posterior lobe of the pituitary gland), corpus luteum, and the uterus endometrium via 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.