Secretory Phase

During the secretory phase continued pituitary secretions of LH cause the cells of the ruptured follicle to develop into a yellowish bump called the corpus luteum. The cor­pus luteum secretes progesterone, which inhibits the production of the cervical mucus during ovulation. Together with estrogen produced by the ovaries, progesterone causes the endometrium to thicken and engorge with blood in preparation for implantation of the blastocyst. The secretory phase is so named because glands and ducts in the endometrium produce and secrete nutrients to ensure development of the blastocyst. If implantation does not occur, the pituitary gland responds to high estrogen and pro­gesterone levels in the bloodstream by shutting down production of LH and FSH. This action causes the corpus luteum to degenerate, and estrogen and progesterone production decreases. This reduction of hormone levels triggers the sloughing off of the endometrium, initiating the menstrual phase once again.