27. An ovarian follicle is an immature egg (oocyte) surrounded by one or more layers of follicle cells. Ovulation occurs when the vesicular follicle is mature and the developing egg is ready to be ejected by the ovary.
28.Follicle-stimulating hormone stimulates a small number of primary follicles to grow and mature each month. FSH causes follicles to grow larger and accumulate fluid in the antrum. The primary oocyte replicates its chromosomes and begins meiosis. Meiotic division produces a secondary oocyte and a polar body. Mature follicles contain a secondary oocyte protruding from the surface of the ovary. This takes about fourteen days and ovulation occurs about the same time in response to release of luteinizing hormone from the anterior pituitary hormone.
30. Menstrual Cycle
Day 1-5: Menstrual Phase - Superficial functional layer of thick endometrial lining of uterus is detaching from uterine wall accompanied by three to five days of bleeding. Detached tissues and blood pass through vagina as the menstrual flow. Average blood loss 50-150 ml. By day 5 estrogen is being produced by growing ovarian follicles.
Day 6-15: Proliferative Phase - The rising estrogen levels produced by the growing follicles stimulates the basal layer of the endometrium to regenerate the functional layer. Glands form and the endometrial blood supply increases. Endometrium becomes well vascularized and thick. At the end of this stage there is a sudden surge of luteinizing hormone in the blood and ovulation occurs.
Day 15-28: Secretory Phase - Blood supply is increased due to rising levels of progesterone productions by corpus luteum of the ovary acting on the estrogen-primed endometrium. Progesterone also causes endometrial glands to increase in size and to begin secreting nutrients into uterine cavity. A developing embryo would be sustained by these nutrients until it has implanted.