Human ovulatory cycle n ~28 days in length n includes menses n ovulation occurs spontaneously n cycles occur throughout the year n one egg produced / month n three phases: –follicular phase (proliferative, estrogenic) –luteal phase (secretory, progestational) –menstrual phase (destructive)

Menstrual phase n 4-5 days in length n uterine lining degenerates n follicle development continues –several antral follicles exist –follicles stimulated by LH, FSH pulses –estrogen levels in blood begin to rise

Follicular phase n begins on last day of menses n phase lasts until ovulation (~day 14) n follicles grow, secrete increasing amounts of estrogen n during most of this phase, estrogen exerts “negative feedback” n by days 10-12, a few ripe tertiary follicles exist n by day 13, 1 Graafian present n in uterus, endometrium thickens n uterine glands enlarge n uterine blood supply increases

Ovulation n occurs at end of follicular phase (day 14) n triggered by peak in estrogen (days 12-13; “positive feedback”) n estrogen causes GnRH surge, LH surge n estrogen also “primes” (sensitizes) pituitary gland to GnRH n FSH surge also occurs n LH surge initiates meiosis in oocyte of Graafian follicle (now a preovulatory follicle) n LH also triggers ovulation (9-12 hrs after LH surge peaks) n other antral follicles doomed to atresia

Luteal phase n last half of menstrual cycle (~14 days) n follicle is transformed to corpus luteum (CL) by LH surge n CL secretes progesterone, estradiol n high progesterone, estradiol exert negative feedback on hypothalamus n uterine lining becomes thick, spongy n LH, FSH levels are negligible > atresia n if fertilization takes place and conceptus implants, placental hCG rescues corpus luteum n no hCG > menstrual phase