C) The Menstrual Cycle The female menstrual cycle takes an average of 28 days. (Variation is common). The cycle occurs in four phases if no fertilization occurs:

1. Menstrual Phase (Day 1-5) i) Thick endometrium tissues are shed from the mucosa of the uterus. ii) Produces menstrual flow.

2. Follicular Phase (Day 6-13) The hypothalamus stimulates the pituitary to release FSH which causes a follicle containing an ovum to develop. One follicle develops the fastest & this follicle secretes estrogen which is responsible for female secondary sex characteristics.

Female secondary sex characteristics: - development of breasts & body hair - increased thickening of the endometrium Increased estrogen levels also stimulate the release of LH from pituitary. This triggers ovulation. LH levels rise near day 14. The follicle migrates to the surface of the ovary.

3) Ovulation - Day 14 Dominant follicle pushes on the outer wall of ovary. The outer wall of the ovary bursts and the ovum is released from ruptured follicle.

The ovum is picked up by fimbrae which sweep ovum into fallopian tube. Follicular cells (from ruptured follicle), differentiate into the corpus luteum. Accompanied by a brief rise in body temp.

Follicular Phase Ovulation Luteal Phase

4) Luteal Phase (Day 15-28) Marked by development of corpus luteum Estrogen levels decreased when the oocyte left the ovary but are restored somewhat when the corpus luteum forms.

Corpus luteum secretes both: estrogen and progesterone. - Further thickens & maintains the endometrium. - inhibits further ovulation – why birth control pills contain high amounts of progesterone. - prevents uterine contractions.

If fertilization does not occur, the corpus luteum disintegrates into a scar called the corpus albicans. The concentrations of estrogen & progesterone decrease, thereby causing weak uterine contractions. These contractions cause the endometrium to pull away from the uterine wall. This marks the beginning of the flow phase. If fertilization has occurred, the corpus luteum remains in order to keep estrogen & progesterone levels high until the placenta forms.

Menstruation begins as endometrium is lost due to lack of estrogen and progesterone.

Hormonal Control of the Female Reproductive System dev. of breasts & body hair increased thickening of endometrium Prepares uterus for embryo by stimulating endometrium inhibits further ovulation. prevents uterine contractions.

The production of estrogen and progesterone (hormones of the ovary) is regulated by the hypothalamus-pituitary complex. Gonadotropins (female FSH and LH) regulate the control of hormones produced by the ovary. The onset of female puberty causes the release of GnRH (gonadotropin releasing hormone) from the hypothalamus. GnRH stimulates the anterior pituitary to release FSH and LH, which are then directed to the ovaries.

Follicular phase: During the follicular phase of the menstrual cycle, FSH secretions are carried by the blood to the ovary, where follicle development is stimulated. The follicles secrete estrogen, which stimulates the development of secondary sexual characteristics and initiates the development of the endometrium. As estrogen levels rise, a negative feedback message is sent to the pituitary to turn off FSH secretions.

Estrogen also causes a positive feedback mechanism on the secretion of LH; LH is released when estrogen levels are high. Ovulation occurs at the end of the follicular phase.

Luteal Phase: LH causes the corpus luteum to secrete estrogen and progesterone. Estrogen and progesterone further the development of the endometrial lining. As progesterone and estrogen build up, another negative feedback mechanism is activated which inhibits the release of FSH, LH and GnRH. The corpus luteum will disintegrate, estrogen and progesterone levels will drop, and menstruation will occur.