FEMALE REPRODUCTIVE CYCLE

A. The general term female reproductive cycle encompasses the ovarian and uterine cycles, the hormonal changes that regulate them, and cyclical changes in the breasts and the cervix.

1. The ovarian cycle is a series of events associated with the maturation of an ovum.

2. The uterine (menstrual) cycle involves changes in the endometrium to prepare for the reception of a fertilized ovum.

B. Hormonal Regulation of the Female Reproductive Cycle

1. The menstrual and ovarian cycles are controlled by GnRH (from hypothalamus), which stimulates the release of FSH and LH by the anterior pituitary gland (Figure 28.23).

2. FSH stimulates the initial development of ovarian follicles and secretion of estrogens by the ovaries.

3. LH stimulates further development of ovarian follicles, ovulation, and the secretion of estrogens and progesterone by the ovaries.

4. Estrogens have several important functions:

1) Promotion of the development and maintenance of female reproductive structures, secondary sex characteristics, and the breasts.

2) Increase protein anabolism and build strong bones.

3) Lower blood cholesterol.

4) Moderate levels of estrogens in the blood inhibit the release of GnRH by the hypothalamus and secretion of LH and FSH by the anterior pituitary gland. This is a negative feedback effect.

5) High levels of estrogens in the blood stimulate the release of LH and GnRH and cause ovulation. This is a positive feedback effect.

3. Progesterone works with estrogens to prepare the endometrium for implantation and the mammary glands for milk synthesis.

4. A small quantity of relaxin is produced monthly to relax the uterus by inhibiting contractions (making it easier for a fertilized ovum to implant in the uterus). During pregnancy, relaxin relaxes the pubic symphysis and helps dilate the uterine cervix to facilitate delivery.

5. Inhibin inhibits secretion of FHS and GnRH and, to a lesser extent, LH. It might be important in decreasing secretion of FSH and LH toward the end of the uterine cycle.

C. Phases of the Female Reproductive Cycle

1. The female reproductive cycle may be divided into four phases (Figure 28.24).

a. The menstrual phase (menstruation, menses) lasts for approximately the first 5 days of the cycle.

1) Ovaries: During this phase, small secondary follicles in each ovary begin to develop.

2) Uterus: Also during this phase, the stratum functionalis layer of the endometrium is shed, discharging 50-150 ml of blood, tissue fluid, mucus, and epithelial cells.

b. The preovulatory phase (uterus: proliferative phase) is the time between menstruation and ovulation. This phase is more variable in length that the other phases, lasting from days 6-13 in a 28-day cycle.

1) Ovaries: During this phase, primary follicles develop into secondary follicles and a single secondary follicle (occasionally more than one) develops into a vesicular ovarian (Graafian) follicle, or mature follicle. This follicle produces a bulge on the surface of the ovary. The dominant follicle continues to increase its estrogen production under the influence of an increasing level of LH.

2) Uterus: During this phase, endometrial repair occurs.

c. Ovulation is the rupture of the vesicular ovarian (Graafian) follicle with release of the secondary oocyte into the pelvic cavity, usually occurring on day 14 in a 28-day cycle.

1) The high levels of estrogen during the last part of the preovulatory phase exert a positive feedback on both LH and GnRH to cause ovulation (Figure 28.25). (Remember, low to moderate levels of estrogen exert negative feedback on LH and GnRH, but high levels of estrogen exert a positive feedback on these hormones)

a) GnRH promotes release of FSH and more LH by the anterior pituitary gland.

b) The LH surge brings about the ovulation. This surge can be detected with ovulation test kits.

2) Following ovulation, the vesicular ovarian follicle collapses (and blood within it forms a clot) to become the corpus hemorrhagicum The clot is eventually absorbed by the remaining follicle cells. In time, the follicular cells enlarge, change character, and form the corpus luteum, or yellow body, under the influence of LH. Stimulated by LH, the corpus luteum secretes estrogens and progesterone.

d. The postovulatory (luteal) phase (Uterus: secretory phase) is the most constant in duration and lasts from days 15-28 in a 28-day cycle, the time between ovulation and onset of the next menstrual period.

1) Ovaries: Both estrogen and progesterone are secreted in large quantities by the corpus luteum.

A) If fertilization and implantation do not occur, the corpus luteum degenerates and becomes the corpus albicans, or white body. The decreased secretion of progesterone and estrogens then initiates another menstrual phase (uterine and ovarian cycle).

b) If fertilization and implantation do occur, the corpus luteum is maintained until the placenta takes over its hormone-producing function. During this time, the corpus luteum, maintained by human chorionic gonadotropin (hCG) from the developing placenta, secretes estrogens and progesterone to support pregnancy and breast development for lactation. Once the placenta begins its secretion, the role of the corpus luteum becomes minor. hCG can be detected in the urine of a pregnant woman as soon as two weeks after fertilization and is what is measured in the home pregnancy test kits.

2) Uterus: This phase is also called the secretory phase because of the secretory activity of the endometrial glands as the endometrium thickens in anticipation of implantation.

2. Figure 28.26 summarizes the hormonal interactions during the ovarian and uterine cycles.