Chapter 28 : The Female Reproductive System

Female Reproductive system

• The primary sex organs are the ovaries. The secondary sex organs include the internal reproductive tract (uterine tubes* , uterus, and vagina)

• The primary sex organs are the ovaries. The secondary sex organs include the internal reproductive tract (uterine tubes*, uterus, and vagina) and the external genitalia (clitoris, labia minora, and labia majora).
  *The uterine tubes are also known as the Fallopian tubes.

Ovaries

• The ovaries are the female gonads. They produce egg cells and sex hormones.

• Most of the ovarian tissue consists of ovarian follicles, each of which consists of one developing ovum (egg) surrounded by numerous small follicular cells.

• Once per month, some follicles undergo follicular development, which consists of growth of the egg cell and proliferation, the growth of the surrounding follicular cells, and the accumulation of fluid within the follicle.

The Uterus

• The uterus is a thick, muscular chamber that opens into the top of the vagina.

• The function, nourish it, and expel the fetus at the appropriate time.

Hormonal Control of Function

• As in males, GnRH is produced by the hypothalamus, and it causes the release of LH and FSH from the anterior pituitary.

• FSH causes the development of the overian follicles, which secrete estrogens, progesterone, inhibin, and low levels of androgens.

• LH is responsible for the process of ovulation.

• Progesterone act primarily on the uterus, preparing it for potential pregnancy. it also tends to maintain a pregnancy (the levels of progesterone drop sharply just before childbirth).

• Estrogens are feminizing hormones that have widespread effects. The three most common estrogens are estradiol, estriol, and estrone.

• For example, estradiol causes growth of the ovaries and secondary sex organs during development. It stimulates the monthly proliferation of cells in the lining of the uterus.

• Most of the visible changes at puberty are due to the effects of estradiol and androgens.

• Estradiol stimulates growth hormone secretion, which leads to a rapid increase in height and a widening of the pelvis. It stimulates fat deposition in the hips, thighs, buttocks, and breasts.

• Androgens are responsible for the growth of pubic and axillary hair and the development of sebaceous glands

• Estrogen, progesterone, and inhibin all regulate FSH and LH secretion through negative feedback inhibition of the anterior pituitary (inhibin gets released by follicular cells and selectively inhibits the secretion of FSH).

The Sexual Cycle

• The sexual cycle, which is about 28 days in length, is related to the activities of both the ovaries and the uterus.

• The events in the ovaries are described as the ovarian cycle, and the events in the uterus are described as the menstrual cycle (or the uterine cycle).

The Ovarian Cycle

1. Follicular Phase – Extends from the beginning of menstruation (Day 1) until ovulation (Day 14). During this time, FSH stimulates the growth of follicles. FSH also stimulates the secretion of estradiol from the granulosa cells. At the end of this phase, there is a spike in LH secretion (the LH surge). The LH causes an increase in the production of follicular fluid, which causes the follicle to swell and enlarge.

2. Ovulation – Ovulation, which consists of a bursting of the follicle due to the increased pressure from the build-up of follicular fluid, takes only a few minutes. Normally, the egg, which is released from the surface of the ovary, is taken up by the uterine tube. However, it is not unusual for oocytes to remain in the pelvic cavity and die.

3. Luteal Phase – Extends from ovulation until the end of the cycle (Day 28). If pregnancy does not occur, the following events takes place: Under the direction of LH, the ovulated follicle becomes a structure called the corpus luteum. LH stimulates the corpus luteum to secrete both estradiol and, especially, progesterone. The corpus luteum starts to shrink at about Day 22 and by Day 26 has turned into a scar tissue called the corpus albicans. Due to the reduction in estrogen and progesterone levels near the end of this phase, negative feedback on FSH production is relaxed, and FSH levels begin to increase again to stimulate a new cohort of follicles.

The Menstrual Cycle

• Proliferative Phase – This phase starts at about Day 5. Estrogen that has been secreted by the follicles stimulates mitosis in the basal layer to cause the regrowth of the functional layer. Estrogen also causes the proliferation of blood vessels and endometrial glands.

3. Secretory Phase - The endometrium continues to thicken, but mostly due to secretion and fluid accumulation instead of mitosis. The number of progesterone receptors in the endometrium increases under the direction of estrogen, and progesterone from the corpus luteum causes the endometrial glands to grow and to secrete glycogen and more fluid. Near the end of this phase, the endometrium starts to degenerate due to the fact that the corpus luteum in the ovary has degenerated and is no longer producing progesterone.

Hormones of Pregnancy

• If fertilization occurs, the 2nd meiotic division of the ovum occurs, followed by fusion of the two gametes. Implantation in the wall of the uterus occurs after about 6 days. The endometrium will form an implantation chamber, and the outer layer of cells around the zygote that are closest to the endometrium fuse into a structure called the syncytiotrophoblast. This structure, which is an invasive mass, establishes contact with the underlying capillaries of the uterine wall to form a primitive circulation.

• The syncytiotrophoblast produces a hormone called Human Chorionic Gonadotropin (HCG). The presence of HCG is the indicator for pregnancy in most pregnancy tests. The function of this hormone is to maintain the corpus luteum so that it continues to produce large amounts of progesterone. In other words, HCG prolongs the life of the corpus luteum. (The presence of high levels of progesterone is important to the maintenance of pregnancy). The levels of HCG reach their peak at 9-14 weeks of gestation, and then decline to very low levels by about 20 weeks. The corpus luteum also disappears at that time.

• After the corpus luteum disappears, the production of estrogens and progesterone is taken over by the placenta.

• Estrogen stimulates tissue growth in the fetus. In the mother, it causes the uterus and external genitalia to enlarge, causes the mammary ducts to grow, and causes the breasts to grow. It also makes the pubic symphysis more elastic so that the pelvis can widen during childbirth.

• Progesterone suppresses uterine contractions so early labor is not as likely to occur. It also contributes to the development of the mammary glands.