The Female Reproductive System

The female gonads. the ovaries, are located in the lower pan of the abdominal cavity. These olive-sized organs produce the female gametes, called am or eggs. Below the ovaries is the uterus, a hoilow. thick-walled, muscular organ about the size of a fist. The uterine walls are lined with mucous membrane that contains many glands and blood vessels. An unborn child devel­ops here. A duct, caiied a Fallopian tube, or oviduct, extends from each side of the titerus. Fringed projections at the upper end of each Fallopian tube surround each ovary. Cilia lining these projections propel the mature egg to the uterus. A tube called ihe vagina leads from the uterus to the outside of the body. Between ihe uterus and the vagina is a muscular ring, the cervix.

At birth a female has about 2 million immature eggs in her ovaries. Beginning at puberty a single egg matures and is re­leased from an ovary each month. In general, the ovaries alter­nate in releasing eggs. The left ovary releases an egg one month, and the right ovary releases one the next month. Approximately 400 eggs mature during a female's lifetime. During pubeny the ovaries aiso begin to secrete ihe female hormone estrogen. Estrogen causes development of secondary female sex characteristics, such as wide hips, body hair, and enlarged breasts.

The Menstrual Cycle

The female reproductive system has three primary functions: the production of eggs, the secretion of female sex hormones, and the nourishment and protection of a new individual. Ap­proximately every 28 days, one egg matures and is released from the ovary, and the uterus is prepared to receive it. These activities are controlled by hormones operating on a feedback system. The entire process of ovuiation and related changes in the uterus operates on a regular, repeating pattern called the menstrual cycle.

The cycle starts with the release of FSH and LH from the anterior pituitary. These hormones trigger the maturing of an egg and its follicle, the fluid-filled chamber around the egg. The follicle secretes estrogen, which causes the uterine lining to thicken. Estrogen also triggers an increase in LH. The LH causes the follicle to rupture and release the mature egg from the ovary surface. The process of releasing a mature egg from the ovary is called mutation.

Following ovuiation LH stimulates the follicle to enlarge and fill with blood vessels. The follicle is now called the corpus luteum, or "'yellow body." It functions temporarily as an endocrine gland, producing the ovarian hormones estrogen and progesterone. Progesterone continues the process of thickening the uterine lining in preparation for the uterus to receive a fertilized egg. Both estrogen and progesterone inhibit the production of GnRH (gonadotropin-releasing hormone) by the hypothalamus. The lower GnRH level inhibits the production of FSH and LH, thus keeping another follicle from maturing.

If an egg is not fertilized, the corpus luteum disintegrates. The lining of the uterus collapses, (is cells die and are sloughed off through the vagina. This discharge of dead tissue and the unfertilized egg is called menstruation. The decreased levels of FSH and LH, plus disintegra­tion of the corpus luteum. lead to lower progesterone and estrogen levels. The Eower hormone levels trigger GnRH, which stimulates FSH and LH secretion. Thus a new cycle begins.

Males normally produce healthy sperm until about age 80. Females, however, cease to produce mature egg cells afier men­opause, the time at which the menstrual cycle ceases. This pe­riod usually begins between the ages 45 and 55. After meno­pause the pituitary does not secrete LH and follicles do not mature. Therefore, estrogen and progesterone are not produced.

Fetal Development and Birth

The fusing of a sperm nucleus and an egg nucleus is called fertilization. When an egg is fertilized, menstruation does not occur. Instead, a nine-month gestation period begins. This period, also called pregnancy, is the time during which a fertilized egg develops into a child inside the mother.

Fertilization

During sexual intercourse semen is ejaculated through the male's penis into the female's vagina. Of some 400 million sperm ejaculated, less than 3,000 sperm get as far as a Fallopian tube and, of these, only about 50 reach the mature egg. In fertil­ization the head and middle section of the sperm enter the ovum. Only one sperm can fertilize an egg. Changes occur in the egg cell membrane to prevent other sperm from penetrating it.

Since both gametes are haploid, each parent contributes half the normal chromosome number. In human beings the haploid number is 23 chromosomes. When the gametes fuse, the zygote, or fertilized egg, has 46 chromosomes.