The Reproductive System - Workings: how the reproductive system functions
The main functions of the male reproductive system are to produce sperm and to introduce that sperm into the female reproductive tract. The main functions of the female reproductive system are to produce ova, receive sperm from the male penis, house and provide nutrients to the developing embryo and fetus, give birth, and produce milk to feed offspring.
The activities of both reproductive systems are controlled by hormones (chemicals produced by the body that affect various bodily processes) released from the pituitary gland, which is located at the base of the brain behind the nose, and from the gonads in each system.
Hormones and the male reproductive system: Sperm production
In males, the testes produce male sex hormones called androgens (from the Greek word andros , meaning "man"). Testosterone is the most important of these. It spurs the growth of the male reproductive organs and the production of sperm. In addition, testosterone brings about the male secondary sex characteristics: deepening of the voice; appearance of hair under the arms, on the face, and in the genital area; and increased growth of muscles and heavy bones.
Testosterone is produced in the male fetus to stimulate the formation of the male duct system and accessory organs. Its production then declines and does not increase until puberty.
At puberty (usually occurring between the ages of twelve and sixteen in boys), the anterior portion of the pituitary gland releases luteinizing hormone. This hormone activates the cells surrounding the seminiferous tubules in each testis to begin secreting testosterone. The anterior pituitary then releases follicle-stimulating hormone, which stimulates the cells of the seminiferous tubules to begin producing sperm. In the presence of testosterone, the sperm are able to mature.
This process, begun at puberty, continues throughout a man's life. Millions of sperm are formed each day in the testes. Although this number decreases as a man ages, the production of sperm never completely stops.
Hormones and the female reproductive system: The menstrual cycle
In women, the ovaries secrete two groups of steroid hormones, estrogens and progesterone. Estrogens spur the development of the female secondary sex characteristics: enlargement of the breasts, appearance of hair under the arms and in the genital area, and the accumulation of fat in the hips and thighs. Estrogens also act with progesterone to stimulate the growth of the lining of the uterus, preparing it to receive a fertilized egg.
The ovaries do not begin to function until puberty (usually occurring between the ages of eleven and fourteen in girls). At this time, the anterior pituitary gland secretes follicle-stimulating hormone (FSH). This hormone stimulates a small number of ovarian follicles in an ovary to grow and mature. It also stimulates the follicle cells in those follicles to secrete estrogens. Together, FSH and estrogens help the oocyte (immature ovum) in each stimulated follicle to mature.
About fourteen days after the anterior pituitary secreted FSH, only one follicle in the ovary has developed to maturity with a mature ovum inside. The anterior pituitary then releases luteinizing hormone (LH), which causes the mature follicle to burst open and release its ovum through the ovary wall. This event is known as ovulation.
Reproductive biologist Min-Chueh Chang (1908–1991) is best known for developing an oral contraceptive—commonly known as the birth control pill—in collaboration with American endocrinologist Gregory Goodwin Pincus (1903–1967) and American gynecologist and obstetrician John Rock (1890–1984).
In the early 1950s, Chang and Pinkus began to study the female hormone progesterone, believing it could provide them with clues about how to create an oral contraceptive (a substance that prevents conception). Through their research the scientists soon realized that increasing blood levels of progesterone could stop ovulation. They experimented with more than two hundred substances in order to find natural compounds that imitated the combined actions of progesterone and another female hormone, estrogen. At that point, they began a collaboration with John Rock of the Rock Reproduction Clinic in Brookline, Massachusetts. The team developed a pill made of three compounds, including estrogen and progesterone derived from a wild Mexican yam. They began testing their pill on human subjects, using groups of women in Brookline, as well as in Haiti and Puerto Rico.
Their trials were successful in stopping ovulation, but the researchers decided to slightly alter the pill they had developed. They wanted to try to eliminate estrogen from the pill because they considered it to be unnecessary. They thought that progesterone was the key ingredient they needed. Trials of this no-estrogen pill, however, had seriously negative results, including pregnancies and breakthrough bleeding (bleeding between menstrual periods) in the women they tested. The scientists restored estrogen to the pill, creating a form of contraception that was over 99 percent effective and relatively safe. The combined estrogen-progesterone pill was approved by the U.S. Food and Drug Administration in 1960.
The cilia carpeting the inner surfaces of the fallopian tubes wave back and forth, creating a current in the fluid that fills the tubes and spaces around the ovaries. The mature ovum that is released is drawn by these currents into the fallopian tube attached to the ovary. Muscle contractions in the walls of the tube also help propel the ovum along.
While the ovum is in a fallopian tube, the ovarian follicle that burst undergoes another transformation. LH, which triggered ovulation, also causes the ruptured follicle to change into another structure. This small yellow structure is known as the corpus luteum. It begins secreting another hormone, progesterone. The new hormone prevents the other stimulated follicles with their eggs from reachig full maturity. In combination with estrogens, it also causes the endometrium (lining of the uterus) to grow in size and secrete nutrients into the cavity of the uterus (to help nourish a developing embryo, if one is present). As the endometrium grows, so does its supply of blood vessels.
The mature ovum in the fallopian tube can only survive for twelve to twenty-four hours. Fertilization of the ovum by male sperm must occur within the fallopian tube within that time. If fertilization does not occur, the ovum enters the uterus and begins to break down. Blood levels of LH and FSH decrease dramatically. The corpus luteum continues to secrete progesterone for about ten to twelve days after ovulation. It then stops and begins to degenerate or shrivel up.
Without the presence of progesterone, the thickened endometrium begins to detach from the uterus and break apart. After a while, the tissue pieces of the endometrium (along with the blood vessels that nourished it) are discharged from the body in a flow of blood through the vaginal opening. This event is referred to menstruation or menses. The flow of blood, which is commonly called a period, usually lasts four or five days. The average amount of blood lost during a period ranges from 1.7 to 5 ounces (50 to 150 milliliters). Once menstruation ends, the endometrium begins to regenerate.
The beginning of menstruation marks the end of what is known as the menstrual cycle. The cycle begins with the secretion of FSH spurring ovarian follicles to mature. The time period of a menstrual cycle is normally about twenty-eight days (the length can vary from nineteen to thirty-seven days). As menstruation is occurring, new follicles in the other ovary are being stimulated by FSH and a new menstrual cycle is underway.
The first menstruation or menstrual period a girl undergoes at puberty is known as menarche. Menstrual cycles continue one after the other (unless a woman becomes pregnant) from puberty until a woman reaches her forties or fifties. At that time, the number of ovarian follicles in the ovaries has been exhausted and the ovaries no longer respond to FSH. Ovulation and menstruation cease, and blood levels of estrogens and progesterone decline. A decline in estrogen levels leads to a reduction in the size of the uterus and breasts. The walls of the vagina and uterus also become thinner. This change in a woman's reproductive system and organs is known as menopause.
The only time a woman does not experience a menstrual cycle between puberty and menopause is when she is pregnant (certain illnesses, emotional stress, and extensive physical activity can also stop menstrual cycles from occurring). For pregnancy to occur under normal conditions, an ovum must be fertilized by a sperm in a fallopian tube.
When a man ejaculates semen into a woman's vagina, the millions of sperm swim through the watery fluid inside the vagina and uterus. Of the multitude of sperm, only a relative few will reach an ovum in a fallopian tube, if one is present. A vast majority of sperm will be destroyed by white blood cells in the walls of the vagina and uterus. Of those that survive, many may not be able to swim to the top of the uterus. Of those that are able, half then swim into the wrong fallopian tube.
Birth control or contraception is the deliberate effort to keep a woman from becoming pregnant. Attempts to prevent pregnancy date back to ancient times and cultures. Some form of contraception is currently used by more than half the women in the United States. Although widespread, contraception remains controversial, with some religious and political groups opposed to the distribution of contraceptives.
There are a number of contraceptive devices to prevent pregnancy, some more effective than others. The following are a few common devices:
Birth control pills: The most popular contraceptive in the United States. The pills contain hormones that are released into a woman's system on a regular basis to prevent pregnancy, either by inhibiting ovulation, preventing implantation of a fertilized egg, or by thickening the secretions throughout a woman's reproductive system so that her partner's sperm has less of a chance to meet her egg.
Condom: A sheath used to cover the penis to prevent semen from entering a woman's reproductive tract. Modern condoms are made of latex (rubber). Also used as an effective barrier against sexually transmitted diseases.
Contraceptive sponge: Device used to cover the cervix to prevent the passage of semen into the uterus. Was taken off the market in the United States in the early 1990s because of potential health risks, but has recently begun to be reintroduced.
Diaphragm: Rubber, cap-shaped device that fits over the cervix and prevents the passage of semen into the uterus.
IUD: Intrauterine device, a small plastic device placed inside the uterus of a women by a physician that prevents an embryo from developing in the wall of the uterus.
A permanent form of birth control—sterilization—can be achieved through surgical procedures. Men can undergo an operation called a vasectomy, in which both vas deferens are cut and tied so that sperm cannot be ejaculated. Women can undergo an operation called a tubal ligation, in which the fallopian tubes are either cut or tied so that an egg cannot pass down into the uterus.
Probably the most common contraceptive method in the world is coitus interruptus, in which a man withdraws his penis from a woman's vagina before ejaculating. Unfortunately, this practice has resulted in numerous accidental pregnancies. The rhythm method (in which intercourse is avoided on the days of the month when a woman is most likely to become pregnant) was and remains the only form of birth control approved by the Roman Catholic Church. Abstinence, or the complete avoidance of sexual activity, remains the most effective way to prevent pregnancy.
About 100 sperm eventually reach an ovum in a fallopian tube. They then attached themselves to the ovum's outer membrane. Enzymes in the acrosomes of the sperms begin to break down the membrane. Eventually, one sperm makes it through to the interior of the ovum. After it does, electrical changes occur in the membrane of the ovum that prevent the other sperm from entering.
The sperm and ovum each carry the genetic material that is necessary to generate and maintain human life. The threadlike structures that carry that genetic material (or genes) are called chromosomes. While all other body cells contain 46 chromosomes (23 pair), a sperm and an ova each contain only 23 single chromosomes. When the genetic material of a sperm combines with that of an ovum, fertilization or conception occurs. The fertilized egg, called a zygote, contains the normal human number of 46 chromosomes.
The zygote then travels down the fallopian tube and enters the uterus. By the time it has reached the uterus, the zygote (now called an embryo) is
a tiny ball of cells that are continually growing and dividing. About a week after fertilization, the embryo becomes embedded in the endometrium. Cells of the embryo then release a hormone that prevents the corpus luteum from degenerating. Estrogens and progesterone continue to be secreted and the endometrium remains intact. Menstruation does not occur and the menstrual cycle is stopped.
Other cells from the embryo combine with tissues from the uterus to form a temporary organ called the placenta. The placenta serves as the site where oxygen and nutrients pass from a woman to the embryo and where
English gynecologist Patrick Steptoe (1913–1988) is best known for helping develop the technique of in vitro fertilization, a process by which an egg is fertilized by sperm outside of the body. Steptoe and his colleague, English physiologist Robert G. Edwards, received international recognition (both positive and negative) when their work led to the birth of the first so-called test tube baby in 1978.
In 1966, while working on fertility problems (the inability of a woman and man to conceive a child), Steptoe teamed up with Edwards. Using ovaries that had been removed for medical reasons, Edwards had previously pioneered the fertilization of eggs outside of the body, a procedure called in vitro fertilization. In this operation, a mature egg is removed from the female ovary and is fertilized with sperm in a petri (glass) dish. After a short incubation or development period, the fertilized egg is implanted in the uterus, where it develops as in a typical pregnancy. This procedure gives hope to women who cannot become pregnant because their fallopian tubes are damaged or missing.
For a decade, Steptoe and Edwards pursued their research, but met with little success. In addition, as news of their work began to spread, Steptoe and Edwards faced intense criticism. Scientists and religious leaders raised ethical and moral questions about tampering with the creation of human life. Members of British Parliament demanded an investigation, and the scientists' research funds were cut off. Nevertheless, Steptoe and Edwards continued their work.
Finally, in 1976, Steptoe met Leslie Brown, a thirty-year-old woman who had experienced problems with her fallopian tubes and could not conceive a child. Steptoe removed a mature egg from her ovary, and Edwards fertilized the egg in a petri dish using the sperm of her husband, Gilbert. After two days, Steptoe implanted the fertilized egg in Brown's uterus, where it continued to thrive as a normal pregnancy. On July 25, 1978, Brown gave birth to Joy Louise, a healthy 5.75 pound (2.61 kilogram) baby girl, the first human conceived outside of a woman's body.
wastes are taken away from the embryo. The placenta also produces hormones—estrogens, progesterone, and others—that help maintain pregnancy.
A fluid-filled sac, the amnion or amniotic sac, develops around the embryo. In this sac, the embryo will continue to develop, connected to the placenta by a blood vessel-containing tube called the umbilical cord. Nine weeks after fertilization, the developing embryo is referred to as a fetus. Approximately 270 days after fertilization, the fetus is ready to be born.