REPRODUCTIVE SYSTEM, HUMAN

Male and female human reproductive systems develop from a similar set of embryological structures, and many structural and functional parallels exist. Individual differences occur in the shape and structure of genitals, but without interfering with reproductive or sexual function. Also, the sexual anatomy of a young child differs from that of a mature adult and, in turn, adult sexual anatomy alters in the later years of life.

MALE REPRODUCTIVE SYSTEM

The male reproductive system basically is designed to produce and transport sperm cells. At the same time, the male genitals play an obvious role in sexual behavior, because reproduction cannot take place unless sperm cells are deposited in the female reproductive system. The major organs of the male reproductive system are the testes (testicles), the prostate, the seminal vesicles, the vas deferens, the epididymis, and the penis. Although the bladder empties through a duct that runs through the length of the penis, it is not considered part of the reproductive system.

Testes

The testes, which are contained in a pouch of skin called the scrotum, are located outside of the body because they require a lower temperature than the rest of the body in order to accomplish one of their main functions: the production of sperm cells (spermatogenesis). If the temperature of the scrotum rises by only a few degrees, the process of spermatogenesis may be seriously impaired. Each of the two testes are suspended from the body and held in place by a spermatic cord; the skin of the scrotum contains numerous sweat glands that assist in the cooling process.

Spermatogenesis

Within the scrotum, each testis is contained by a thick protective capsule, within which is a network of tightly coiled tubes called the seminiferous tubules; if uncoiled, these tubules would stretch to almost a mile in length. Spermatogenesis takes place within the seminiferous tubules. Sperm production fully occurs usually by the age of 16, even though it can begin before a boy reaches puberty. The male continues to produce sperm throughout his life but with a marked slowing of the process in the later years. A man often is able to father children when he is into his seventies or eighties, but the peak of his fertility is usually earlier in the life cycle. The testes also produce the male hormones, or androgens, in a number of large cells called the interstitial cells of Leydig, which are located between the seminiferous tubules. Like the production of sperm, the manufacture and secretion of these hormones begins about the time of puberty and continues throughout life. The hormones are manufactured in the testes and circulate throughout the body, affecting various organs.

Pathway of the Sperm

After sperm are produced in the seminiferous tubules, they move through the testes into another system of ducts called the epididymis. Although the epididymis is only about 3.8 cm (1.5 in) in length, it is so tightly coiled that it would measure about 12 m (20 ft) in length if extended. The sperm cells remain in this duct system and continue to mature for about 2 weeks and then pass into a longer transportation duct called the vas deferens. The male sterilization procedure, or vasectomy, is named for this duct because the operation involves cutting the vas deferens so that the sperm cells cannot travel from the testes to the penis.

Seminal Fluids

Before the sperm cells reach the penis, they travel through a number of internal organs: the prostate, the seminal vesicles, and Cowper's glands. The major function of these internal organs is to produce fluids that will provide the sperm cells with a nourishing and balanced environment. Only a very small proportion of the male ejaculate is actually made up of sperm cells; the remainder consists of the seminal fluids secreted by these internal organs. Because of this factor, a male who undergoes vasectomy will continue to ejaculate about the same volume of fluid as a fertile male. Although seminal fluids are not absolutely necessary in order for a man to be fertile, these fluids allow the sperm cells to live longer within the acidic environment of the vagina.

The Penis

In the final phase of their journey, the sperm cells pass through the urethral duct, which runs through the center of the penis. The penis contains a large number of arteries, veins, and small blood vessels as well as erectile tissue, the last of which consists of three hollow, spongelike cylinders of tissue. When a male has an erection, these spongy tissues fill with blood and become firm. Unlike a number of other mammals--for instance, the whale--no bones are located within the human penis. Erection is caused solely by the relaxation of the blood vessels within the penis. Thus, when a man is under emotional or physical stress, he may experience some difficulty achieving a firm erection because the blood vessels may not relax sufficiently.

FEMALE REPRODUCTIVE SYSTEM

The female reproductive organs are designed for conception, pregnancy, and childbirth. Like the male, female anatomy includes both internal and external sexual organs. The internal organs are more closely related to reproductive function and the external organs tend to be related to sexual function.

Function of the Female System

Just as the testes produce sperm, which are the basic reproductive material in men, ovaries produce eggs, or ova. The two ovaries are located to the right and the left of the uterus, to which they are connected by Fallopian tubes. During each menstrual cycle the ovaries mature and release an egg cell. When a female infant is born, she already has in her ovaries all the egg cells that will be released during her life. These egg cells, however, do not mature until the time of puberty, when the menstrual cycle begins under the influence of female hormones. This process continues until the menopause, when the menstrual cycle ceases and hormone production diminishes.

The second major function of the ovaries is the production of the female hormones estrogen and progesterone. Like the male, the female produces hormones that circulate throughout the body and serve to keep the sexual system ready for reproduction. Unlike the male, however, the female secretes varying amounts of different hormones in a distinct, monthly cycle. During the first half of the cycle, hormones cause a single egg cell to mature; at midcycle, ovulation, or the release of the egg cell from the ovary, takes place. Also during the first half of the cycle, the lining of the uterus (endometrium) prepares to receive and nourish a fertilized egg. If fertilization and pregnancy have not occurred, hormone levels tend to decrease, the uterine lining is shed, and the cycle begins anew. The pattern of ovarian hormone release is regulated by the pituitary gland, which is located in the brain; for this reason, physical or emotional stress can affect the length or regularity of the menstrual cycle.

Ovulation

In the adult woman each of the two ovaries is about the size and shape of an unshelled almond and is grayish in color. During ovulation the egg cell breaks through or ruptures the wall of the ovary, at which time a small amount of bleeding may occur. Within the ovary are a number of compartments called follicles, which contain egg cells at various stages of development. After the egg cell is released, the follicle, referred to in this instance as the corpus luteum, remains within the ovary and continues to secrete hormones. If no pregnancy develops, the corpus luteum shrinks and disappears by the next menstrual cycle; if pregnancy does occur, the corpus luteum continues to secrete hormones for about 6 months.

Pathway of the Egg

After the mature egg is released from the ovary, it must travel a short distance to the entrance of the Fallopian, or uterine, tubes, which work as a transport system, moving the egg from the ovary to the uterus, and at the same time, secreting substances that nourish the egg. If fertilization--the meeting of egg and sperm--is to occur, it must take place within the Fallopian tube. Generally, the egg takes about 3 days to make the short journey from ovary to uterus; however, fertilization can take place only within the first 24 hours of this passage.

The Uterus

The uterus, or womb, is about the size and shape of an inverted pear in women who have never borne children. During pregnancy, this muscular container increases enormously in size and weight. After childbirth the uterus tends to remain slightly larger in size but retains the same shape.

The bulk of the uterus is a network of dense muscular fibers interlaced in all directions; the contractions of childbirth begin when these muscles work to move the fetus toward the vagina.

At the bottom of the uterus is the cervix, a passageway between the uterus and vagina. It is normally very small and, at times, is blocked entirely by mucus that is secreted by the cervical glands. During birth, however, it opens wide enough to allow for passage of the fetus.

The Vagina

Like the uterus, the vagina is normally a small organ with little internal space: in the resting state, the walls of the vagina are touching each another. Also like the uterus, the muscles of the vagina are able to expand greatly during intercourse or childbirth, after which they return to their normal size. In shape, the vagina resembles an elongated "S" and is usually about 10 cm (4 in) long in the mature female. The vagina, in addition to muscular tissue, contains a rich network of blood vessels; when a woman is sexually aroused this network fills with blood in much the same way that the male's penis becomes erect due to increased blood flow. In turn, the pressure of this blood causes the mucous lining of the vagina to secrete drops of fluid; this lubrication response is a primary sign of female sexual arousal.

Female external sexual anatomy consists of the labia majora, labia minora, and clitoris. The labia majora, or outer lips, are two folds of skin that normally enclose the external genitals. The labia minora, or inner lips, are two smaller skin folds containing a rich network of blood vessels. They are normally pinkish in color but may show a variety of color changes when a woman is sexually stimulated.

The Clitoris

According to sex researchers William Masters and Virginia Johnson, the clitoris is a unique organ in human anatomy, having as its major purpose the sensation of sexual pleasure.

The clitoris, like the penis, to which it is homologous, is made up of erectile tissue--that is, when a woman is sexually stimulated, the clitoris fills with blood and becomes firm. Normally, the clitoral body is covered by a fold of skin called the clitoral hood.

In attempting to understand female sexuality, many people at one time thought that the size of the clitoris was related to the intensity of a woman's sexual response. Another theory suggested that the distance between the clitoris and the vaginal opening determined the ease of reaching orgasm during intercourse. Research, however, has shown that sexual arousal and orgasm may have little relationship with the size, shape, or position of the clitoris.

PATHOLOGY

Male and female reproductive systems do not always function properly. On occasion, a child is born with a part of the reproductive system malformed or missing, although the problem may not be detected immediately at birth. Girls are sometimes born without ovaries or a uterus, and boys may have an undescended testicle. Both sexes are subject to congenital conditions involving improper amounts of hormones, affecting the proper expression of sexual characteristics. Although external defects can often be changed through surgery and hormone treatments, the person is often sterile.

Physical trauma can damage the reproductive system: the penis and testes, located outside the body, are particularly vulnerable to damage from blows or occupational hazards from machines. Because the testes are outside the body, sperm development can be affected by external factors, such as temperature. Sperm production drops even at normal body temperature, so tight clothing or even a high fever can cause a temporary drop in sperm count, temporarily reducing fertility. Sperm count and the health of the sperm has been found to be sensitive to a man's exposure to toxic chemicals in the workplace. Infection with mumps after reaching puberty can affect sperm production and leaves some men sterile.

Women can also sustain physical damage to reproductive organs from blows to the body, as well as from accidental cutting of the uterus or ovaries during abdominal surgery or extensive internal scarring after abdominal surgery. Improperly performed abortions and improperly inserted intrauterine devices can perforate the uterus and lead to infection. amenorrea, the cessation of menstruation, is a symptom of a number of conditions. Hormone regulation problems, lack of ovulation, and extreme loss of weight, from anorexia or from physically demanding activities such as running or ballet dancing, can cause menstruation to stop.

Bulges in the rectum (rectocele) and bladder (cystocele) can put pressure on the nearby reproductive organs in both males and females and cause dysfunction. Prolapse of the uterus, which sometimes occurs in postmenopausal women, due to the drop in hormone levels, can cause bladder control problems. Men can get a type of varicose vein called a varicocele in the testes, which can block the pathway of the sperm, thus impairing fertility.

Bleeding at the wrong time of the menstrual cycle characterizes a number of problems in the female reproductive system. Uterine lining tissue, called endometrium, can grow on the surface of the uterus, ovaries, Fallopian tubes, bladder, or rectum, a condition called endometriosis. Like normal uterine tissue, these growths are responsive to monthly hormonal cycles, bleeding and in some cases causing great pain during menstruation. Endometrial tissue can impair fertility by covering ovaries and blocking Fallopian tubes. Another problem related to uterine tissue is fibroid tumors, in most cases nonmalignant growths in the uterus, which can cause severe menstrual cramps and bleeding between periods. Ovaries can also develop noncancerous tumors and cysts that interfere with hormone production and the normal menstrual cycle.

Sexually transmitted diseases (STDs, or venereal disease) are a significant cause of damage to the human reproductive system, especially the female system. Diseases such as syphilis, gonorrhea, chlamydia, herpes, genital warts, vaginitis (including trichmoniasis, yeast, and bacterial infections), and pelvic inflammatory disease (PID includes any number of infections of the female reproductive organs) infect both partners and can cause scarring and blockage of the narrow tubes of the reproductive system, particularly the female's Fallopian tubes, and problems such as ectopic pregnancy. Repeated infection with PID, syphilis, and other STDs can lead to infertility in women. AIDS does not directly affect fertility, but it does make the HIV-infected person more susceptible to infections, and studies of HIV-positive women have found a pattern of increased infection of reproductive organs before diagnosis with AIDS.

Cancer can develop in male and female reproductive systems. The type of cancer and mortality rate depends on the affected organ and early diagnosis. Most treatments for cancers of the reproductive tract involve removal of the cancerous part, as well as chemotherapy or radiation. The most common reproductive cancers in women are cancer of the uterus, cervix, and ovaries. A link has been found between occurrence of cervical cancer and infection with the human papilloma virus, which causes genital warts. Cancer of the vulva, vagina, and Fallopian tubes is very rare. The most common reproductive cancers in men are prostate cancer, testicular cancer, and penile cancer. The PROSTATE gland can cause problems even if it is not cancerous, because it tends to swell as men age, putting pressure on the urethra and causing bladder problems. Removal of an enlarged prostate gland can cause impotence in some men.

Linda Rosen for GROLIER

Bibliography: Bain, J., et al., eds., Andrology (1978); Betzig, L. L., et al., eds., Human Reproductive Behavior (1988); Elder, Reproduction, Obstetrics and Gynecology (1989); Genazzami, A. R., ed., The Brain and Female Reproductive Function (1988); Grabowski, C., Human Reproduction and Development (1983); Knepp, T. H., Human Reproduction, rev. ed. (1967); Mader, S., Human Reproductive Biology (1980).