The Reproductive System

uman reproduction, always an object of the most intense interest, has lately become the darling of the media, the subject of innumerable television talk shows, magazine articles, and newspaper editorials. With each new medical breakthrough in fertility and family planning, the noise level grows higher. From elementary school onward we're now deluged with information—some factual, some not—on menstruation and menopause, conception and contraception.

Sorting it all out may seem impossible. But a reasonable understanding of the basics of reproduction can make the job easy. As you weigh your options, whether to encourage pregnancy or forestall it, your best resource is a working knowledge of the organs, glands, and hormones that prepare your body for motherhood.

A Quick Review of the Reproductive Anatomy

Our overview of the reproductive system begins at the external genital area— or vulva—which runs from the pubic area downward to the rectum. Two folds of fatty, fleshy tissue surround the entrance to the vagina and the urinary opening: the labia majora, or outer folds, and the labia minora, or inner folds, located under the labia majora. The clitoris, is a relatively short organ (less than one inch long), shielded by a hood of flesh. When stimulated sexually, the clitoris can become erect like a man's penis. The hymen, a thin membrane protecting the entrance of the vagina, stretches when you insert a tampon or have intercourse.

From this point onward, the reproductive system leads deeper and deeper into the body.

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The Vagina

The vagina is a muscular, ridged sheath connecting the external genitals to the uterus, where the embryo grows into a fetus during pregnancy. In the reproductive process, the vagina functions as a two-way street, accepting the penis and sperm during intercourse and roughly nine months later, serving as the avenue of birth through which the new baby enters the world .

The Cervix

The vagina ends at the cervix, the lower portion or neck of the uterus. Like the vagina, the cervix has dual reproductive functions.

After intercourse, sperm ejaculated in the vagina pass through the cervix, then proceed through the uterus to the fallopian tubes where, if a sperm encounters an ovum (egg), conception occurs. The cervix is lined with mucus, the quality and quantity of which is governed by monthly fluctuations in the levels of the two principle sex hormones, estrogen and progesterone.

When estrogen levels are low, the mucus tends to be thick and sparse, which makes it difficult for sperm to reach the fallopian tubes. But when an egg is ready for fertilization and estrogen levels are high the mucus then becomes thin and slippery, offering a much more friendly environment to sperm as they struggle towards their goal. (This phenomenon is employed by birth control pills, shots and implants. One of the ways they prevent conception is to render the cervical mucus thick, sparse, and hostile to sperm.)

Later, at the end of pregnancy, the cervix acts as the passage through which the baby exits the uterus into the vagina. The cervical canal expands to roughly 50 times its normal width in order to accommodate the passage of the baby during birth.

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The Uterus

The uterus is the muscular organ which holds the developing baby during the nine months after conception. Like the cervical canal, the uterus expands considerably during the reproductive process. In fact, the organ grows to from 10 to 20 times its normal size during pregnancy.

A CLOSER LOOK AT THE UTERUS

Note the thick muscular walls—crucial when the baby is ready for delivery—and the lush inner lining, or endometrium, which nurtures the developing egg. From this angle, you can also see how the fallopian tubes cradle the ovaries in their feathery fimbria, ready to conduct a mature egg away from the ovary and on into the uterus.

Each month the uterus goes through a cyclical change, first building up its endometrium or inner lining to receive a fertilized egg, then, if conception does not occur, shedding the unused tissue through the vagina in the monthly process called menstruation.

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The Fallopian Tubes

Beyond the uterus, the fallopian tubes connect the rest of the system to the ultimate source of the eggs, the two ovaries. Each of these tubes is roughly five inches long and ranges in width from about one inch at the end next to the ovary, to the diameter of a strand of thin spaghetti.

The trumpet-shaped part near the ovary has about 20 to 25 feathery projections called fimbria, one of which is attached to the ovary. It is the fimbria that each month urge an egg to exit the ovary and begin its trip towards the uterus.

The Ovaries

The ovaries are a woman's storehouse of egg cells. They are among the first organs to be formed as a female baby develops in the uterus. At the 20-week mark, the structures that will become the ovaries house roughly 6 to 7 million potential egg cells. From that point on, the number begins to decrease rapidly. A newborn infant has between 1 million to 2 million egg cells. By puberty the number has plummeted to 300,000. For every egg that matures and undergoes ovulation, roughly a thousand will fail, so that by menopause, only a few thousand remain. During the course of an average reproductive lifespan, roughly 300 mature eggs are produced for potential conception.

The egg cells remain inactive until puberty, when the reproductive system is activated by a cascade of substances called sex hormones. Then, each month about 20 egg cells, each encased in a sac called a follicle, begin to ripen. Responding selectively to the sex hormones, one follicle becomes dominant while the others shrink away. The egg within the dominant follicle continues ripening to maturity. Then, helped by the feathery fimbria, it exits the ovary and enters the adjacent fallopian tube to be either fertilized or, if conception fails to occur, expelled from the body during menstruation.

If fertilization is to occur, it usually happens when the egg's journey is about one-third complete. Once a sperm unites with the egg, its surrounding gelatinous coat releases substances that prevent more sperm from entering.

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The Corpus Luteum

The fertilized egg then continues on its journey through the fallopian tube. About four or five days after fertilization, it enters the uterus and implants itself on the endometrium, which has been primed by the sex hormones to accept and nurture it.

FROM FOLLICLE TO “YELLOW BODY”

Host to a lifetime supply of eggs, the ovaries each month launch about 20 contenders towards potential conception. Each ripens in a supporting follicle, growth of which is triggered by the aptly named “follicle-stimulating hormone.” In turn, the winning follicle gives off increasing amounts of the hormone estrogen, which prepares the lining of the uterus for pregnancy. Once a mature egg has begun its trip through the fallopian tube, remnants of the winning follicle form the corpus luteum, or “yellow body.” Progesterone from the corpus luteum halts development of the remaining follicles and brings the lining of the uterus to peak preparedness.

Meanwhile, the follicle that held the egg still has a critical role to play. First it shrinks markedly, then begins to accumulate fatty substances, or lipids, that give it a yellowish tinge. The resulting structure, now called the corpus luteum (yellow body), produces progesterone and estradiol, two of the hormones critical to reproduction.

In a non-pregnant woman, the corpus luteum lasts for about 14 days, after which it shrinks and dries up, eventually becoming a speck of fibrous scar tissue. If conception occurs, however, a hormone from the developing placenta, which surrounds the baby in the uterus, stimulates the corpus luteum to maintain its production of progesterone during the first trimester of pregnancy.

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