Sex and reproduction

Sex is probably surrounded by more interest and perhaps confusion than any other area of human activity. On a purely biological level, sex is the mechanism by which humans reproduce their young a process that involves transferring a male reproductive cell (sperm) into the female reproductive tract, where it must join with a female reproductive cell (ovum) to produce a single fertilized egg (zygote). Over a period of nine months, this fertilized cell then develops within the female into a new individual.

Among humans, however, sex has also become a way of expressing profound emotions such as love and passion, a form of emotional expression unique to humans and which, in every age and culture, has been an integral part of human behavior.

Male and female reproductive systems

Male sex organs are shown in cross section, above. The male gametes the sperm are produced in the testes, which lie in the scrotum. They travel from the epididymis of the testis along the vas deferens to the seminal vesicles, which are located at the rear of the prostate gland. The spermatic ducts join with the urethra, the tube through which urine passes out from the bladder. The testes also produce the male sex hormone testosterone, which is secreted into the bloodstream and is responsible for initiating and maintaining male sexual characteristics.

The male and female reproductive systems are perfectly designed for the job they have to do. The female system produces the ova and contains an organ, the uterus, or womb, in which the developing offspring is accommodated. The male system produces sperm and includes an organ, the penis, which deposits sperm within the female. Although both systems differ in structure, housing, and function, both develop from the same embryonic tissue and, in their adult forms, contain many counterpart or corresponding organs.

The female reproductive organs are housed inside the body within the pelvis. From the outside, all that can be seen are the external genitals, known collectively as the vulva. At the front, as if looking through a woman’s open legs, is the mons veneris or mons pubis, a pad of fatty tissue that lies over the pubic bone and which from puberty onward is covered by pubic hair. Running down and back are two folds of skin, the labia majora, which surround two smaller folds, the labia minora. At their front, the labia minora form a hood under which lies the clitoris, a small, highly sensitive organ corresponding to the man’s penis and similarly formed of erectile tissue. Below the clitoris lies the urethra, then below this, the vaginal opening. In women who have not had sexual intercourse, this opening is partly closed by the hymen, a thin membrane that is usually torn or ruptured the first time that a woman has coitus, although it may be stretched or torn earlier.
The vagina itself is a muscular passage about 4 inches (10 centimeters) long, which leads up from the vulva to the uterus. It is capable of great distention; during childbirth it distends greatly to allow a child to be born. A minute opening, the os uteri, forms the entrance to the cervix, or neck, of the uterus. Lined by the endometrium, the uterus is a small pear-shaped organ within which the growing fetus is sheltered during pregnancy.

From either side of the uterus, the Fallopian tubes reach back to the ovaries. These ovalshaped organs are the female gonads, or reproductive glands, equivalent to the male testes. The ovaries release a ripe ovum every month and also produce the female sex hormones progesterone and estrogen. These play a vital role in the female reproductive cycle and are also responsible for the development of such secondary sexual characteristics as breasts, fat, and body hair.

The male reproductive system lies both inside and outside the body and, unlike the female system, is linked to the urinary system. Visible organs are the penis and testes. The penis, whose size and shape may vary considerably, is normally flaccid. Behind and below the penis are the two testicles, or testes, the male gonads. These produce the sperm cells and the male sex hormone testosterone, responsible for such secondary sexual characteristics as facial hair and deep voice. The testes are flattened oval-shaped organs that lie inside the baglike scrotum. Within them, sperm cells are continually produced inside coiled seminiferous tubules. Once formed, the sperm cells mature inside epididymides, two tubular organs adjacent to the testes.

Female sex organs are shown in cross section. The vagina leads to the cervix of the uterus (the neck of the womb), and the uterus itself connects, by means of the Fallopian tubes, with the ovaries. These structures lie above and behind the uterus. During ovulation, at about the middle of the menstrual cycle, an egg is released from an ovary and travels down a Fallopian tube to the uterus. The ovaries also produce the female sex hormones progesterone and estrogen, which initiate and maintain female sexual characteristics

Birth control

When reproduction is not the desired result of sex, some form of conscious intervention is required. This is commonly referred to as birth control. Although conception the fertilization of an ovum by a sperm is the natural result of sex, it does not necessarily happen every time two people have sexual intercourse. Because of her monthly reproductive, or menstrual, cycle, a woman is only able to conceive during a limited number of days each month near the time of ovulation. These days can be identified, although not always with certainty, by counting the days between menstrual periods and also by observing certain associated physical changes, such as small alterations in body temperature or changes in vaginal discharge. Avoiding sexual activity for 5 days before and after the calculated day of ovulation is regarded by many as the most natural and by some, as the only acceptable form of birth control. The technique is sometimes referred to as the rhythm method of birth control.

Other methods of birth control range from using hormonal contraceptives, through physical or chemical barriers, to surgical sterilization. Hormonal methods, such as the oral contraceptive pill or hormone injections or implants, work by preventing ovulation. Physical barriers, such as the condom or diaphragm, prevent sperm from reaching an ovum. Chemical barriers usually in the form of foam or gel kill sperm before they reach the uterus. Another technique, involving an intrauterine device (IUD), seems to prevent the ovum from implanting in the lining of the uterus. Surgical sterilization involves cutting or tying off the tubes through which the sperm or the ovum must pass the vas deferens in a man, the Fallopian tubes in a woman. In both cases, pregnancy is prevented because the partner operated on is made sterile. This form of contraception is permanent.

Sexual activity

It is still not known exactly what determines sexual activity in humans. It would seem there are physiological rules that may be linked to changes within the body, such as changes in hormonal level that possibly interact with the central nervous system. But age, social, cultural, and psychological factors all play a part in determining human sexual behavior.

From various investigations into human sexual activity, such as the Kinsey Reports of the late 1940’s and 1950’s, the studies conducted over the last four decades by Masters and Johnson, and, more recently, surveys by Shere Hite, it is clear that individual requirements and practices vary enormously, ranging from those who live active sexual lives, whether with many partners or one, to those who seem content with little or no sexual involvement.

any one time there may be millions of mature sperm available in the seminal vesicles that can be released when a man ejaculates. By contrast, however, the ovaries produce a few thousand eggs, or ova, but only a few hundred of them are released during the female’s lifetime. Normally, only one egg at a time is released each month during ovulation.

Female reproductive cycle

A woman is born with a full complement of immature ova contained within ovarian follicles. Some 2 million are present at birth, declining to about 300,000 by puberty. Every month, a woman’s body undergoes a routine cycle of physical changes during which an ovum is released and the body prepares for possible pregnancy.

This cycle is known as the menstrual cycle. It begins at puberty sometime between the ages of 10 and 16 when its onset is known as the menarche; the menstrual cycle repeats itself every 24 to 35 days in most women, unless an egg is fertilized. This cycle continues until the menopause, which usually occurs between the ages of 45 and 55 in most women.

The monthly cycle is affected by various hormones controlled by the hypothalamus. During the first phase the follicular phase the follicle-stimulating hormone (FSH) produced by the pituitary causes ovaries and ovarian follicles to enlarge; it also causes the ovaries to produce estrogen. One ovarian follicle outstrips the others in growth, ruptures and then, due to the action of luteinizing hormone (LH), releases the single egg. This is called ovulation and occurs midway through the cycle.

The mature ovum then enters the Fallopian tube and is propelled into the uterus. There, due to the action of estrogen and progesterone, the endometrium has become thickened and vascular in preparation for the arrival of a fertilized egg. If the egg is not fertilized by a sperm within about 3 days, it dies.

During the second half of the cycle the luteal phase LH causes the follicular remains to form a bright yellow structure, the corpus luteum, and the endometrium continues to thicken under the influence of progesterone and estrogen. If a fertilized egg fails to arrive, the corpus luteum degenerates and the endometrial lining is shed together with blood, passing out of the body as menstrual flow. Menstruation lasts 3 to 7 days, then the entire cycle begins again.

The female reproductive cycle depends on the menstrual cycle, which is based on monthly ovulation. During the menstrual cycle, the lining of the uterus thickens in preparation for the implantation of a fertilized ovum. If no ovum implants, the lining breaks down and is expelled as menstrual bleeding. An ovum, grown in a follicle of the ovary, is then released and is guided into the Fallopian tube by the tube’s fingerlike fimbria. As it moves toward the uterus, it may encounter sperm (A), one of which may penetrate and fertilize it (B), causing cell division (C, D) to begin. After a few days, a fertilized ovum—now developed to the blastocyst stage (E)—reaches the uterus lining and implants there.

Fertilization and implantation

Fertilization occurs in the Fallopian tube and can only happen if both ovum and sperm are present.

When the man ejaculates, some 400 million sperm are deposited into the woman’s vagina. Propelled by tail-like structures, the sperm make their way fairly rapidly up the vagina, through the cervix, and into the uterus. This takes less than an hour, but at least half the number die in the acidic conditions of the vagina; others die as the sperm continue to travel to the top of the uterus and into the Fallopian tube. Here, conditions are favorable for the few hundred sperm that remain and they can survive for up to 72 hours.

Fertilization occurs immediately if an ovum is already present. It is accomplished when one male sperm penetrates the surface of the ovum. The cell wall then becomes impenetrable to other sperm, and the nuclei of the two cells fuse together. Cell division begins almost at once, the ovum subdividing or segmenting, first into two and then doubling with each division until it becomes a rounded mass of cells the morula. As the cells increase in number, they also differentiate to form the different cells that make up the human body. As the process continues, a fluid-filled cavity develops in the morula, now called the blastocyst; the outer layer forms a cellular wall—the trophoblast, which will form the placenta-while the remaining cells form a mass from which the fetus and amniotic sac will develop.

During this process, the fertilized ovum has been making its way toward the uterine cavity, and about seven days after fertilization, the blastocyst implants in the endometrium. Small projections on the trophoblast, called chorionic villi, burrow into the uterine wall so that the blastocyst becomes completely embedded, obtaining its nourishment by diffusion from the uterus. Once implantation has occurred, conception is complete, the normal menstrual cycle is suspended, and pregnancy is established. Occasionally, the fertilized ovum fails to reach the uterus and instead implants elsewhere in the reproductive tract. This is known as ectopic pregnancy and almost invariably requires medical treatment.

Boy or girl?

The sex of the new infant is determined at the time of fertilization and depends on the sex chromosome present in the nucleus of the sperm. The nucleus of every cell in the body, except the germ cells (sperm or ovum), contains a “blueprint” of information determining how that cell functions. Forty-four of these chromosomes are somatic not concerned with reproduction while two are sex chromosomes. These are of two types, X and Y; females have two X chromosomes (XX), and males have one X and one Y chromosome (XY). Germ cells, however, only contain 23 chromosomes, half the normal number. Each female ovum therefore contains 22 somatic chromosomes and one X chromosome; each male sperm contains 22 somatic chromosomes and either an X or a Y chromosome.

At fertilization, the fusion of the cells ensures that the newly fertilized ovum contains its full complement of chromosomes but its sex depends on the sperm. An ovum fertilized by an X-carrying sperm will develop into a female embryo; one fertilized by a Y-carrying sperm will develop into a male.

An ovum is a female reproductive, or germ, cell. This photograph shows its nucleus, approximately central, surrounded by cytoplasm, which in turn is surrounded by the zona pel-lucida, in which a number of sperm can be seen.

Infertility

Infertility the inability to produce children affects about one in ten couples. There are many causes. In about 40 per cent of cases, infertility is due to male sterility low sperm count, abnormal sperm, or impotence. In women, the most common cause is blockage of the Fallopian tubes. Other causes are failure to ovulate and cervical disorders. In one case in ten, no cause will be found, but in other cases, medical investigation will reveal a cause that can be treated successfully.

Sex chromosomes in the nuclei of body cells are of two types, called X and Y. All cells in a female have two X chromosomes; all in a male, one X and one Y. Germ cells have only one sex chromosome, however: an ovum has an X, but a sperm can have either an X or a Y. When a sperm fertilizes an ovum, the resulting cells will therefore have either two X chromosomes and be female or one X and one Y and be male.