Sexual Reproduction in Humans

Index to this page

The Problems to be Solved

Sexual reproduction is the formation of a new individual following the union of two gametes. In humans and the majority of other eukaryotes - plants as well as animals - the two gametes Gametes need In animals (and some plants), these two rather contrasting needs are met by anisogametes:

Sex Organs of the Human Male

The reproductive system of the male has two major functions: Sperm production - spermatogenesis - takes place in the testes.

Each testis is packed with seminiferous tubules (laid end to end, they would extend more than 20 meters) where spermatogenesis occurs.


The walls of the seminiferous tubules consist of diploid spermatogonia, stem cells that are the precursors of sperm.


Meiosis of each spermatocyte produces 4 haploid spermatids. These then differentiate into sperm, losing most of their cytoplasm in the process.

For simplicity, the figure shows the behavior of just a single pair of homologous chromosomes with a single crossover. With 23 pairs of chromosomes and several crossovers possible between each pair, the variety of gene combinations in sperm is very great.


Sperm cells are little more than flagellated nuclei. Each consists of

This electron micrograph (courtesy of Dr. Don W. Fawcett and Susumu Ito) shows the sperm cell of a bat. Note the orderly arrangement of the mitochondria. They supply the ATP to power the whiplike motion of the tail.

An adult male manufactures over 100 million sperm cells each day. These gradually move into the epididymis and the first portion of the vas deferens, where they undergo further maturation and are stored.

In addition to making sperm, the testis is an endocrine gland. Its principal hormone, testosterone, is responsible for the development of the secondary sex characteristics of men such as the beard, deep voice, and masculine body shape. Testosterone is also essential for making sperm.
Link to more on testosterone.
Testosterone is made in the interstitial cells that lie between the seminiferous tubules.


Interstitial cells are, in turn, the targets for a hormone often called interstitial cell stimulating hormone (ICSH). It is a product of the anterior lobe of the pituitary gland. However, ICSH is identical to the luteinizing hormone (LH) found in females, and I prefer to call it LH.


Follicle-stimulating hormone (also named for its role in females) acts directly on spermatogonia to stimulate sperm production (aided by the LH needed for testosterone synthesis). [Discussion]

Sex Organs of the Human Female

The responsibility of the female mammal for successful reproduction is considerably greater than that of the male.

She must


Egg formation takes place in the ovaries. In contrast to males, the initial steps in egg production occur prior to birth. By the time the fetus is 25 weeks old, all the oogonia that she will ever possess have been formed by mitosis. Hundreds of these diploid cells No further development occurs until years later when the girl becomes sexually mature. Then the oocytes complete their development, usually one at a time and once a month.

The primary oocyte grows much larger and completes the first meiotic division, forming a large secondary oocyte and a small polar body that receives little more than one set of chromosomes. Which chromosomes end up in the egg and which in the polar body is entirely a matter of chance. In humans, the first polar body does not complete meiosis.

In humans (and most vertebrates) the second meiotic division occurs after fertilization, converting the secondary oocyte into an egg (and a second polar body). Strictly speaking, then, it is secondary oocytes that are fertilized rather than eggs (but I shall not be so strict).

As in the diagram for spermatogenesis, the behavior of the chromosomes is greatly simplified.

The photomicrograph (courtesy of Turtox) shows polar body formation during oogenesis in the whitefish. Even allowing for the fact that fish eggs are larger than mammalian eggs, you can readily see how the polar body gets little more than one set of chromosomes.

These events take place within a follicle, a fluid-filled envelope of cells surrounding the developing egg. The ripening follicle also serves as an endocrine gland. Its cells make a mixture of steroid hormones collectively known as estrogen. Estrogen is responsible for the development of the secondary sexual characteristics of a mature woman, e.g., Estrogen continues to be secreted throughout the reproductive years of women During this period, it plays an essential role in the monthly menstrual cycle.
Link to a discussion of the menstrual cycle and the hormones that regulate it.


Ovulation occurs about two weeks after the onset of menstruation. In response to a sudden surge of LH, the follicle ruptures and discharges a secondary oocyte. This is swept into the open end of the fallopian tube and begins to move slowly down it.

Several sexually-transmitted diseases (STDs), especially gonorrhea and infections by chlamydia can cause scarring and blocking of the tubes and are a major cause of infertility.
In tubal ligation, the fallopian tubes are surgically cut and their ends tied to prevent pregnancy.

Copulation and Fertilization

For fertilization to occur, sperm must be deposited in the vagina within a few days before or a day or two after ovulation. Sperm transfer is accomplished by copulation. Sexual excitation dilates the arterioles supplying blood to the penis. Blood accumulates in three cylindrical spongy sinuses that run lengthwise through the penis. The resulting pressure causes the penis to enlarge and erect and thus able to penetrate the vagina. Movement of the penis back and forth within the vagina causes sexual tension to increase to the point of ejaculation. Contraction of the walls of each vas deferens propels the sperm along. Fluid is added to the sperm by the seminal vesicles, Cowper's glands, and the prostate gland. [View] These fluids provide a source of energy (fructose) and perhaps in other ways provide an optimum chemical environment for the sperm.

The mixture of sperm and accessory fluids is called semen. It passes through the urethra and is expelled into the vagina.

Physiological changes occur in the female as well as the male in response to sexual excitement, although these are not as readily apparent. In contrast to the male, however, such responses are not a prerequisite for copulation and fertilization to occur.

Once deposited within the vagina, the sperm proceed on their journey into and through the uterus and on up into the fallopian tubes. It is here that fertilization may occur if an "egg" is present (strictly speaking, it is still a secondary oocyte until after completion of meiosis II).

Although sperm can swim several millimeters each second, their trip to and through the fallopian tubes may be assisted by muscular contraction of the walls of the uterus and the tubes. There is also evidence that the egg releases a chemical attractant for sperm. In any case, sperm may reach the egg within 15 minutes of ejaculation. The trip is also fraught with heavy mortality. An average human ejaculate contains several hundred million sperm but only a few hundred complete the journey. And of these, only one will succeed in entering the egg and fertilizing it. Fertilization begins with the binding of a sperm cell to the outer coating of the egg (called the zona pellucida). Enzymes released by the acrosome at the tip of the sperm head digest a path through the zona and enable the sperm to enter the cytoplasm of the egg.

The same path allows other enzymes to leave the cytoplasm of the egg. Within moments, these act on the zona making it impermeable to the other sperm that arrive.

Soon the head of the successful sperm enlarges into the male pronucleus. At the same time, the egg (secondary oocyte) completes meiosis II forming a second polar body and the female pronucleus.

The male and female pronuclei move toward each other. Their nuclear envelopes disintegrate. A spindle is formed, and a full diploid set of chromosomes assembles on it. The fertilized egg or zygote is now ready for its first mitosis.


Embryonic development begins while the fertilized egg is still within the fallopian tube. The developing embryo travels down the tube, reaching the uterus in three or four days. As a result of repeated mitotic divisions and some migration of cells, a hollow ball of cells is formed called the blastocyst. Approximately one week after fertilization, the blastocyst embeds itself in the thickened wall of the uterus, a process called implantation, and pregnancy is established.

The blastocyst produces two major divisions of cells:
The extraembryonic membranes form the amnion, placenta and umbilical cord. The placenta grows tightly fused to the wall of the uterus. Its blood vessels, supplied by the fetal heart, are literally bathed in the mother's blood. Although there is normally no mixing of the two blood supplies, the placenta does facilitate the transfer of a variety of materials between the fetus and the mother. But the placenta is not simply a transfer device. Using raw materials from the mother's blood, it synthesizes large quantities of proteins and also some hormones.
Link to discussion of the placenta
as an endocrine gland.
The metabolic activity of the placenta is almost as great as that of the fetus itself.

The umbilical cord connects the fetus to the placenta. It receives deoxygenated blood from the iliac arteries of the fetus and returns oxygenated blood to the liver and on to the inferior vena cava.

Because its lungs are not functioning, circulation in the fetus differs dramatically from that of the baby after birth. While within the uterus, blood pumped by the right ventricle bypasses the lungs by flowing through the foramen ovale and the ductus arteriosus.

Although the blood in the placenta is in close contact with the mother's blood in the uterus, intermingling of their blood does not normally occur. However, some of the blood cells of the fetus usually do get into the mother's circulation - where they have been know to survive for decades. This raises the possibility of doing prenatal diagnosis of genetic disorders by sampling the mother's blood rather than having to rely on the more invasive procedures of amniocentesis and chorionic villus sampling (CVS).

Far rarer is the leakage of mother's blood cells into the fetus. However, it does occur. A few pregnant women with leukemia or lymphoma have transferred the malignancy to their fetus. Some babies have also acquired melanoma from the transplacental passage of these highly-malignant cells from their mother.

During the first 2 months of pregnancy, the basic structure of the baby is being formed. This involves cell division, cell migration, and the differentiation of cells into the many types found in the baby. During this period, the developing baby - called an embryo - is very sensitive to anything that interferes with the steps involved. Virus infection of the mother, e.g., by rubella ("German measles") virus or exposure to certain chemicals may cause malformations in the developing embryo. Such agents are called teratogens ("monster-forming"). The tranquilizer, thalidomide, taken by many pregnant European women between 1954 and 1962, turned out to be a potent teratogen and was responsible for the birth of several thousand deformed babies.

By 3 months, all the systems of the baby have been formed, at least in a rudimentary way. From then on, development of the fetus, as it is now called, is primarily a matter of growth and minor structural modifications. The fetus is less susceptible to teratogens than is the embryo.

Pregnancy involves a complex interplay of hormones. These are described in a separate page. [Link to it.]

The placenta is an allograft

One of the greatest unsolved mysteries in immunology is how the placenta survives for 9 months without being rejected by the mother's immune system. Every cell of the placenta carries the father's genome (a haploid set of his chromosomes); including one of his #6 chromosomes where the genes for the major histocompatibility antigens are located.

One partial exception: none of the genes on the father's X chromosome are expressed. While X-chromosome inactivation is random in the cells of the fetus, it is NOT random in the cells of the trophoblast. In every cell of the trophoblast - and its descendants - it is the paternal X chromosome that is inactivated. [Discussion of X-chromosome inactivation.] But this does not solve our problem because the genes for all the major histocompatibility antigens are located on chromosome 6, which is not inactivated.
Discussion of the human major histocompatibility complex (MHC)

Thus the placenta is immunologically as foreign to the mother as a kidney transplant would be.

Yet it thrives.

Despite a half-century of research, the mechanism for this immunologically privileged status remains uncertain. But one thing is clear:

The mother is not intrinsically tolerant of the father's antigens.

Some evidence:

So what accounts for the phenomenon? Some possibilities:

Birth and Lactation

Exactly what brings about the onset of labor is still not completely understood. Probably a variety of integrated hormonal controls are at work.
Link to a discussion of hormones involved in birth and lactation.
The first result of labor is the opening of the cervix. With continued powerful contractions, the amnion ruptures and the amniotic fluid (the "waters") flows out through the vagina. The baby follows, and its umbilical cord can be cut.

The infant's lungs expand, and it begins breathing. This requires a major switchover in the circulatory system. Blood flow through the umbilical cord, ductus arteriosus, and foramen ovale ceases, and the adult pattern of blood flow through the heart, aorta, and pulmonary arteries begins. In some infants, the switchover is incomplete, and blood flow through the pulmonary arteries is inadequate. Failure to synthesize enough nitric oxide (NO) is one cause.

Shortly after the baby, the placenta and the remains of the umbilical cord (the "afterbirth") are expelled.

At the time of birth, and for a few days after, the mother's breasts contain a fluid called colostrum. It is rich in calories and protein, including antibodies that provide passive immunity for the newborn infant.

Three or four days after delivery, the breasts begin to secrete milk.
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11 June 2001