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How Human Reproduction Works
by Craig C. Freudenrich, Ph.D.

Photo courtesy Georgia Reproductive Specialists
Fertilized egg
From the time we are teenagers through mid-life or longer, we are capable of sexually reproducing. Sex plays a major role in much of our culture, constantly manifesting itself in our fashion, literature, music, television and movies.

From a biological standpoint, the purpose of sex is to merge two sets of genetic information, one from the father (sperm) and one from the mother (egg), to make a baby that is genetically different from either parent.

In this edition of HowStuffWorks, we will explore the biology of sex -- otherwise known as human sexual reproduction. We will examine the body's sexual organs, the biological cycles relevant to sex, and the process of fertilization.

The primary goal of sex is to merge the sperm and egg (fertilization) to make a baby. In many organisms, sex occurs outside of the body. For example, in most fish or amphibians, females lay eggs somewhere (usually on the sea/river bed), the male comes along and sprays the eggs with sperm and fertilization takes place.

In reptiles and mammals (including humans), fertilization takes place inside the body of the female (internal fertilization). This technique increases the chances of successful sexual reproduction. Because we use internal fertilization, our sexual organs are specialized for this purpose. Let's take a closer look at the sexual organs in males and females.

Male Sex Organs
From the outside, the male has two visible sex organs, the testes and penis. The testes (singular: testis) are the primary male sexual organs in that they make the sperm and produce testosterone. The sperm cell is the male sex cell (gamete). Testosterone is the hormone that causes male secondary sex characteristics such as facial and pubic hair, thickened vocal cords and developed muscles.

The testes are housed outside of the main part of the male's body, in a sac called the scrotum. This location is important because in order for the sperm to develop properly, they must be kept at a slightly lower temperature (95 to 97 degrees Fahrenheit, 35 to 36 degrees Celsius) than normal body temperature (98.6 F, 37 C).

The immature sperm travel from each testis to a coiled tube on the outer surface of each testis called the epididymis, where they mature in about 20 days. The sperm exit the body through the penis.

The penis is made of soft, spongy tissue. When engorged with blood during sexual excitation and intercourse, the spongy tissue stiffens and causes the penis to become erect, which is important for the penis's main function -- to place the sperm inside the female.

Male sex organs

As mentioned above, sperm are made in the testes. During sexual intercourse, smooth muscles contract and propel mature sperm from the end portions of the epididymis through a long tube (vas deferens or ductus deferens) inside the body, just beneath the bladder. From there, the sperm get mixed with nutrient-rich fluids from the seminal vesicles and a milky secretion from the prostate gland. This combination of sperm and fluids is called semen. The semen does three things:

  • Provides a watery environment in which the sperm cells can swim while outside the body
  • Provides nutrients for the sperm cells (fructose, amino acids, vitamin C)
  • Protects the sperm cells by neutralizing acids in the female's sexual tract
Once the semen is made, it passes through another tube (urethra) within the penis, exiting the body through the opening of the penis.

One last male organ is a tiny, pea-sized set of glands inside the body at the base of the penis, called the bulbourethral or Cowper's glands. During sexual excitation, and just prior to the ejection of sperm (ejaculation), the Cowper's glands secrete a tiny amount of fluid that neutralizes any traces of acidic urine that may be leftover in the urethra. It is also believed that these secretions may lubricate the penis and female tract during sexual intercourse.

Female Sex Organs
All of the female's sexual organs are located within her body except the vulva. The vulva consists of two sets of folded skin (labia major, labia minor) that cover the opening to the reproductive tract, and a small nub of sensitive, erectile tissue (clitoris), which is the remnant of the fetal penis (see sidebar).

The two ovaries are the major female sex organs, the counterpart of the male testes. The ovaries make the eggs, or oocytes, which are the female gametes, and produce estrogen, the female sex hormone. Estrogen causes female secondary sexual characteristics such as pubic hair, breast development, widening of the pelvis and deposition of body fat in hips and thighs. The ovaries are located in the abdomen.

Female sex organs

Eggs develop inside the ovary and are released upon ovulation into a tube (the oviduct or Fallopian tube) lined with fingerlike projections. The egg travels through the Fallopian tube, where fertilization can take place, to a muscular chamber called the uterus.

The uterus is where the baby develops. It is made of smooth muscle and is normally about the size and shape of a small pear turned upside down. During pregnancy, it can stretch to about the size of a basketball to hold the developing baby. The base of the uterus (neck of the pear) is a muscular wall called the cervix. In the cervix is a tiny opening, about the size of a pinhead, called the external os. The external os is filled with a thick plug of protein (mucus) that serves as a barrier to the entrance of the uterus. The cervix leads into a smooth-muscle-walled tube called the vagina, or birth canal.

The vagina connects the uterus to the outside of the body, and its opening is covered by the labia. The vagina receives the male's penis during sexual intercourse and delivers the baby during childbirth. The vagina is normally narrow (except around the cervix), but can stretch during intercourse and childbirth.

Finally, two sets of glands, the greater vestibular gland (Bartholin's gland) and the lesser vestibular gland, are located on either side of the vagina and empty into the labial folds of skin. The secretions from these glands lubricate the labial folds during sexual excitation and intercourse.

Development of Sex Organs

When we first develop, we have two sets of organs: one that can develop into the female sex organs (Mullerian duct) and one that can develop into the male sex organs (Wolffian ducts). Which sex organs develop depends on the presence of the male hormone testosterone (in humans, the default sex is female):
  • If the embryo is a male (XY chromosomes), then testosterone will stimulate the Wolffian duct to develop male sex organs, and the Mullerian duct will degrade.
  • If the embryo is female (XX), then no testosterone is made. The Wolffian duct will degrade, and the Mullerian duct will develop into female sex organs. The female clitoris is the remnants of the Wolffian duct.
  • If the embryo is a male (XY), but there is a defect such that no testosterone is made, then the Wolffian duct will degrade, and the Mullerian duct will develop into non-functional female sex organs.
Sex-organ development is determined by the third month of development.

Other Organs Involved in Sex
Although not located in the reproductive systems, two other organs are important for sexual function in both males and females:

  • The hypothalamus in the brain - The hypothalamus has nerve cells that secrete a hormone called gonatotropin releasing hormone (GnRH) into the blood vessels leading to the anterior pituitary gland.
  • The anterior pituitary gland just beneath the brain - GnRH causes the anterior pituitary cells to release two hormones, luteinizing hormone (LH) and follicle stimulating hormone (FSH), into the general blood circulation. LH and FSH act on the testes/ovaries to stimulate the making and maturation of the sex cells and the production of sex hormones (testosterone, estrogen, progesterone).

The nerve cells time-release small, low-level spurts of GnRH every 90 minutes, which causes the anterior pituitary to secrete small pulses of LH and FSH. The sex hormones from the testes/ovaries give feedback to the hypothalamus and anterior pituitary to regulate the secretion of GnRH, LH and FSH -- this interplay is called the negative feedback control system. The chemical interplay between the hypothalamus, anterior pituitary gland and the testes/ovaries is important for sexual development, maintaining sexual function and sexual reproduction. An error in this chemical interplay can be a cause of infertility.

Production of Sex Cells
From the time of puberty on, men make sex cells (in the form of sperm cells) continuously. In contrast, by the time a female is born, she has made all of the eggs that she will ever have. As she reaches puberty, the eggs begin to develop and get released, and this process continues until menopause. In both males and females, the production of sex cells involves meiosis, a type of cell division whereby our two sets of genetic instructions are reduced to one set for the sex cell.

Photo courtesy U.S. DOE, Human Genome Project

Each cell in your body contains a set of chromosomes from your mother (her egg) and your father (his sperm). When you look at human chromosomes, they are X-shaped, except for the male Y chromosome. When your body produces sex cells (sperm or egg, depending on whether you are a male or female), your body must reduce the number of chromosomes by half to go into the sex cells. To do this, it randomly sorts chromosomes from both sets in one cell division and then reduces them by half in another. Therefore, each sperm or egg that your body produces is unique and different -- it contains a different mix of your mother's and father's genes. This is why two brothers in the same family can look and act totally different from one another even though they come from the same parents -- it all depends on which genes (chromosomes) were randomly chosen when producing the sex cells of the mother and father.

For details on meiosis, see The Biology Project: Meiosis Tutorial and this Question of the Day.

Cycles: Timing is Everything
Remember that, biologically, the main goal of sexual reproduction is to have the sperm combine with the egg to make a baby. With respect to the man, age is not critical. Men are capable of producing sperm that can fertilize an egg at any time from the onset of puberty until they die (there have been many cases in which men in their 70s and 80s have conceived children with younger women). In contrast, women typically can release fertilizable eggs from the time of puberty until their late 40s or early 50s. After that time, their ovaries stop releasing eggs and they undergo various biochemical and physiological changes, the sum of which is called menopause.

Although women are capable of conceiving a child from puberty to menopause, the timing of sexual intercourse is critical to the success of sexual reproduction. Women have an ovulatory cycle, or menstrual cycle, made up of complex physiological and biochemical changes that are linked to the timing of reproduction.

In the first phase of the menstrual cycle, a follicle (the cell complex that surrounds and nurtures the egg) grows in the ovary, while the lining of the uterus builds up to receive a fertilized egg. At mid-cycle, when the egg is ready, the ovary releases the egg. This stage is called ovulation, and is the optimum time for fertilization to take place. The egg enters the Fallopian tube on its way to the uterus. In the last phase of the menstrual cycle, one of two things happens: If the egg is fertilized, it continues on to the uterus, attaches to the lining and pregnancy begins; if the egg is not fertilized, the uterus sheds its lining (and the now-dead egg), menses begins and another menstrual cycle ensues.

For more on the menstrual cycle, see How Menstruation Works.

Come Together
When a man and a woman engage in sexual activity, each goes through an arousal phase. In both the man and the woman, nerve impulses from the brain cause their heart rates to increase and dilate peripheral blood vessels. They feel warm, and they begin to sweat. The Cowper's glands in the man and the vestibular glands in the woman secrete fluid that lubricates the man's urethra and the woman's labial area and vagina.

The man's brain sends nerve impulses to the blood vessels in his penis and tells the arterioles to dilate and the venules to constrict. The blood flow engorges the spongy tissue of his penis, causing it to become erect. As the couple engages in intercourse, the man inserts his erect penis into the woman's vagina. As intercourse continues, the man reaches a point at which muscle contractions in the epididymis, prostate and seminal vesicles propel semen from the penis into the woman's vagina (ejaculation) at the base of the uterine cervix. Muscle contractions in the woman's body periodically dip her cervix into the semen.

Once the semen is deposited at the base of the uterus, the sperm begin a long journey to fertilization. This journey must be completed within 12 to 48 hours, before the sperm die. They must first cross the barrier of the cervix, which will be thin and watery if the woman has just ovulated (for our purposes, we will assume that sexual intercourse has occurred within a couple of hours after ovulation).

Once the sperm have traversed the cervical mucus, they travel up the moist lining of the uterus into the Fallopian tubes (only one of the Fallopian tubes contains an egg, so many sperm travel in the wrong direction). Fewer than 1,000 sperm out of the millions in the semen actually reach the Fallopian tubes.

Many sperm surround the egg in the Fallopian tube. The head of each sperm (acrosome) releases enzymes that begin to break down the outer, jelly-like layer of the egg's membrane, trying to penetrate the egg. Once a single sperm has penetrated, the cell membrane of the egg changes its electrical characteristics (depolarizes). This electrical signal causes small sacs just beneath the membrane (cortical granules) to dump their contents into the space surrounding the egg. The contents swell, pushing the other sperm far away from the egg (cortical reaction). The other sperm die within 48 hours. The cortical reaction ensures that only one sperm fertilizes the egg.

Photo courtesy Georgia Reproductive Specialists
Fertilized egg, showing two pronuclei beginning to divide (left) and reaching an eight-celled stage within 72 hours (right)

The fertilized egg is now called a zygote. The depolarization caused by sperm penetration results in one last round of division in the egg's nucleus, forming a pronucleus containing only one set of genetic information. The pronucleus from the egg merges with the nucleus from the sperm. Once the two pronuclei merge, cell division begins immediately.

The dividing zygote gets pushed along the Fallopian tube. By approximately four days after fertilization, the zygote has about 100 cells and is called a blastocyst. When the blastocyst reaches the uterine lining, it floats for about two days, finally implanting in the uterine wall by the sixth day after fertilization. Once implanted, the blastocyst secretes hCG, which rescues the corpus luteum and signals that a successful pregnancy has begun.

Sometimes, two dominant follicles develop eggs and ovulate. If both are fertilized and subsequently implanted in the uterus, two embryos develop: twins. Because they developed from separate eggs that were fertilized by different sperm, they are called fraternal twins. Fraternal twins do not share any more genetic information than siblings born separately.

In contrast, the two daughter cells that remain after a fertilized egg undergoes its first division may separate and divide independently of each other. When this happens, they remain loosely connected while in the Fallopian tube, and the two blastocysts implant together in the uterine wall. They develop into two separate embryos. Because these embryos came from the same fertilized egg, they share identical genetic material and are called identical twins.

The implanted blastocyst continues developing in the uterus for nine months. As the baby grows, the uterus stretches until it is about the size of a basketball.

Contraception and STDs
As you can see from the process of sexual reproduction, there are several ways to prevent the sperm and egg from coming together. These methods of contraception fall into the following categories:

  • Not engaging in sexual activity - abstinence
  • Preventing a follicle from developing - birth control pills
  • Placing a barrier between sperm and egg - condoms (male/female), cervical caps, diaphragms
  • Killing the sperm - spermicides
  • Surgery - blocking the sperm or egg with surgical procedures like tubal ligations (in women) or vasectomies (in men)
  • Timing - avoiding intercourse during the period of maximum fertility

See Epigee Birth Control Guide for details, including information on the effectiveness of various contraception methods.

Sexual activity carries certain risks of diseases caused by:

  • Bacteria (gonorrhea, syphilis, chlamydia)
  • Protozoa (trichomoniasis)
  • Viruses (genital herpes, HIV/AIDS)

See The Naked Truth About Sexually Transmitted Diseases for details.

For more information on human reproduction and related topics, check out the links on the next page.

Lots More Information!

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