After you have finished reading this chapter, you should be able to: Describe the structures and functions of the male and female reproductive systems. Explain the importance of hormones to the functioning of reproductive systems. Discuss issues that affect reproductive health: contraception, infertility, and STDs. Where did you come from, Baby dear? Out of the everywhere into the here. George Macdonald, “Baby,” At the Back of the North Wind Introduction Everyone loves the news stories about the taxi driver who delivers a baby for the woman who could not get to the hospital on time. At some point each of us asks ourselves with awe for the first time, “Where did I come from?” It has been said of the body that it is “fearfully and wonderfully made.” How true this is of the reproductive systems on which the survival of our species depends. In this chapter , you will study the systems of the human body responsible for our being here: the male and the female reproduc- tive systems. ■ ■ THE MALE REPRODUCTIVE SYSTEM The male reproductive system has two main functions: It produces hap - loid male gametes, the sperm cells; and it must be able to deposit the sperm cells it produces inside the female. (As described in Chapter 16, 360 Human Reproduction 17 fertilization in all mammals, including humans, occurs internally.) In addition, the male reproductive system provides a pathway for the removal of urine. (See Figure 17-1.) The first function, sperm cell formation, occurs in the two testes. The formation of sperm requires a temperature that is a few degrees cooler than the temperature of the rest of the body. How is this temperature reduction produced? The testes are suspended within a sac called the scrotum. Because it is not located within the body cavity, the temperature of the scrotum is a few degrees lower than normal body temperature. The scrotum is an adaptation that has evolved to increase the chances of pro- ducing healthy sperm. Inside the testes are a great many tiny tubes, or tubules. In fact, if all the tubules in a single testis were laid end-to-end, they would reach from one end of a football field to the other, seven times over! As normal diploid cells move through these tubules, they undergo meiotic cell division. The production of haploid gametes occurs in a process called gametogenesis. The formation of sperm cells is called spermatogenesis. Nowhere else in the male’ s body does meiotic cell division occur. In the tubules, the cells go through two divisions that produce four haploid spermatids from each diploid cell. The spermatids are actually immature sperm and are pushed from the tubules into the epididymis, a tubule about 6 meters C hapter 17 / Human Reproduction 3 61 Seminal vesicle Coccyx Large intestine Abdominal cavity Ureter Bone of pelvis Prostate gland Urinary bladder Sperm duct Urethra Penis Twisted tubules a. Sectional view Testis Scrotum Sperm cells Sperm-forming cells b. Front view c. Microscopic view of cross section of a tubule of testis LIVING ENVIRONMENT BIOLOGY, 2e/fig. 17-1 s/s (rev.10/22/03) Figure 17-1 The male reproductive system. long packed into each testis. The spermatids remain there for several weeks, completing their development into mature sperm cells. Sperm cells are highly specialized cells that are able to move. Each sperm cell must be able to deliver a haploid set of chromosomes from the male to an egg cell in the female reproductive tract. The structure of a mature sperm cell is well adapted to its function. Almost the entire head of the sperm is the haploid nucleus, the all-important genetic information that is delivered to the egg. Attached to the head of the sperm is a long tail that propels the sperm along. Also present are large numbers of mito- chondria that produce ATP, which yields the energy the sperm use to pro- pel themselves to the egg. (See Figure 17-2.) From the epididymis, sperm move into the vas deferens, the begin- ning of their pathway out of the body. As sperm move through the vas deferens, a number of glands add fluids. Sperm and these added fluids make up the semen. In fact, most of the semen is not sperm. Instead, it consists mostly of fructose, a sugar that provides an additional source of energy for the sperm. Prostaglandins are also found in the semen. Prostaglandins are chemicals that cause muscle contractions to occur in the female reproductive system. These contractions help move the sperm toward the egg. In addition, semen has an alkaline pH. This high pH neu- tralizes the acid in the male’ s urethra and in the female reproductive sys - tem that might damage the sperm. The male reproductive system is adapted for internal fertilization. The penis is a structure that has evolved to deposit sperm safely within the female’s body. The tube that passes through the penis, the urethra, has two functions. It is the pathway for sperm out of the body as well as the pathway for urine to leave the body after having been stored in the uri - nary bladder. At the time of sexual excitement, a valve closes. This valve prevents the bladder from releasing urine. At the same time, nerve impulses from the brain cause blood vessels in the penis to relax. Blood rushes in and fills empty spaces in the spongy tissue in the penis. The 3 62 R eproduction, Growth, and Development Mitochondria Head Tail LIVING ENVIRONMENT BIOLOGY, 2e/fig. 17-2 s/s Figure 17-2 The mature sperm cell is well adapted to deliver the haploid set of chromosomes from the male to the egg cell of the female. penis enlarges and becomes rigid. The result is an erection. Strong mus- cular contractions occur in a sequence of events called an orgasm. Semen is forced from the body during ejaculation. The semen in a single ejacu- lation contains up to 300 million sperm. Any one of these sperm cells could fertilize an egg and lead to the birth of a new human. ■ ■ THE FEMALE REPRODUCTIVE SYSTEM Three important functions are performed by the female reproductive sys- tem. First, gametes (eggs) are produced in the ovaries. Second, a pathway is provided for sperm cells to reach an egg. Third, the female reproduc- tive system provides a temporary home for the developing embryo. (See Figure 17-3.) In females, gametogenesis produces haploid egg cells in the ovaries, a pair of reproductive organs. The production of egg cells, a process called oogenesis, actually begins in a woman’s body before she is born. Approx- imately 2 million cells in the female fetus have already begun the first phases of meiosis in the immature ovaries by the time of birth. Nothing further happens to these cells until the female reaches puberty. In an adult male, sperm production occurs all the time; an average of 30 million sperm are produced each day. In a female, all potential eggs are present when she is born. Throughout her reproductive life, a female releases only a few hundred of these eggs. Usually only a single egg matures and is released each month. During egg formation, meiosis produces four cells after two divisions. However, only one mature egg is made. Along with this single mature egg, three other smaller structures are formed. Because C hapter 17 / Human Reproduction 3 63 Abdominal cavity Fallopian tube Ovary Uterus Vaginal canal Bone of pelvis Urethra Urinary bladder a. Sectional view Ureter Coccyx Large intestine LIVING ENVIRONMENT BIOLOGY, 2e/fig. 17-3 s/s b. Ovary (microscopic view) Blood vessel Ripe follicle Ovum Germinal epithelium Young follicles Figure 17-3 The female reproductive system. an egg cell needs a relatively large amount of raw materials, nutrients stored in the cytoplasm are not divided equally between the egg and the three other structures produced during meiosis. Instead, just the large, single egg is packed with the nutrients needed to nourish the embryo right after fertilization. This development of egg cells occurs within the ovary once a month. One mature egg cell is released from one of the ovaries. The ovaries con- tain follicles. A follicle is a hollow bundle of cells with an egg inside. The follicle helps the egg mature. As the egg matures, the follicle enlarges and fills with fluid. The greatly enlarged follicle moves to the edge of the ovary and bursts, releasing the egg and the fluid. This event is ovulation, the release of an egg from the ovary. (See Figure 17-4.) The egg cell gets swept into the oviduct (or fallopian tube), a long tubular structure found next to each ovary. If fertilization occurs, the sperm usually joins the egg in the oviduct. The egg continues to move along the oviduct to the uterus, a pear-shaped organ with thick muscu- lar walls. If the egg cell was fertilized, the embryo becomes attached to the inside wall of the uterus and continues to develop. If fertilization did not 3 64 R eproduction, Growth, and Development LIVING ENVIRONMENT BIOLOGY, 2e/fig. 17-4 s/s Primary follicles Developing follicles Mature follicle Ovary Oocyte Corpus luteum Disintegrating corpus luteum Ovulation Ovary Uterus Oviduct Figure 17-4 Ovulation is the release of a mature egg from the ovary into the oviduct. occur, the egg cell breaks down within 24 hours of ovulation and is passed from the body, along with some blood and tissue from the uterine lining. (See Figure 17-5.) At the lower end of the uterus is the cervix, a narrow opening. It is through the cervix that the sperm traveled on their way to the egg cell. Connecting the cervix to the outside of the body is the vagina. The vagina is made up of muscular tissue. It is into the vagina that sperm are ejaculated from the penis. Also, the vagina is the birth canal, through which the infant passes as it leaves the mother’s body during childbirth. Unlike in males, the reproductive pathway in females is not combined with the pathway for excretion. The urethra, through which urine passes from the urinary bladder, ends at an opening near the vagina. ■ ■ HORMONES AND SEXUAL REPRODUCTION During one’s life, many changes and events occur in the body to make sexual reproduction possible. Hormones coordinate these changes. The main endocrine gland in charge of producing these hormones is the ante- rior pituitary gland in the brain. The pituitary gland is controlled by the C hapter 17 / Human Reproduction 3 65 LIVING ENVIRONMENT BIOLOGY, 2e/fig. 17-5 s/s Anterior lobe of pituitary FSH Follicle Follicle stage Ovum Estrogen Ovum Ovulation stage P rogesterone Corpus luteum stage Menstruation stage Inhibits LH production LH I n h i b i t s F S H p r o d u c t i o n Blood vessels Lining of uterus 1st 14th 28th Day of menstrual cycle Figure 17-5 Ovulation occurs in the middle of the menstrual cycle. hypothalamus, a part of the brain. This connection suggests that, in some way, thoughts stimulate the release of hormones in the brain that affect the reproductive system. Two main hormones, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), are secreted from the anterior pituitary. These hormones were first named for their functions in females. Today, we know that these hormones have important functions in males, too. The effects of a hormone depend not on the hormone itself but on the hormone’s target tissue. In males, the target tissues for FSH and LH are the testes. FSH is required for sperm formation to occur. LH causes the testes to produce testosterone, the main male sex hormone. The effects of testosterone include the development of the male sex organs before and after birth. Without the presence of testosterone, the fetus would develop into a female. Around the age of 11, the level of testosterone sud- denly increases in a boy’s body. As a result, sperm production begins. This event is the beginning of puberty. During puberty, the penis and the testes begin to mature. Testosterone also affects various other tissues in the male. Testosterone causes pubic and underarm hair growth, causes changes in body propor- tions, increases muscle tissue, and deepens the voice. Testosterone may influence the development of acne and the beginning of body odors from bacteria attracted to sweat glands. Because none of these characteristics are directly related to sexual reproduction, they are called secondary sex characteristics. In males, the level of testosterone in the body remains much the same for about 40 years after puberty. After that time, the level of testosterone gradually begins to decrease. In females, FSH and LH from the anterior pituitary stimulate the release of the major sex hormones, estrogen and progesterone, from the ovaries. Estrogen and progesterone are produced in the ovaries. The onset of puberty in females occurs somewhat earlier than in males. At about age 10, the levels of estrogen and progesterone increase dramatically, causing the uterus, vagina, and ovaries to mature. Secondary sex characteristics, including the growth of pubic and underarm hair, breast development, acne, and body odors, also are influenced by estrogen and progesterone. In addition, a monthly cycle of events, known as the menstrual cycle, begins. Remember, in males, sperm production occurs all the time after puberty . In females, the menstrual cycle occurs every month after puberty. Part of this cycle includes the release of an egg cell (or egg cells) from the ovaries. (See Figure 17-6.) If you take a close-up look at the ovary, you can see that an egg devel- ops in its own follicle. The development of the egg is stimulated by 3 66 R eproduction, Growth, and Development FSH. The follicle releases estrogen which, in turn, stimulates a sudden release of LH from the anterior pituitary. The release of LH happens about 14 days into the menstrual cycle and causes ovulation—the release of the egg—to occur. Eggs can be fertilized for up to 10 to 15 hours after ovulation. In addition to ovulation, another critical function occurs during the menstrual cycle. The woman’s body must be prepared in case fertilization occurs. Everything must be ready to nurture the developing embryo. Dur- ing the first two weeks of the cycle, estrogen causes the lining of the uterus to thicken. There is also an increase in the amount of blood that flows to this area. During the second half of the cycle, after ovulation has occurred, the follicle (now without the egg cell) develops into a yellow-colored body, the corpus luteum. The corpus luteum begins to secrete progesterone, the pregnancy hormone, which prepares the uterus for an embryo. Sperm can survive in the female for up to 72 hours after sexual intercourse. Therefore, sexual intercourse a few days before or a few days after ovula- tion may result in pregnancy. If pregnancy occurs, the embryo becomes attached to the inner lining of the uterus. The growing tissue then begins to release more hormones to keep everything in the right condition. How- ever, if fertilization does not occur, the continued preparations in the uterus are unnecessary. The body realizes this near the end of the four- week period of the menstrual cycle. At this time, the level of LH decreases, the corpus luteum breaks down, and the level of progesterone decreases. Without this hormone, the uterine lining no longer remains intact. Instead, the lining breaks down. The built-up tissue along with some blood and the unfertilized egg are released from the body . This fl ow of blood, called menstruation, lasts for about four days. Then the levels of FSH again begin to increase. A new follicle releases an egg. The cycle con- tinues. (See table on page 368, which describes the menstrual cycle.) C hapter 17 / Human Reproduction 3 67 Estrogen Ovulation Length of one menstrual cycle (about 28 days) Hormone concentration in blood Progesterone LIVING ENVIRONMENT BIOLOGY, 2e/fig. 17-6 s/s Figure 17-6 The menstrual cycle and ovulation are controlled by the release of hormones. THE FOUR PHASES OF THE MENSTRUAL CYCLE Corpus Luteum Follicle Phase Ovulation Phase Menstruation In most women, the menstrual cycle continues for at least 40 years from puberty. Between the ages of 45 and 55, the levels of hormones change; the menstrual cycle becomes less regular and eventually stops. This stage, called menopause, marks the point at which a female is no longer capable of reproducing. Menopause is a normal occurrence in all women; however, the effects of menopause vary widely from one woman to another. In men, by contrast, sperm production continues throughout life, although the number of healthy sperm likely declines with age. ■ ■ SEXUAL REPRODUCTION IN OTHER MAMMALS Only human females and some other higher primates have a regular monthly menstrual cycle. Humans, however, are able to mate at any time. Females of almost all other mammal species will mate only at specific times during a year. These times are known as estrus or “heat.” It is only during estrus that egg cells are released for fertilization. Estrus occurs in wolves and deer once a year , in dogs usually twice a year , in cows and horses once a month, and in rats and mice every few days. Ovulation, mating, and fertilization occur on a specific timetable. In many animals, such as deer or sheep, estrus occurs during the autumn. This mating pattern evolved through natural selection and allows the development of the embryo to occur during the winter. The young are Check Your Understanding How is it true that “the effects of a hormone depend not on the hor- mone itself but on the hormone’s target tissue”—specifically in the case of FSH and LH in both males and females? If unfertilized, ovum passes through uterus → corpus luteum disintegrates, so it releases less estrogen and prog- esterone into blood → uterine lining is shed and dis- charged. (3–7 days) Ruptured follicle (corpus luteum) continues to release estrogen and starts to release proges- terone → uterine lining fully ready to receive fertilized ovum. (2–3 days) Pituitary gland secretes burst of FSH and LH → follicle ruptures → mature ovum is released from ovary into the oviduct (where it can be fertilized). (3–4 days) Pituitary gland secretes FSH and LH into blood → follicle grows and matures within ovary → estrogen levels rise and uterine lining thickens. (10–14 days) 3 68 R eproduction, Growth, and Development born in early spring, when food is more plentiful. The newborn then have plenty of time to grow before the next harsh winter sets in. Human females are among the few animals that allow mating at times when no eggs have been released. In other words, sexual intercourse in humans can occur even when fertilization is not possible. It has been sug- gested that this sexual pattern evolved in the human species in order to develop strong bonds between males and females. Such ties between the sexes are the basis of the human family unit. Is there a connection among biology, behavior, and the strength of family ties? ■ ■ BIRTH CONTROL Over the years, a wide variety of methods have been developed to reduce the chances of becoming pregnant. These methods are called birth con- trol or contraception. Birth control helps a couple plan if and when to have a child. However, birth control gained even more importance in the late twentieth century for two additional reasons. The world’s human population is growing faster than at any other time in history. For example, in 1800, Earth supported a population of almost 1 billion people. By the year 2000, the human pop- ulation reached 6 billion. If the current rate of growth continues, that num- ber will double within the next 40 years. Today, the highest rates of population growth are in the poorest, least-developed countries. Popula- tion growth is a matter of great concern throughout the world and is of crit- ical concern in certain countries. As a result, family planning, including the use of various birth control methods, is being arranged not only by individual couples but also by government agencies. In countries such as India, Indonesia, Mexico, and China, the popu- lation growth rate is very high. The governments in these countries are concerned about their ability to feed and care for an ever-increasing pop- ulation. China has given the most attention to family planning. In 1970, China began an official “one child–one family” policy. In each commu- nity in China, government agencies work with couples to reach this goal. The rate of population growth has declined in China, but other social problems are developing. For example, how will a country take care of its older population if there are fewer younger people working to support them? Other countries are watching China closely to see if it succeeds in meeting its population growth goals. Indeed, because China has almost one-quarter of Earth’ s total human population, the success of family plan- ning in China is an important matter for the entire world. (See table on page 370, which lists the most populous countries.) C hapter 17 / Human Reproduction 3 69 [...]... more than 20 percent of the time The name “test-tube babies” has been used to describe children born through this method Chapter 17 / Human Reproduction 373 It was reported that Mrs Chukwu had taken two hormone-based fertility drugs—follicle-stimulating hormone and human chorionic gonadotropic hormone—to produce multiple eggs The occurrence of this woman giving birth to eight babies raises some serious... canal Both diseases can cause blindness in infants In the United States, Chapter 17 / Human Reproduction 375 every baby born in a hospital is given antibiotic eyedrops immediately after birth to prevent possible infections Before modern antibiotics were developed, syphilis was one of the most terrible diseases in humans The infectious bacterium that causes syphilis destroys nerve tissue, including... infections Therefore, an understanding of the stages of development that occur during pregnancy is important Pregnancy and human embryonic development are the topics of this investigation MATERIALS NOVA video The Miracle of Life, handouts “Cutouts of Stages of Human Development” and “Stages of Human Development” chart (from the Teacher’s Manual), scissors, blank paper, tape PROCEDURE 1 Your class will view... class: smoking crack; hereditary disorders; the use of nicotine and caffeine by the mother while pregnant; fetal alcohol syndrome; diseases such as measles, rubella, or AIDS Chapter 17 / Human Reproduction 377 378 Reproduction, Growth, and Development II CHAPTER 17 REVIEW Answer these questions on a separate sheet of paper VOCABULARY The following list contains all of the boldfaced terms in this chapter... disease during sex The possibility of contracting serious, life-threatening diseases has increased the need for individuals to practice “safe sex.” A diaphragm is a latex barrier placed over Chapter 17 / Human Reproduction 371 the cervix, the entrance to the uterus A diaphragm prevents sperm from reaching the area of the female’s reproductive tract where fertilization of an egg is possible The use of condoms... “Cutouts of Stages of Human Development.” Cut out each diagram and tape it to a sheet of paper As a group, prepare a description of each diagram Write your description beneath the diagram Now study the diagrams and place them in order from the earliest to the latest Number them from 1 to 6 3 Join another group and share your findings 4 Your teacher will now hand out the “Stages of Human Development” chart... sequence you developed compare? 5 Read and study the following questions Discuss them with your group before you present your answers to the class 376 Reproduction, Growth, and Development INTERPRETIVE QUESTIONS 1 How is the knowledge of the prenatal stages of human development used by physicians to monitor a pregnancy? 2 How does a sonogram help monitor a woman’s pregnancy? 3 What are some problems that... yourself, and the people you care for, from contracting an STD Responsible behaviors are important and may save lives LABORATORY INVESTIGATION 17 What Are Some Stages in the Development of Human Embryos? INTRODUCTION In humans, the embryonic development of organs occurs in the early stages of pregnancy During its development, an embryo may encounter risks from faults present in its genes It may also be... occurs in the a oviduct b uterus c vagina d follicle 9 Meiosis in females produces a four spermatids b four eggs c one egg and three small structures d two eggs and two small structures Chapter 17 / Human Reproduction 379 10 The corpus luteum produces the hormone a progesterone b estrogen c testosterone d luteinizing hormone 11 In most mammals, egg cells are released only during a menopause b estrus... LIVING ENVIRONMENT BIOLOGY, 2e/fig 17-Q16 s/s 18 Briefly describe the events in gamete production in humans 19 Briefly describe five STDs What are some ways to avoid contracting STDs? 20 Why might a fertility specialist recommend that a would-be father switch from wearing briefs to wearing boxer shorts? 380 Reproduction, Growth, and Development PART C—READING COMPREHENSION Base your answers to questions . and may save lives. C hapter 17 / Human Reproduction 3 75 376 Reproduction, Growth, and Development INTRODUCTION In humans, the embryonic development of. Development of Human Embryos? Chapter 17 / Human Reproduction 377 INTERPRETIVE QUESTIONS 1. How is the knowledge of the prenatal stages of human development used