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The Human
Reproductive System
This scanning electron microscope image enlarges human sperm cells on a human egg cell thousands of times. You will read in this chapter that after one sperm cell enters an egg cell, other sperm cells cannot.
-ViViÊÊ -ViViÊÊ +PVSOBM+PVSOBM Imagine that you are a sperm cell entering an egg cell.
Describe your journey to the egg cell’s nucleus.
Sorry, but only one sperm per egg!Sorry, but only one sperm per egg!
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In humans, fertilization and embryo development take place inside a female’s body.
LESSON 1 Reproductive Systems
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LESSON 2
Development Before Birth
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5.d, 7.a
5.e, 7.a, 7.b, 7.c, 7.d
Visit to: ▶▶ (^) view ▶▶ (^) explore Virtual Labs ▶▶ (^) access content-related Web links ▶▶ (^) take the Standards Check
Start-Up Activities
Is it a boy or a girl?
Each human’s genetic makeup comes from his or her parents. Who determines a child’s gender? Procedure
1. Obtain a bag containing two flat, wooden craft sticks from your teacher. Open your bag and remove the sticks. You will have sticks labeled X and X (female) or X and Y (male). These are chromosomes. 2. Find someone with the opposite pair. This is your partner. 3. Put your sticks back in your own bag. 4. Each of you should draw a stick from your own bag. Record the letters and the gender of the offspring they represent. 5. Repeat steps 3 and 4 nine more times. 6. Calculate the number of male offspring and the number of female offspring.
Think About This
- Infer which parent determines gender. - Compare the numbers of male offspring and female offspring for the entire class.
STEP 1 Fold a sheet of paper in half lengthwise. Make the back edge about 2 cm longer than the front edge.
STEP 2 Fold in half again.
STEP 3 Unfold the paper once. Cut along the fold of the top flap to make two flaps. Label the flaps as shown.
Reproductive Organs Make the following Foldable to organize information about the organs of the male and female reproductive systems and functions.
Monitoring Your Comprehension As you read Lesson 1, list each reproductive organ and its function under the appropriate tab. ca7.msscience.com
2.b, 7.c
Target Your Reading
Use this to focus on the main ideas as you read the chapter.
(^1) Before you read the chapter, respond to the statements
below on your worksheet or on a numbered sheet of paper.
- Write an A if you agree with the statement.
- Write a D if you disagree with the statement. (^2) After you read the chapter, look back to this page to see if
you’ve changed your mind about any of the statements.
- If any of your answers changed, explain why.
- Change any false statements into true statements.
- Use your revised statements as a study guide.
1 Every child is genetically related to one male and one female parent.
2 A female’s reproductive system does not begin producing eggs until she reaches sexual maturity.
3 Both males and females begin to lose the ability to reproduce after age 54.
4 Millions of sperm fertilize one egg.
5 A developing fetus’ heart does not begin to beat until after 24 weeks of development.
6 The umbilical cord and placenta supply the fetus with oxygen and nutrients from the mother’s blood.
7 A mother’s use of drugs or alcohol can lead to health problems for her child as the child grows and develops.
Before You Read A or D
Statement After You Read A or D
Read one or two paragraphs first and take notes after you read. You are likely to take down too much infor- mation if you take notes as you read.
Print a worksheet of this page at ca7.msscience.com.
LESSON 1
504 Chapter 13 • The Human Reproductive System
Reproductive Systems
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Real-World Reading Connection It’s the first day of school.
You notice that some students changed a lot over the summer but others hardly changed at all. Although you and your class- mates are about the same ages, you are at different stages of physical development. Your reproductive systems are at different stages of development, too.
Male Reproductive System Adult males have many body characteristics that differ from adult females. Men usually have more body hair, deeper voices, and larger, more muscular bodies than women do. These fea- tures develop as boys get older and their reproductive systems grow toward maturity. As you read in Chapter 3, testes (singular, testis) are male animal organs that produce sperm. Human males have two testes, as shown in Figure 1. A human male’s testes do not begin to produce sperm until his reproductive system matures. Which human male reproductive organ produces sperm?
Reading Guide
What You’ll Learn ▼
(^) List the organs of the male and female reproductive systems. ▼
(^) Compare and contrast the development of sperm and eggs. ▼ Sequence the path traveled by sperm from formation to fertilization. ▼
(^) Describe ovulation and the menstrual cycle.
Why It’s Important Knowing how human reproductive systems work will enable you to make well- informed decisions about your reproductive health.
Vocabulary
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Figure 1 The repro- ductive system of a human male produces and then delivers sperm to the reproduc- tive system of a female.
Science Content Standards 5.d Students know how the reproductive organs of the human female and male generate eggs and sperm and how sexual activity may lead to fertilization and pregnancy. 7.a Select and use appropriate tools and technology (including calculators, computers, balances, spring scales, microscopes, and binoculars) to perform tests, collect data, and display data.
506 Chapter 13 • The Human Reproductive System
Sperm Production Males start producing sperm during puberty (PYEW bur tee), which usually begins when they are 10–16 years of age. Sperm production occurs by meiosis in cells that line the seminiferous tubules, as shown in Figure 3. It takes 65–75 days to produce a mature sperm cell, also shown in Figure 3. A male can continue to make healthy sperm for the rest of his life.
Sperm and Semen Each sperm consists of a head, a midpiece, and a tail. The head contains a nucleus, and the midpiece contains mitochondria that release energy. The tail, or flagellum, whips back and forth and propels the sperm forward. A male ejaculates about 2–5 mL of semen (SEE mun) on aver- age. Semen contains a liquid made by glands in a male’s reproduc- tive system and about 100 to 650 million sperm. The seminal vesicles are a pair of glands that makes most of the liquid in semen. They produce a thick, yellowish liquid that contains mucus, ascorbic acid (vitamin C), hormonelike substances that control cell activity, and an enzyme that helps thicken the semen. This liquid also contains sugar that is an energy supply for sperm. The prostate gland also makes some of the liquid in semen. It pro- duces a thin, milky liquid containing enzymes and nutrients.
Which two glands make the liquid in semen?
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Figure 3 Primary spermatocytes in the seminiferous tubules of the testes divide by meiosis to produce four haploid cells that will mature into sperm cells. This photo- graph shows sperm cells being produced in a seminiferous tubule. A mature sperm cell is made of three parts—head, midpiece, and a tail, or flagellum.
Midpiece
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Figure 4 In the female reproductive system, eggs are produced by meiosis in ovaries. An egg travels from an ovary, into a fallopian tube, then toward the uterus. Compare and contrast the way a sperm moves and the way an egg moves.
Female Reproductive System
A female’s reproductive system produces eggs. This system is also the place where a fertilized egg can grow and develop into a baby. Recall that a male begins producing sperm when he reaches puberty. A female begins producing eggs before she is born.
Female Reproductive Organs Unlike a male, all the reproductive organs of a female are located inside her abdomen, as shown in Figure 4. Two folds of skin, called labia (LAY bee uh), protect the opening to a female’s reproductive system. Beyond the opening, inside the female’s body is a thin-walled chamber called the vagina (vuh JI nuh). This is where semen is deposited.
Uterus Above the vagina, further inside the body, is the uterus (YEW tuh rus). It is a thick, muscular organ inside which a fertil- ized egg can develop. A uterus is normally about the size and shape of a pear, but it enlarges during pregnancy. A tissue called the endometrium (en doh MEE tree um) lines the uterus. The neck, or opening, of the uterus into the vagina is called the cervix (SUR vihks). During childbirth, the cervix gets wider, or dilates. This enables the baby to move into the vagina and out of the mother’s body.
Ovaries and Fallopian Tubes A pair of organs called ovaries (sin- gular, ovary) produces eggs. An egg released from an ovary moves into a fallopian tube (fuh LOH pee un • TOOB) or oviduct that connects the ovary to the uterus, also shown in Figure 4. Fertiliza- tion usually occurs while the egg is in a fallopian tube. An egg cell has no flagellum, so it cannot move on its own like a sperm cell can. Recall from Chapter 1 that the surface of a cell can have hairlike structures called cilia that move back and forth. The cells on the inside surface of a fallopian tube have cilia. These cilia move an egg toward the uterus.
WORD ORIGIN uterus from Latin uterus; means womb, belly
Lesson 1 • Reproductive Systems 509
Menstrual Flow During the menstrual cycle, the endometrium thickens and the number of blood vessels in it increases to support a fertilized egg. However, if a released egg is not fertilized, the endometrium breaks down and sloughs off. This tissue, some blood, and the unfertilized egg leave the vagina as menstrual flow. Menstrual flow usually lasts four to seven days. After menstrual flow stops, the endometrium thickens and its blood vessels regrow.
Ovulation About two weeks after the first day of menstrual flow, ovulation occurs. Usually, only one egg is released from one of a female’s ovaries during a menstrual cycle. It takes about 24 to 48 hours for an egg to move down the fallopian tube and into the uterus. If the egg is fertilized, a zygote forms, cell divisions begin, and an embryo begins to develop. When the embryo enters the uterus, it attaches to, or implants in, the endome- trium. If this happens, menstrual bleeding does not occur. The absence of menstrual bleeding is usually one of the first signs of pregnancy.
Hormones Chemical messengers called hor- mones regulate the timing of the menstrual cycle and ovulation. Some of the glands and organs that produce hormones, including hormones that regulate menstrual cycles, are shown in Figure 7 on the following pages.
Which hormones control ovulation? The hormones LH and FSH help to control the menstrual cycle and ovulation.
Source: Carlson, Bruce M. Foundations of Embryology. New York: McGraw-Hall. 1996.
Data Analysis
1. Graph the data. Use different colors to
graph LH and FSH.
2. Highlight The day of highest hormone
concentration is called the LH or FSH surge. Highlight each surge on your graph.
3. Compare your graph to Figure 6.
Relate the amounts of LH and FSH to the day of ovulation.
4. Infer what might happen if a woman
produced too little LH and FSH.
Day of Cycle
Units of LH in Blood
Units of FSH in Blood 1 10 9 4 15 11 7 13 11 10 12 14 13 13 22 16 9 17 19 8 13 22 6 10 25 5 9 28 9 6
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5.d, 7.a
MA7: AF 1.5, MR 2.
510 Chapter 13 • The Human Reproductive System
Visualizing Hormones
Figure 7 Your hormones regulate and coordinate many body func- tions, from reproduction to growth and development. They circulate in the blood and affect only specific cells. Glands and organs, including the nine shown here, produce hormones.
Hormones related to body
▲
activities, from reproduction to growth, are produced by the pituitary (pih TEW uh ter ee) gland. It is about the size of a pea and is attached to the hypothalamus (hi poh THAL uh mus) of the brain.
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▲
▲ The thymus (THI mus)— a gland in the upper chest behind the sternum—pro- duces hormones that stimu- late the production of certain infection-fighting cells.
Testosterone (tes TAHS tuh rohn)
▲
is a hormone that controls the development and maintenance of male sexual traits and plays an important role in the production of sperm. Testes produce testosterone.
Contributed by National Geographic
512 Chapter 13 • The Human Reproductive System
Menopause Sometime between the ages of 46 and 54, most women stop ovulating and no longer have menstrual cycles. This stage of life is called menopause (MEN uh pawz). Many women begin to lose the ability to reproduce before menopause, as early as their mid-30s. As a woman gets older, the eggs she produces decrease in quality, and it becomes more difficult for her to have a successful preg- nancy. An important difference between males and females is that a male’s reproductive system continues to function throughout his lifetime, but a female’s reproductive system does not.
Fertilization Have you ever heard people say they were in the right place at the right time? The same might be said about sperm and fertiliza- tion. In humans, for a sperm to fuse with an egg cell, the sperm must swim to the right place—a fallopian tube—and at the right time—near the time of ovulation. Sperm deposited in or near a female’s vagina can swim into her reproductive tract, as shown in Figure 8. Most of these sperm will not make it to an egg. Some sperm swim into a fallopian tube that does not contain an egg. Some sperm swim in the opposite direc- tion, away from the fallopian tubes. Other sperm might have genetic or physical defects that prevent them from fertilizing an egg even if they reach it. These facts help explain why millions of sperm are ejaculated to fertilize just one egg. Normally, only one sperm fertilizes an egg, as shown in Figure 8. Once a sperm attaches to an egg, chemical reactions occur that block other sperm from entering the same egg. Sperm can live inside a female’s reproductive tract for up to three days. Therefore, a female can become pregnant even if sexual inter- course occurs a couple days before she ovulates.
SCIENCE USE V. COMMON USE
fertilization Science Use the joining of a sperm cell and an egg cell, forming a zygote. Fertilization usually occurs in a woman’s fal- lopian tubes. Common Use the application of a substance to soil to increase the soil’s nutrients. Fertilization of a garden’s soil with aged animal manure can improve plant growth.
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To see an animation of fertilization, visit ca7.msscience.com.
LESSON 1 Review
Lesson 1 • Reproductive Systems 513
Reproductive Systems Summary In this lesson, you learned that the male reproductive system includes several organs, such as the testes where sperm are pro- duced. Other male organs contribute fluid to the semen in which sperm move. The female reproductive system also consists of several organs, including ovaries in which eggs develop. A menstrual cycle lasts about 28 days. It includes the building up of the endometrium, ovulation, and, if fertilization does not take place, menstrual flow. If an egg is fertilized and a zygote implants in the endometrium, the menstrual cycle stops and pregnancy begins.
Science nline For more practice, visit Standards Check at.
Summarize
Create your own lesson summary as you organize an outline.
1. Scan the lesson. Find and list the first red main heading. 2. Review the text after the heading and list 2– details about the heading. 3. Find and list each blue subheading that follows the red main heading. 4. List 2–3 details, key terms, and definitions under each blue subheading. 5. Review additional red main headings and their supporting blue subhead- ings. List 2–3 details about each.
ca7.msscience.com
Standards Check
Using Vocabulary
1. Distinguish between vagina and uterus. 5.d 2. Write the definition of men- strual cycle in your own words. 5.d
Understanding Main Ideas
3. Which is not part of the female reproductive system? 5.d A. epididymus B. uterus C. ovary D. endometrium 4. Compare the roles of a male and a female in reproduction. **5.d
- Describe** what happens to an egg from ovulation to implan- tation in the wall of the uterus. **5.d
- Explain** why a male produces millions of sperm when usually only one egg is available to be fertilized? 5.d 7. What structures move an egg through a fallopian tube? (^) 5.d A. cilia B. flagella C. muscles D. seminal vesicles 8. Compare sperm production and egg production. (^) 5.d
Applying Science
9. Create a table listing the male and female reproduc- tive organs and their func- tions. (^) **5.d
- Sequence Information** Draw a graphic organizer like the one below to create a time line about the menstrual cycle. (^) 5.d
ELA7: W 2.
LESSON 2
Lesson 2 • Development Before Birth 515
Development Before Birth
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the good health of its mother.
Real-World Reading Connection Did you know that some-
one might have taken your picture before you were born? A sonogram uses sound waves to produce a video image of a fetus. It can help a medical provider determine if the fetus is develop- ing normally and whether it is a girl or a boy.
Fetal Development Recall from Chapter 3 that all sexually produced organisms begin life as a zygote that forms when a sperm fertilizes an egg. Cell divisions of a human zygote begin about 24 hours after fer- tilization. Cells continue to divide and, after about seven days, a hollow ball of more than 100 cells has formed, as shown in Figure 9. This ball of cells is the embryo that implants into the endometrium. After two weeks of growth, the cells begin to arrange themselves into three layers, also shown in Figure 9. Different body structures eventually form from each layer. Over a period of about nine months, a human embryo develops into a baby.
When do cell divisions of a human zygote begin?
Reading Guide
What You’ll Learn ▼
(^) Explain the importance of the placenta and the umbilical cord to a fetus. ▼
(^) List the major developmental stages of a fetus. ▼ Infer how a mother’s lifestyle can affect her fetus.
Why It’s Important Knowing how a fetus devel- ops helps you understand how important it is for a pregnant woman to take good care of her health.
Vocabulary
pregnancy placenta trimester umbilical fetus cord prenatal care
Review Vocabulary
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Figure 9 From the outer layer, nerves and skin develop; from the middle layer, heart, kid- neys, bones, and muscles; and from the inner layer, lungs, liver, and digestive system.
After 7 days
After 14 days
Science Content Standards 5.e Students know the function of the umbilicus and placenta during pregnancy. Also covers: 7.a, 7.b, 7.c, 7.d
Table 1 Stages of Pregnancy Stage Example Development First trimester By the end of the first trimester of pregnancy, an embryo has grown from a microscopic ball of cells to about 7.5 cm long, weighing about 23 g. Its heart is beating, and it can move its arms and legs.
Second trimester At the end of the second trimester, a fetus is about 25–30 cm long and frequently makes kicking movements. The fetus shown here is about 24 weeks old. A baby born at this stage of development cannot survive without intensive medical care.
Third trimester During the third trimester, a fetus usually triples in size. This fetus is 36 weeks old. A baby born at this stage of development probably would survive but might require medical care.
516 Chapter 13 • The Human Reproductive System
Growth and Development of Body Systems The development of a baby within a female’s uterus is called pregnancy. In humans, pregnancy usually lasts for 38 weeks after fertilization, or about 40 weeks after the beginning of the last menstrual cycle. When describing the many changes that take place during pregnancy, it is helpful to divide the nine months of pregnancy into three parts, called trimesters. The first trimester is the first twelve weeks of pregnancy. By the end of the first trimester, an embryo has all the structures that will become the major organ systems of an adult. During the sec- ond and third trimesters, an embryo is called a fetus (FEE tus). A fetus changes as it continues to develop, as shown in Table 1. During the second trimester, the pregnant female can feel the fetus’s movements. During the third trimester, the fetus grows rapidly, nearly tripling in size in preparation for birth. Table 1 When might a fetus survive with intensive medical care?
Interactive Table To explore more about the stages of pregnancy, visit Tables at ca7.msscience.com.
Figure 11 A healthy diet for a pregnant woman includes foods such as those shown here. Infer why it is important for a woman who might become pregnant to consume these types of food.
518 Chapter 13 • The Human Reproductive System
Fetal Health Everything that happens in a woman’s body has an effect on her developing fetus. Anything she does that could harm her health before or during her pregnancy could also harm her fetus. It is important for any woman who might become pregnant to take good care of her health. If she is in good health before she becomes pregnant, she has a better chance of having a healthy pregnancy and a healthy baby.
Prenatal Care Health care designed to protect the health of a pregnant woman and prevent problems in her developing fetus is called prenatal care. Research has shown that a pregnant woman who receives prenatal care from a certified health care provider has a better chance of delivering a healthy baby. A pregnant woman’s prenatal care includes advice and information about nutrition, about viral infections, and about substances that could harm her fetus.
Why is prenatal care important?
Nutrition All the energy and nutrients a fetus needs for normal develop- ment must come from its mother. Vitamins, minerals, proteins, fats, and carbohydrates pass from mother to fetus through the pla- centa. To support her growing fetus, a pregnant woman needs to eat a healthy diet that includes dairy products, proteins, fruits, vegetables, and whole grains, such as those shown in Figure 11.
Calories A pregnant woman is usually advised to add about 300 extra calories a day to her diet. The added calories supply the extra energy needed for the development of the fetus. However, it’s best to choose healthy foods and avoid high-calorie foods that contain large amounts of sugar or fat, but few other nutrients.
ACADEMIC VOCABULARY supplement (SUH pluh muhnt) (noun) something that adds to something else Vitamin tablets and other types of food supplements contain the same vitamins and minerals found in grains, fruits, vegeta- bles, and other foods.
Figure 12 A pregnant woman is advised to avoid substances that could harm her fetus.
Lesson 2 • Development Before Birth 519
Folic Acid A fetus’s spinal cord forms during the first weeks of pregnancy. Without a certain amount of folic acid, a form of vitamin B, spinal cord formation is abnormal. Doctors often rec- ommend that pregnant women take vitamin
supplements containing folic acid, in addition
to eating a balanced diet.
Caffeine A pregnant woman should avoid caffeine, or consume it only in small amounts. Caffeine can increase a woman’s blood pressure and heart rate, which can be stressful to her fetus.
Environmental Factors A pregnant woman can encounter substances in her environment, such as those in Figure 12, that present health risks for her fetus. She might inhale harmful substances, consume them with food or water, or absorb them through her skin. These substances can then pass through the pla- centa and into the fetus. For example, a pregnant woman is usually advised to avoid using pesti- cides or insect repellents. Chemicals in insecti- cides and other pesticides can cause premature birth, birth defects, or miscarriage—the loss of an embryo during the first trimester. Lead is a chemical element sometimes found in air pollution, old paint, and electronics. It can be harmful to anyone, but is especially harmful to a fetus, an infant, or a young child. Pregnant women who have been exposed to high levels of lead have a higher risk of miscarriage, premature delivery, and low birth-weight babies.
Can folic acid prevent birth defects? Ten of every 10,000 babies in the United States are born with a neural tube birth defect, such as spina bifida. It occurs when the bones of the spine do not form prop- erly during the first month of pregnancy. The data table below shows how folic acid affects a woman’s risk of having a baby with a neural tube defect.
from CDC
Data Analysis
1. Calculate the percentage of babies with
neural tube defects born to women who took folic acid.
2. Calculate the percentage of babies
born with neural tube defects to women who did not take folic acid.
3. Compare the percentages of babies
with neural tube defectsß for the two groups.
4. Analyze Does folic acid prevent neural
tube defects? Explain why or why not.
5. Conclude What conclusion can be
made from the data?
Effects of Folic Acid Supplements on Neural Tube Defects Folic Acid Taken Before or During Pregnancy
Babies Born with Neural Tube Defects
Babies Born Without Neural Tube Defects Yes 6 497 No 21 581
MA7: NS 1.0, NS 1.
5.e, 7.a
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