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25.3: From Fertilization to Old Age

Difficulty Level: At Grade Created by: CK-12
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Lesson Objectives

  • Outline the events that occur between fertilization and the embryonic stage.
  • Explain how the embryo forms specialized cells and organs.
  • Identify major events in the growth and development of the fetus.
  • Explain the role of the placenta during fetal development.
  • Describe pregnancy and childbirth.
  • List milestones in growth and development from birth to adulthood.
  • Describe the stages of adulthood, and explain why aging occurs.


period of transition between the beginning of puberty and adulthood during which significant physical, mental, emotional, and social changes occur
amniotic sac
enclosed membrane containing fluid that surrounds and protects a fetus
fluid-filled ball of cells that develops a few days after fertilization in humans
process by which unspecialized cells become specialized into one of many different types of cells, such as neurons or epithelial cells
stage of growth and development that occurs from implantation through the eighth week after fertilization in humans
developing human organism between weeks 8 and 38 after fertilization
process in which a blastocyst embeds in the endometrium lining the uterus
first year of life after birth in humans
carrying of one or more offspring from fertilization until birth


A day or two after an ovary releases an egg, the egg may unite with a sperm. Sperm are deposited in the vagina during sexual intercourse. They propel themselves through the uterus and enter a fallopian tube. This is where fertilization usually takes place.

Cleavage and Implantation

When a sperm penetrates the egg, it triggers the egg to complete meiosis. The sperm also undergoes changes. Its tail falls off, and its nucleus fuses with the nucleus of the egg. The resulting cell, called a zygote, contains all the chromosomes needed for a new human organism. Half the chromosomes come from the egg and half from the sperm.

Morula and Blastocyst Stages

The zygote spends the next few days traveling down the fallopian tube toward the uterus, where it will take up residence. As it travels, it divides by mitosis several times to form a ball of cells called a morula. The cell divisions are called cleavage. They increase the number of cells but not the overall size of the new organism. As more cell divisions occur, a fluid-filled cavity forms inside the ball of cells. At this stage, the ball of cells is called a blastocyst. The cells of the blastocyst form an inner cell mass and an outer cell layer, as shown in Figure below. The inner cell mass is called the embryoblast. These cells will soon develop into an embryo. The outer cell layer is called the trophoblast. These cells will develop into other structures needed to support and nourish the embryo.

Blastocyst. The blastocyst consists of an outer layer of cells called the trophoblast and an inner cell mass called the embryoblast.


The blastocyst continues down the fallopian tube and reaches the uterus about 4 or 5 days after fertilization. When the outer cells of the blastocyst contact cells of the endometrium lining the uterus, the blastocyst embeds in the endometrium. The process of embedding is called implantation. It generally occurs about a week after fertilization. You can watch an animation of a blastocyst implanting at this link: http://embryology.med.unsw.edu.au/notes/week2_3.htm

Growth and Development of the Embryo

After implantation occurs, the blastocyst is called an embryo. The embryonic stage lasts through the eighth week following fertilization. During this time, the embryo grows in size and becomes more complex. It develops specialized cells and tissues and starts to form most organs. For an interactive animation of embryonic development, go to this link: http://health.howstuffworks.com/adam-200129.htm.

Formation of Cell Layers

During the second week after fertilization, cells in the embryo migrate to form three distinct cell layers, called the ectoderm, mesoderm, and endoderm. Each layer will soon develop into different types of cells and tissues, as shown in Figure below.

Cell Layers of the Embryo. The migration of cells into three layers occurs in the 2-week-old embryo. What organs eventually develop from the ectoderm cell layer? Which cell layer develops into muscle tissues?

Differentiation of Cells

A zygote is a single cell. How does a single cell develop into many different types of cells? During the third week after fertilization, the embryo begins to undergo cellular differentiation. Differentiation is the process by which unspecialized cells become specialized. As illustrated in Figure below, differentiation occurs as certain genes are expressed while other genes are switched off. Because of this process, cells develop unique structures and abilities that suit them for their specialized functions. You can explore cell differentiation by watching the video at this link: http://videos.howstuffworks.com/hsw/10313-the-cell-cell-differentiation-video.htm.

Cellular differentiation occurs in the 3-week-old embryo.

Organ Formation

After cells differentiate, all the major organs begin to form during the remaining weeks of embryonic development. A few of the developments that occur in the embryo during weeks 4 through 8 are listed in Figure below. As the embryo develops, it also grows in size. By the eighth week of development, the embryo is about 30 millimeters (just over 1 inch) in length. It may also have begun to move.

Embryonic Development (Weeks 4–8). Most organs develop in the embryo during weeks 4 through 8. If the embryo is exposed to toxins during this period, the effects are likely to be very damaging. Can you explain why? (Note: the drawings of the embryos are not to scale.)

Growth and Development of the Fetus

From the end of the eighth week until birth, the developing organism is referred to as a fetus. Birth typically occurs at about 38 weeks after fertilization, so the fetal period generally lasts about 30 weeks. During this time, as outlined in Figure below, the organs complete their development. The fetus also grows rapidly in length and weight. For detailed videos of growth and development of the fetus birth, go to these links: http://www.youtube.com/watch?v=aR-Qa_LD2m4&#38;feature=related and http://www.youtube.com/watch?v=RS1ti23SUSw&feature=related.

Fetus at 18 weeks after fertilization.

Fetal Development (Weeks 9–38). Organ development is completed and body size increases dramatically during weeks 9–38.

By the 38th week, the fetus is fully developed and ready to be born (see Figure below). A 38-week fetus normally ranges from 36 to 51 centimeters (14–20 inches) in length and weighs between 2.7 and 4.6 kilograms (about 6–10 pounds).

A 38-week-old fetus has completed development and will soon be born.

Placenta and Related Structures

The fetus could not grow and develop without oxygen and nutrients from the mother. Wastes from the fetus must also be removed in order for it to survive. The exchange of these substances between the mother and fetus occurs through the placenta.


The placenta is a temporary organ that begins to form from the trophoblast layer of cells shortly after implantation. (For an animation showing how the placenta forms, go to link below.) The placenta continues to develop and grow to meet the needs of the growing fetus. A fully developed placenta, like the one in Figure above, is made up of a large mass of blood vessels from both the mother and fetus. The maternal and fetal vessels are close together but separated by tiny spaces. This allows the mother’s and fetus’s blood to exchange substances across their capillary walls without the blood actually mixing.

The fetus is connected to the placenta through the umbilical cord, a tube that contains two arteries and a vein. Blood from the fetus enters the placenta through the umbilical arteries, exchanges gases and other substances with the mother’s blood, and travels back to the fetus through the umbilical vein.

Amniotic Sac and Fluid

Attached to the placenta is the amniotic sac, an enclosed membrane that surrounds and protects the fetus (see Figure above). It contains amniotic fluid, which consists of water and dissolved substances. The fluid allows the fetus to move freely until it grows to fill most of the available space. The fluid also cushions the fetus and helps protect it from injury.

Pregnancy and Childbirth

Pregnancy is the carrying of one or more offspring from fertilization until birth. It is the development of an embryo and fetus from the expectant mother’s point of view.

The Mother’s Role

The pregnant mother plays a critical role in the development of the embryo and fetus. She must avoid toxic substances such as alcohol, which can damage the developing offspring. She must also provide all the nutrients and other substances needed for normal growth and development. Most nutrients are needed in greater amounts by a pregnant woman, but some are especially important, including folic acid (vitamin B9), calcium, iron, and omega-3 fatty acids.


Near the time of birth, the amniotic sac breaks in a gush of fluid. Labor usually begins within a day of this event. Labor involves contractions of the muscular walls of the uterus, which cause the cervix to dilate. With the mother’s help, the contractions eventually push the fetus out of the uterus and through the vagina. Within seconds of birth, the umbilical cord is cut. Without this connection to the placenta, the baby cannot exchange gases, so carbon dioxide quickly builds up in the baby’s blood. This stimulates the brain to trigger breathing, and the newborn takes its first breath.

From Birth to Adulthood

For the first year after birth, a baby is called an infant. Childhood begins at age two and continues until adolescence. Adolescence is the last stage of life before adulthood.


Infancy is the first year of life after birth. Infants are born with a surprising range of abilities. For example, they have well-developed senses of touch, hearing, and smell. They can also communicate their needs by crying. During their first year, they develop many other abilities, including those described below. For a video of major milestones in the first year of life, go to this link: http://www.youtube.com/watch?v=5_Ao_3hTS6I&feature=channel.

By 6 weeks after birth, infants typically start smiling (see Figure below) and making vocal sounds. By 6 months, infants are babbling. They have also learned to sit and are starting to crawl. The deciduous (baby) teeth have started to come in. By 12 months, infants may be saying their first words. They usually can stand with help and may even have started to walk.

A baby’s first smile is an early milestone in infant development.

Infancy is the period of most rapid growth after birth. Growth is even faster during infancy that it is during puberty. By the end of the first year, the average baby is twice as long as it was at birth and three times as heavy.


A toddler is a child aged 1 to 3 years. Children of this age are learning to walk, or “toddle.” Growth is still relatively rapid during the toddler years but it has begun to slow down. During the next three years, children achieve many more milestones.

  • By age 4, most children can run, climb stairs, and scribble with a crayon. They know many words and use simple sentences. The majority are also toilet trained.
  • By age five, children are able to carry on conversations, recognize letters and words, and use a pencil to trace letters. They can usually tie their own shoelaces and may be learning to ride a bicycle, swing a bat, kick a ball and play other games (Figure below).
  • By age 6, most children begin losing their deciduous teeth, and their permanent teeth start coming in. They speak fluently and are learning to read and write. They spend more time with peers and develop friendships.

Five year olds can usually play various games.

Older children continue to grow slowly until they start the adolescent growth spurt during puberty. They also continue to develop mentally, emotionally, and socially. Think about all the ways you have changed since you were as young as the child in Figure above. What milestones of development did you achieve during these childhood years?


Adolescence is the period of transition between the beginning of puberty and adulthood. You learned about the physical changes of puberty earlier in this chapter. Adolescence is also a time of significant mental, emotional, and social changes. For example:

  • Adolescents generally develop the ability to think abstractly.
  • Adolescents may have mood swings because of surging hormones.
  • Adolescents usually try to be more independent from their parents.
  • Adolescents typically spend much of their time with peers.
  • Adolescents may start to develop intimate relationships.

Adulthood and Aging

Adulthood does not have a definite starting point. A person may be physically mature by age 16 or 17 but not defined as an adult by law until older ages. In the U.S., you can’t join the armed forces or vote until age 18. You can’t buy or use alcohol or take on many legal and financial responsibilities until age 21.

Early Adulthood

Early adulthood coincides with the 20s and early 30s. During early adulthood, people generally form intimate relationships, both in friendship and love. Many people become engaged or marry during this time. Often they are completing their education and becoming established in a career. Health problems in young adults tend to be minor. The most common causes of death are homicides, car crashes, and suicides.

Middle Adulthood

Middle adulthood lasts from the mid-30s to the mid-60s. During this stage of life, many people raise a family and strive to attain career goals. They start showing physical signs of aging, such as wrinkles and gray hair. Typically, vision, strength, and reaction time start declining. Diseases such as type 2 diabetes, cardiovascular disease, and cancer are often diagnosed during this stage of life. These diseases are also the chief causes of death in middle adulthood.

Old Age

Old age begins in the mid-60s and lasts until the end of life. Most people over 65 have retired from work, freeing up their time for hobbies, grandchildren, and other interests. Stamina, strength, reflex time, and the senses all decline during old age, and the number of brain cells decreases as well. The immune system becomes less efficient, increasing the risk of serious illnesses such as cancer and pneumonia. Diseases such as Alzheimer’s disease that cause loss of mental function also become more common.

Causes of Aging

Why do we decline in all these ways as we age? Generally, it’s because cells stop dividing and die. There are at least two reasons why cells stop dividing:

  1. Cells are programmed to divide only a set number of times.
  2. Mutations accumulate in DNA, and cells with damaged DNA may not divide.

KQED: Embryonic Stem Cell Research

As we have seen in this lesson, you begin life as a single cell. By the time you were born, that cell had gone through millions of divisions. By adulthood, you’re made up of approximately 100 trillion individual cells. There are about 200 different types of cells that make these 100 trillion cells, all all these cells work together to allow you to live. Each cell, or group of cells, has a specific task or specialty. Those specialties allow an organism to function.

During early development, embryonic stem cells are present. These are undifferentiated cells that have the potential to turn into any type of cell. This potential is what makes these cells very good candidates for medical treatments, such as treating individuals with heart disease. To learn more about stem cells, see http://www.kqed.org/quest/television/stem-cell-gold-rush.

Lesson Summary

  • Fertilization is the union of a sperm and egg, resulting in the formation of a zygote. The zygote undergoes many cell divisions before it implants in the lining of the uterus.
  • The embryonic stage begins with implantation. An embryo forms three distinct cell layers, and each layer develops into different types of cells and organs.
  • The fetal stage begins about two months after fertilization and continues until birth. During this stage, organs continue to develop, and the fetus grows in size.
  • The placenta allows nutrients and wastes to be exchanged between the mother and fetus. The fetus is connected to the placenta through the umbilical cord.
  • A pregnant woman should avoid toxins and take in adequate nutrients for normal fetal growth and development. During childbirth, contractions of the uterus push the child out of the body.
  • Growth and development are most rapid during infancy and slower throughout the rest of childhood until adolescence. Adolescence involves mental, emotional, and social changes in addition to the physical changes of puberty.
  • During early adulthood, people form intimate relationships and start careers. Serious health problems start showing up in middle adulthood and old age. Aging occurs as cells lose their ability to divide.

Lesson Review Questions


1. What happens during fertilization? Where does it usually take place?

2. What is implantation? When does it occur?

3. Describe the role of the placenta in fetal development.

4. What causes the fetus to be pushed out of the uterus during birth?

5. When does adulthood begin?

6. Aging is associated with the death of cells. Give two reasons why cells die.

Apply Concepts

7. Make a flow chart of embryonic and fetal development.

8. Create a time line of milestones of growth and development from birth to adulthood.

Think Critically

9. Explain how the embryo forms specialized cells.

10. Why would an embryo be more susceptible than a fetus to damage by toxins?

11. Why is the umbilical cord cut before a newborn has started to breathe on its own?

12. Compare and contrast adolescence and puberty.

Points to Consider

Most diseases become more common as people grow older, but sexually transmitted diseases (STDs) are much more common in teens and young adults. These diseases are now usually called sexually transmitted infections (STIs).

  • Can you name any STIs? Do you know how STIs spread?
  • What might explain why STIs are so common in young people?

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