Chapter 520: Growth And Development

21
520

Chapter 21 Growth and Development

Growth and Development
Outline

PRENATAL PERIOD, 522

Fertilization to Implantation, 522
Periods of Development, 527
Formation of the Primary Germ
Layers, 527
Histogenesis and Organogenesis, 527
Birth Defects, 530
BIRTH, OR PARTURITION, 530
Stages of Labor, 530
POSTNATAL PERIOD, 530
Infancy, 533
Childhood, 534
Adolescence and Adulthood, 534
Older Adulthood, 535
EFFECTS OF AGING, 536
Skeletal System, 536
Integumentary System (Skin), 536
Urinary System, 536
Respiratory System, 538
Cardiovascular System, 538
Special Senses, 538

520

Objectives

AFTER YOU HAVE COMPLETED THIS
CHAPTER, YOU SHOULD BE ABLE TO:
1. Discuss the concept of development as a biological process
characterized by continuous modification and change.
2. Discuss the major developmental changes characteristic of the prenatal stage of life from fertilization to birth.
3. Discuss the three stages of labor that characterize a normal vaginal birth.
4. Identify the three primary germ layers and several derivatives in the adult body that develop from each layer.
5. List and discuss the major developmental changes characteristic of the four postnatal periods of life.
6. Discuss the effects of aging on the major body organ systems.

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M

any of your fondest and most vivid memories are probably associated with your birthdays. The day of birth is an important milestone of life. Most people continue to remember their birthday in some special way each year; birthdays serve as pleasant and convenient reference points to mark periods of transition or change in our lives. The actual day of birth marks the end of one phase of life called the prenatal period and the beginning of a second called the postnatal period. The prenatal period begins at conception and ends at birth; the postnatal period begins at birth and continues until death.
Although important periods in our lives such as childhood and adolescence are often remembered as a series of individual and isolated events, they are in reality part of an ongoing and continuous process. In reviewing the many changes that occur during the cycle of life from conception to death, it is often convenient to isolate certain periods such as infancy or adulthood for study. It is important to remember, however, that life is not a series of stop-and-start events or individual and isolated periods of time. Instead, it is a biological process that is characterized by continuous modification and change.
This chapter discusses some of the events and changes that occur in the development of the individual from conception to death. Study of development during the prenatal period is followed by a discussion of the birth process and a review of

To make the study of human growth and development more efficient, we suggest these tips:
1. Review the concepts of human reproduction from the previous chapter.
2. The term germ in primary germ layer refers to “germinate.” All the structures of the body come from one of these layers. They are named based on their location in the developing embryo. Endoderm means inner skin, mesoderm means middle skin, and ectoderm means outer skin.
3. Genesis means to create. Histogenesis means to create tissues, and organogenesis means to create organs.
4. The early developmental stages can be put on flash cards. You might also want to include on the flash card where in the developmental sequence the particular stage is, in other words, what it developed from. Remember to include the functions of the amnion, chorion, and placenta. In your study group, go over the flash cards of the stages of development, making sure you know the proper sequence.
5. Use flash cards to match the primary germ layers and the structures that come from each of them.
Continued on page 545
521

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Chapter 21 Growth and Development

changes that occur during infancy and adulthood.
Finally, some important changes that occur in the individual organ systems of the body as a result of aging are discussed.

PRENATAL PERIOD
The prenatal stage of development begins at the time of conception, or fertilization (that is, at the moment the female ovum and the male sperm cells unite)
(Figure 21-1). The period of prenatal development continues until the birth of the child about 39 weeks later. The science of the development of the individual before birth is called embryology (em-breeOL-oh-jee). It is a story of miracles, describing the means by which a new human life is started and the steps by which a single microscopic cell is transformed into a complex human being.

Fertilization to Implantation
After ovulation the discharged ovum first enters the abdominal cavity and then finds its way into a uterine (fallopian) tube. Sperm cells “swim” up the uterine tubes toward the ovum. Look at the relationship of the ovary, the two uterine tubes, and the uterus in Figure 21-2. Recall from Chapter
20 that each uterine tube extends outward from the uterus for about 10 cm. It then ends in the abdominal cavity near the ovary, as you can see in
Figure 21-2, in an opening surrounded by fringelike processes, the fimbriae. Using the uterus as a point of reference, anatomists divide each uterine tube into three parts. The innermost part of the tube actually extends through the uterine wall, the middle third extends out into the abdominal cavity, and the outermost third of the tube

FIGURE 21-1
Fertilization. Fertilization is a specific biological event. It occurs when the male and female sex cells fuse. After union between a sperm cell and the ovum has occurred, the cycle of life begins. The scanning electron micrograph shows spermatozoa attaching themselves to the surface of an ovum. Only one will penetrate and fertilize the ovum.

Sperm cell
Ovum
Cytoplasm

Nucleus

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FIGURE 21-2
Fertilization and implantation. At ovulation, an ovum is released from the ovary and begins its journey through the uterine tube. While in the tube, the ovum is fertilized by a sperm to form the single-celled zygote. After a few days of rapid mitotic division, a ball of cells called a morula is formed. After the morula develops into a hollow ball called a blastocyst, implantation occurs.
S
Uterine (fallopian) tube

R
Divided
zygote

L
I

Morula

Fertilization
Discharged
ovum

Spermatozoa

Uterus

First mitosis Blastocyst

Corpus luteum Developing follicles Implantation

Ovary
Fimbriae

Ovulation

ends near the ovary in the dilated, funnel-shaped opening described above.
Sperm cells that are deposited in the vagina must enter and “swim” through the uterus and then move out of the uterine cavity and through the uterine tube to meet the ovum. Fertilization most often occurs in the outer one third of the oviduct, as shown in Figure 21-2. The fertilized ovum, or zygote (ZYE-goht), is genetically complete; it represents a new single-celled individual.
Time and nourishment are all that is needed for expression of characteristics such as sex, body build, and skin color that were determined at the time of fertilization. As you can see in the figure,

the zygote immediately begins mitotic division, and in about 3 days a solid mass of cells called a morula (MOR-yoo-lah) is formed (see Figure 212). The cells of the morula continue to divide, and by the time the developing embryo reaches the uterus, it is a hollow ball of cells called a blastocyst (BLAS-toh-sist).
During the 10 days from the time of fertilization to the time when the blastocyst completes implantation in the uterine lining, no nutrients from the mother are available. The rapid cell division taking place up to the blastocyst stage occurs with no significant increase in total mass compared with the zygote (Figure 21-3). One of the specializations

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FIGURE 21-3
Early stages of human development. A, Fertilized ovum or zygote. B to D, Early cell divisions produce more and more cells. The solid mass of cells shown in D forms the morula—an early stage in embryonic development.

A

B

C

D

of the ovum is its incredible store of nutrients that help support this embryonic development until implantation has occurred.
Note in Figure 21-4 that the blastocyst consists of an outer layer of cells and an inner cell mass.
As the blastocyst develops, it forms a structure with two cavities, the yolk sac and amniotic
(am-nee-OT-ik) cavity. The yolk sac is most important in animals, such as birds, that depend

heavily on yolk as the sole source of nutrients for the developing embryo. In these animals the yolk sac digests the yolk and provides the resulting nutrients to the embryo. Because uterine fluids provide nutrients to the developing embryo in humans until the placenta develops, the function of the yolk sac is not a nutritive one.
Instead, it has other functions, including production of blood cells.

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FIGURE 21-4
Implantation and early development. The hollow blastocyst implants itself in the uterine lining about 10 days after ovulation. Until the placenta is functional, nutrients are obtained by diffusion from uterine fluids. Notice the developing chorion and how the blastocyst eventually forms a yolk sac and amniotic cavity.

Implanted blastocyst Outer layer of cells Developing chorion Inner cell mass

Yolk sac

Yolk sac

Amniotic cavity Amniotic cavity Uterine lining Uterine glands and vessels

The amniotic cavity becomes a fluid-filled, shockabsorbing sac, sometimes called the bag of waters, in which the embryo floats during development. The chorion (KOH-ree-on), shown in Figures 21-4 and
21-5, develops into an important fetal membrane in the placenta (plah-SEN-tah). The chorionic villi shown in Figure 21-5 connect the blood vessels of the chorion to the placenta. The placenta (see Figure
21-5) anchors the developing fetus to the uterus and provides a “bridge” for the exchange of nutrients and waste products between mother and baby.
The placenta is a unique and highly specialized structure that has a temporary but very important series of functions during pregnancy. It is composed of tissues from mother and child and functions not only as a structural “anchor” and nutritive bridge but also as an excretory, respiratory, and endocrine organ (see Figure 21-5).
Placental tissue normally separates the maternal blood, which fills the lacunae of the placenta,

from the fetal blood so that no intermixing occurs.
The very thin layer of placental tissue that separates maternal and fetal blood also serves as an effective “barrier” that can protect the developing baby from many harmful substances that may enter the mother’s bloodstream. Unfortunately, toxic substances, such as alcohol and some infectious organisms, may nonetheless penetrate this protective placental barrier and injure the developing baby. The virus responsible for German measles (rubella), for example, can easily pass through the placenta and cause tragic developmental defects in the fetus.

To learn more about fertilization and implantation, go to AnimationDirect on your CD-ROM.

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FIGURE 21-5
The placenta. Relationship of uterus, developing infant, and placenta. The close placement of the fetal blood supply and the maternal blood in the lacunae of the placenta permits diffusion of nutrients and other substances. It also forms a thin barrier to prevent diffusion of most harmful substances. No mixing of fetal and maternal blood occurs. A, Diagram showing a cross section of the placental structure. B, Photograph of a normal, full-term placenta (fetal side) showing the branching of the placental blood vessels.
Maternal
venule

Maternal arteriole Chorionic villi
Umbilical vein
Umbilical
arteries

A

B

Endometrium

Umbilical cord Maternal blood
Fetal
arteriole

Placenta
Fetal
venule

R esearch, Issues and Trends
In Vitro Fertilization
The Latin term in vitro means, literally, “within a glass.” In the case of in vitro fertilization, it refers to the glass laboratory dish where an ovum and sperm are mixed and where fertilization occurs.
In the classic technique, the ovum is obtained from the mother by first inserting a fiberoptic viewing instrument called a laparoscope through a very small incision in the woman’s abdomen. After it is in the abdominal cavity, the device allows the physician to view the ovary and then puncture and “suck

up” an ovum from a mature follicle. Over the years refinements to this technique have been made, and less invasive procedures are currently being used.
After about 2.5 days’ growth in a temperaturecontrolled environment, the developing zygote, which by then has reached the 8- or 16-cell stage, is returned by the physician to the mother’s uterus.
If implantation is successful, growth will continue and the subsequent pregnancy will progress. In the most successful fertility clinics in the United States, a normal term birth will occur in about 30% of in vitro fertilization attempts.

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R esearch, Issues and Trends
How Long Does
Pregnancy Last?
This seems like a silly question to most of us; the answer is 9 months, isn’t it? Actually, the length of gestation (the amount of time one is pregnant) is defined in different ways in different situations and can vary from one pregnancy to another. The average gestation in humans is 266 days, starting at the day of

Periods of Development
The length of pregnancy (about 39 weeks)—called the gestation period—is divided into three 3-month segments called trimesters. A number of terms are used to describe development during these periods known as the first, second, and third trimesters of pregnancy.
During the first trimester or 3 months of pregnancy, many terms are used. Zygote describes the ovum just after fertilization by a sperm cell. After about 3 days of constant cell division, the solid mass of cells, identified earlier as the morula, enters the uterus. Continued development transforms the morula into the hollow blastocyst, which then implants into the uterine wall.
The embryonic phase of development extends from the third week after fertilization until the end of week 8 of gestation. During this period in the first trimester, the term embryo is used to describe the developing individual. The period of development extending from week 9 to week 39 is termed the fetal phase. During this period, the term embryo is replaced by fetus.
By day 35 of gestation (Figure 21-6, A), the heart is beating and, although the embryo is only 8 mm
(about 3⁄8 inch) long, the eyes and so-called limb buds, which ultimately form the arms and legs, are clearly visible. Figure 21-6, C, shows the stage of development of the fetus at the end of the first trimester of gestation. Body size is about 7 to 8 cm (3.2 inches) long. The facial features of the fetus are apparent, the limbs are complete, and gender can be identified. By month 4 (Figure 21-6, D) all organ systems are complete and in place.

conception. But physicians instead usually count from the beginning of the woman’s last menstrual period, for an average of 280 days. But these are only averages. What is normal in one case can be different from what is normal in another case. In practice, any pregnancy of less than 37 weeks (259 days) is said to be premature, and any lasting more than 42 weeks
(294 days) is said to be postmature. So, as with many statistics regarding human function, what is “normal” can be spoken of only in generalities and averages.

Formation of the Primary Germ
Layers
Early in the first trimester of pregnancy, three layers of specialized cells develop that embryologists call the primary germ layers (Table 21-1).
Each layer gives rise to definite structures such as the skin, nervous tissue, muscles, or digestive organs. Table 21-1 lists a number of structures derived from each primary germ layer called, respectively, endoderm (EN-doh-derm), or inside layer; ectoderm (EK-toh-derm), or outside layer; and mesoderm (MEZ-oh-derm), or middle layer.

Histogenesis and Organogenesis
The process of how the primary germ layers develop into many different kinds of tissues is called histogenesis (his-toh-JEN-eh-sis). The way in which those tissues arrange themselves into organs is called organogenesis (or-gah-no-JENeh-sis). The fascinating story of histogenesis and organogenesis in human development is long and complicated; its telling belongs to the science of embryology. But for the beginning student of anatomy and physiology, it seems sufficient to appreciate that life begins when two sex cells unite to form a single-celled zygote and that the new human body evolves by a series of processes consisting of cell differentiation, multiplication, growth, and rearrangement, all of which take place in a definite, orderly sequence (Figure 21-7).
Development of structure and function go hand in hand, and from 4 months of gestation, when every organ system is complete and in place, until term

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FIGURE 21-6
Human embryos and fetuses. A, At 35 days. B, At 49 days. C, At the end of the first trimester. D, At 4 months.
Branchial arches

Eye

Heart

A

B

Arm bud Leg bud C

Limb buds D

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TABLE 21-1

Primary Germ Layer Derivatives
ENDODERM

ECTODERM

MESODERM

Lining of gastrointestinal tract
Lining of lungs
Lining of hepatic and pancreatic ducts
Kidney ducts and bladder
Anterior pituitary gland (adenohypophysis)
Thymus gland
Thyroid gland
Parathyroid gland
Tonsils
Adrenal medulla

Epidermis of skin
Tooth enamel
Lens and cornea of eye
Outer ear
Nasal cavity
Facial bones
Skeletal muscles in head
Brain and spinal cord
Sensory neurons

Dermis of skin
Circulatory system
Many glands
Kidneys
Gonads
Muscle
Bones (except facial)

FIGURE 21-7
Critical periods of neonatal development. The red areas show when teratogens are most likely to cause major birth defects, and the yellow areas show when minor defects are more likely to arise.
First
trimester
Embryonic
period

Preembryonic period 1

2

Second trimester 3

4

5

6

Fetal period
(week 9—full-term)
7

8

1-celled zygote Morula

Central nervous system
Heart
Upper limbs

Blastocyst

Lower limbs
Ears
Eyes
Teeth

Implantation

Major birth defects
Palate

Spontaneous abortion Third trimester Minor birth defects
External genitals

9

16

20-36

38

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Chapter 21 Growth and Development

(about 280 days), fetal development is mainly a matter of growth. Figure 21-8, A, shows the normal intrauterine placement of a fetus just before birth in a full-term pregnancy.

Birth Defects
Developmental problems present at birth are often called birth defects.
Such abnormalities may be structural or functional, perhaps even involving behavior and personality. Birth defects may be caused by genetic factors such as abnormal genes or inheritance of an abnormal number of chromosomes.
Birth defects also may be caused by exposure to environmental factors called teratogens (TAYR-ahtoh-jenz). Teratogens include radiation (for example, x-rays), chemicals (for example, drugs, cigarettes, or alcohol), and infections in the mother (for example, herpes or rubella). Some teratogens are also mutagens because they do their damage by changing the genetic code in cells of the developing embryo. Nutritional deficiencies during pregnancy also can lead to birth defects.
As Figure 21-7 shows, the period during the first trimester when the tissues are beginning to differentiate and the organs are just starting to develop is the time that teratogens are most likely to cause damage. In fact, teratogens can cause spontaneous abortion (miscarriage) if significant damage occurs during the pre-embryonic stage.

QUICK

1. What is the postnatal period? The prenatal period?
2. What is a zygote? How is it
different from a morula or blastocyst?
3. What are germ layers?
4. What is meant by the term organogenesis?

BIRTH, OR PARTURITION
The process of birth, or parturition (pahr-too-RIshun), is the point of transition between the prenatal and postnatal periods of life. As pregnancy draws to a close, the uterus becomes “irritable” and, ultimately, muscular contractions begin and

cause the cervix to dilate or open, thus permitting the fetus to move from the uterus through the vagina, or “birth canal,” to the exterior. The process normally begins with the fetus taking a headdown position against the cervix (Figure 21-8, A).
When contractions occur, the amniotic sac, or “bag of waters,” ruptures, and labor begins.

Stages of Labor
Labor is the process that results in the birth of a baby. It has three stages (Figure 21-8, B to E):
1. Stage one—period from onset of uterine contractions until dilation of the cervix is complete 2. Stage two—period from the time of maximal cervical dilation until the baby exits through the vagina
3. Stage three—process of expulsion of the placenta through the vagina
The time required for normal vaginal birth varies widely and may be influenced by many variables, including whether the woman has previously had a child. In most cases, stage one of labor lasts from 6 to 24 hours, and stage two lasts from a few minutes to an hour. Delivery of the placenta (stage three) normally occurs within
15 minutes after the birth of the baby.

To learn more about the three stages of birth, go to AnimationDirect on your CD-ROM.

QUICK

1. What is meant by the term parturition?
2. What are the three stages of labor? POSTNATAL PERIOD
The postnatal period begins at birth and lasts until death. Although it is often divided into major periods for study, we need to understand and appreciate that growth and development are continu-

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FIGURE 21-8
Parturition. A, The relation of the fetus to the mother. B, The fetus moves into the opening of the birth canal, and the cervix begins to dilate. C, Dilation of the cervix is complete. D, The fetus is expelled from the uterus. E, The placenta is expelled. Pubic symphysis

Placenta

Urinary bladder

A

Urethra
Vagina
Rectum
Cervix

Placenta
Umbilical
cord

B

C

Amniotic sac Vagina

Ruptured amniotic sac

Cervix
Uterus

Placenta

D

Umbilical cord
Placenta
(maternal side) E
A

Placenta
(fetal side)

S

I
P

ous processes that occur throughout the life cycle.
Gradual changes in the physical appearance of the body as a whole and in the relative proportions of the head, trunk, and limbs are quite noticeable between birth and adolescence. Note in Figure 21-9

the obvious changes in the size of bones and in the proportionate sizes between different bones and body areas. The head, for example, becomes proportionately smaller. Whereas the infant head is approximately one-fourth the total height of the

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Chapter 21 Growth and Development

H ealth and Well-Being
Quickening
Pregnant women usually notice fetal movement for the first time between weeks 16 and 18 of pregnancy.
The term quickening has been used for generations to describe these first recognizable movements of the fetus. From an occasional “kick” during months 4 and 5 of pregnancy, the frequency of fetal movements steadily increases as gestation progresses. The frequency of fetal movements is an excellent indicator of the unborn baby’s health.
Recent studies have shown that simply by recording the number of fetal movements each day

after week 28 of pregnancy, a woman can provide her physician with extremely useful information about the health of her unborn child. Ten or more movements during a daily measurement period are considered normal.
Educating pregnant women about fetal movements and how to monitor their frequency is but one example of expanded interest in prenatal home care.
Assisting pregnant women with making informed judgments about nutrition, exercise, lifestyle adjustments, and birthing options before they enter the hospital for delivery of their baby is an important and growing part of home health-care services.

Research, Issues and Trends
Antenatal
Diagnosis and Treatment
Advances in antenatal (from the
Latin ante, “before,” natus, “birth”) medicine now permit extensive diagnosis and treatment of disease in the fetus much like that in any other patient. This new dimension in medicine began with techniques by which Rh babies could be given transfusions before birth.

Current procedures using images provided by ultrasound equipment (Figures A and B) allow physicians to prepare for and perform, before the birth of a baby, corrective surgical procedures such as bladder repair.
These procedures also allow physicians to monitor the progress of other types of treatment on a developing fetus. Figure A shows placement of the ultrasound transducer on the abdominal wall. The resulting image
(Figure B), called an ultrasonogram, shows a 21-week embryo. A
S

I
P

A

Ultrasound transducer B

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FIGURE 21-9
Changes in the proportions of body parts from birth to maturity. Note the dramatic differences in head proportion.

Newborn

2-year-old

body, the adult head is only about one-eighth the total height. The facial bones also show several changes between infancy and adulthood. In an infant the face is one-eighth of the skull surface, but in an adult the face is half of the skull surface.
Another change in proportion involves the trunk and lower extremities. The legs become proportionately longer and the trunk proportionately shorter. In addition, the thoracic and abdominal contours change, roughly speaking, from round to elliptical. Such changes are good examples of the everchanging and ongoing nature of growth and development. It is unfortunate that many of the changes that occur in the later years of life do not result in increased function. These degenerative changes are certainly important, however, and are discussed later in this chapter. The following are the most common postnatal periods: (1) infancy,
(2) childhood, (3) adolescence and adulthood, and
(4) older adulthood.

Infancy
The period of infancy begins abruptly at birth and lasts about 18 months. The first 4 weeks of infancy are often referred to as the neonatal (nee-oh-NAYtal) period (Figure 21-10). The baby is referred to as

5-year-old

13-year-old

Adult

FIGURE 21-10
The neonate infant. The umbilical cord has been cut.

a neonate. Dramatic changes occur at a rapid rate during this short but critical period. Neonatology
(nee-oh-nay-TOL-oh-jee) is the medical and nursing specialty concerned with the diagnosis and treatment of disorders of the newborn. Advances in this area have resulted in dramatically reduced infant mortality. Many of the changes that occur in the cardiovascular and respiratory systems at birth are necessary for survival. Whereas the fetus totally depends on the mother for life support, the newborn infant must become totally self-supporting in terms of blood circulation and respiration immedi-

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Chapter 21 Growth and Development

ately after birth. A baby’s first breath is deep and forceful. The stimulus to breathe results primarily from the increasing amounts of carbon dioxide
(CO2) that accumulate in the blood after the umbilical cord is cut following delivery.
Many developmental changes occur between the end of the neonatal period and 18 months of age. Birth weight doubles during the first 4 months and then triples by 1 year. The baby also increases in length by 50% by the 12th month. The “baby fat” that accumulated under the skin during the first year begins to decrease, and the plump infant becomes leaner.
Early in infancy the baby has only one spinal curvature (see Figure 6-11). The lumbar curvature appears between 12 and 18 months, and the oncehelpless infant becomes a toddler who can stand
(Figure 21-11). One of the most striking changes to occur during infancy is the rapid development of the nervous and muscular systems. This permits

FIGURE 21-11
The toddler spine. Photograph showing the normal curvature of the vertebral column in a toddler. The dark shadow emphasizes the distinct lumbar curvature that develops with the ability to walk.

the infant to follow a moving object with the eyes
(2 months); lift the head and raise the chest
(3 months); sit when well supported (4 months); crawl (10 months); stand alone (12 months); and run, although a bit stiffly (18 months).

Childhood
Childhood extends from the end of infancy to sexual maturity or puberty—12 to 14 years in girls and 14 to
16 years in boys. Overall, growth during early childhood continues at a rather rapid pace, but month-tomonth gains become less consistent. By the age of
6 the child appears more like a preadolescent than an infant or toddler. The child becomes less chubby, the potbelly becomes flatter, and the face loses its babyish look. The nervous and muscular systems continue to develop rapidly during the middle years of childhood; by 10 years of age, the child has developed numerous motor and coordination skills.
The deciduous teeth, which began to appear at about 6 months of age, are lost during childhood, beginning at about 6 years of age. The permanent teeth, with the possible exception of the third molars (or wisdom teeth), have all erupted by age 14.

Adolescence and Adulthood

S
A

P
I

The average age range of adolescence varies, but generally the teenage years (13 to 19) are used. The period is marked by rapid and intense physical growth, which ultimately results in sexual maturity. Many of the developmental changes that occur during this period are controlled by the secretion of sex hormones and are classified as secondary sex characteristics. Breast development is often the first sign of approaching puberty in girls, beginning about age 10. Most girls begin to menstruate at 12 to
13 years of age, which is about 3 years earlier than a century ago. In boys the first sign of puberty is often enlargement of the testicles, which begins between
10 and 13 years of age. Both sexes show a spurt in height during adolescence (Figure 21-12). In girls the spurt in height begins between the ages of 10 and
12 and is nearly complete by 14 or 15 years. In boys the period of rapid growth begins between 12 and 13 and is generally complete by 16.
Many developmental changes that began early in childhood are not completed until the early or

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R esearch, Issues and Trends
Freezing Umbilical Cord
Blood
The concept of development of blood cells from red bone marrow, a process called hematopoiesis, was introduced in Chapter 11. Ultimately, the presence of
“stem cells” is required for bone marrow to produce blood cells. The fact that umbilical cord blood is rich in these stem cells has great clinical significance.
In the past, if the stem cells in the bone marrow of a child were destroyed as a result of leukemia or by chemotherapy, death would result unless a bone marrow transplant was possible. Infusion of stored umbilical cord blood obtained from the child at the time of

middle years of adulthood. Examples include the maturation of bone, resulting in the full closure of the growth plates, and changes in the size and placement of other body components such as the sinuses. Many body traits do not become apparent for years after birth. Normal balding patterns, for example, are determined at the time of fertilization by heredity but do not appear until maturity. As a general rule, adulthood is characterized by maintenance of existing body tissues. With the passage of years the ongoing effort of maintenance and repair of body tissues becomes more and more difficult. As a result, degeneration begins. It is the process of aging, and it culminates in death.

birth is an attractive alternative. The blood is rich in stem cells and can be obtained without risk; this procedure is much more cost-effective than a bone marrow transplant.
Removing and freezing umbilical cord blood at the time of birth may become a type of biological insurance against some types of leukemia that may affect a child later in life. Cord blood is readily available at birth and is a better source of stem cells than bone marrow.
When the umbilical cord is cut after birth, the blood that remains in the cord is simply drained into a sterile bag (see photo), frozen, and then stored in liquid nitrogen in one of about a dozen cord-blood centers in the
United States.

Older Adulthood
Most body systems are in peak condition and function at a high level of efficiency during the early years of adulthood. As a person grows older, a gradual but certain decline takes place in the functioning of every major organ system in the body. The study of aging is called gerontology. The remainder of this chapter deals with a number of the more common degenerative changes that frequently characterize senescence (seh-NES-ens), or older adulthood. Many of the biological changes associated with advancing age are shown in Figure 21-13. The illustration highlights the proportion of remaining function in a number of organs in older adulthood when compared with a 21-year-old person.

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Research, Issues and Trends

FIGURE 21-12

Height gain (cm per year)

Growth in height. The figure shows typical patterns of gain in height to adulthood for girls and boys. Notice the rapid gain in height during the first few years, a period of slower growth, then another burst of growth during adolescence— finally ending at the beginning of adulthood.
24
23
22
21
20
19
18
17
16
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1

Girls
Boys

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
Age (years)

QUICK

1. How do the proportions of the human body change during postnatal development?
2. What is the neonatal period of development? Senescence?
3. During which phase of development do the deciduous teeth appear?
4. What biological changes happen during puberty?

Fetal Alcohol Syndrome
Consumption of alcohol by a woman during her pregnancy can have tragic effects on a developing fetus.
Educational efforts to inform pregnant women about the dangers of alcohol use are now receiving national attention. Even very limited consumption of alcohol during pregnancy poses significant hazards to the developing baby because alcohol can easily cross the placental barrier and enter the fetal bloodstream.
When alcohol enters the fetal blood, the potential result, called fetal alcohol syndrome (FAS), can cause tragic congenital abnormalities such as
“small head,” or microcephaly (my-kroh-SEF-ahlee), low birth weight, developmental disabilities such as mental retardation, and even fetal death.

and shaggy-appearing margins with spurs—a process called lipping. This type of degenerative change restricts movement because of the piling up of bone tissue around the joints. With advancing age, changes in calcification may result in reduction of bone size and in bones that are porous and subject to fracture. The lower cervical and thoracic vertebrae are the site of frequent fractures. The result is curvature of the spine and the shortened stature so typical of late adulthood. Degenerative joint diseases such as osteoarthritis (OS-tee-oh-arTHRY-tis) are also common in elderly adults.

Integumentary System (Skin)
With advancing age the skin becomes dry, thin, and inelastic. It “sags” on the body because of increased wrinkling and skinfolds. Pigmentation changes and the thinning or loss of hair are also common problems associated with the aging process.

EFFECTS OF AGING

Urinary System

Skeletal System

The number of nephron units in the kidney decreases by almost 50% between the ages of 30 and 75. Also, because less blood flows through the kidneys as an individual ages, there is a reduction in overall function and excretory capac-

In older adulthood, bones undergo changes in texture, degree of calcification, and shape. Instead of clean-cut margins, older bones develop indistinct

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537

FIGURE 21-13
Some biological changes associated with maturity and aging. Insets show proportion of remaining function in the organs of a person in late adulthood compared with that of a 20-year-old.

85%

65%

Brain weight
Cardiac output at rest

80%

55%

Citric acid cycle

Respiratory capacity of lungs
Basal metabolic rate

50%

Liver blood flow

65%

Kidney mass

63%
85%

Liver weight

Conduction velocity of nerve fiber

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Chapter 21 Growth and Development

S cience Applications
Embryology
Rita Levi-Montalcini
(b. 1909)
Rita Levi-Montalcini had just finished a medical degree in her native Italy when in 1938 the Fascist government under Mussolini barred all
“non-Aryans” from working in academic and professional careers. Being Jewish, Levi-Montalcini was forced to move to Belgium to work. But when Belgium was about to be invaded by the Nazis, she decided to return home to Italy and work in secret.
Her home laboratory was very crude but in it she made some important discoveries about how the nervous system develops during embryonic development. After World War II, she was invited to Washington University in St. Louis to work. There, she discovered the existence of nerve growth factor

ity or the ability to produce urine. In the bladder, significant age-related problems often occur because of diminished muscle tone. Muscle atrophy (wasting) in the bladder wall results in decreased capacity and inability to empty or void completely. Respiratory System
In older adulthood the costal cartilages that connect the ribs to the sternum become hardened or calcified. This makes it difficult for the rib cage to expand and contract as it normally does during inspiration and expiration. In time the ribs gradually become “fixed” to the sternum, and chest movements become difficult. When this occurs the rib cage remains in a more expanded position, respiratory efficiency decreases, and a condition called
“barrel chest” results. With advancing years a generalized atrophy or wasting of muscle tissue takes place as the contractile muscle cells are replaced by connective tissue. This loss of muscle cells decreases the strength of the muscles associated with inspiration and expiration.

(NGF), for which she later won the 1986 Nobel Prize.
Her discovery of a chemical that regulates the growth of new nerves during early brain development has led to many different paths of investigation. For example, by learning more about growth regulators we now know more about how the nervous system develops, as well as other tissues, organs, and systems of the body.
Today, many professions make use of the discoveries of embryology—the study of early development.
Not only are these discoveries important for health professionals such as obstetricians, obstetric nurses, and others involved in prenatal health care, they are also important in understanding adult medicine more fully. In fact, even gerontology (study of aging) and geriatrics (treatment of the aged) have benefited from embryological research. How? By providing insights on how tissue development is regulated in the embryo, scientists can better understand how to possibly stimulate damaged tissue in older adults to repair or regenerate themselves.

Cardiovascular System
Degenerative heart and blood vessel disease is one of the most common and serious effects of aging. Fatty deposits build up in blood vessel walls and narrow the passageway for the movement of blood, much as the buildup of scale in a water pipe decreases flow and pressure. The resulting condition, called atherosclerosis (ath-er-oh-sklehROH-sis), often leads to eventual blockage of the coronary arteries and a “heart attack.” If fatty accumulations or other substances in blood vessels calcify, actual hardening of the arteries, or arteriosclerosis (ar-tee-ree-oh-skleh-ROH-sis) occurs.
Rupture of a hardened vessel in the brain (stroke) is a frequent cause of serious disability or death in the older adult. Hypertension, or high blood pressure, is also more common.

Special Senses
The sense organs, as a group, all show a gradual decline in performance and capacity as a person ages. Most people are farsighted by age 65 because

Chapter 21 Growth and Development

eye lenses become hardened and lose elasticity; the lenses cannot become curved to accommodate for near vision. This hardening of the lens is called presbyopia (pres-bee-OH-pee-ah), which means
“old eye.” Many individuals first notice the change at about 40 or 45 years of age, when it becomes difficult to do close-up work or read without holding printed material at arm’s length. This explains the increased need, with advancing age, for bifocals
(glasses that incorporate two lenses) to assist the eye in accommodating for near and distant vision.
Loss of transparency of the lens or its covering capsule is another common age-related eye change. If the lens actually becomes cloudy and significantly impairs vision, it is called a cataract (KAT-ah-rakt) and must be removed surgically. The incidence of glaucoma (glaw-KOH-mah), the most serious agerelated eye disorder, increases with age. Glaucoma causes an increase in the pressure within the eyeball and, unless treated, often results in blindness.
In many elderly people a very significant loss of hair cells in the organ of Corti (inner ear) causes a

539

serious decline in the ability to hear certain frequencies. In addition, the eardrum and attached ossicles become more fixed and less able to transmit mechanical sound waves. Some degree of hearing impairment is universally present in the older adult.
The sense of taste is also decreased. This loss of appetite may be caused, at least in part, by the replacement of taste buds with connective tissue cells. Only about 40% of the taste buds present at age 30 remain in an individual at age 75.

QUICK

1. What are some changes that occur in the skeleton as one ages?
2. How is kidney function affected during old age?
3. What changes in the cardiovascular system occur in older adults?
4. How does one’s eyesight change during late adulthood?

R esearch, Issues and Trends
Extending the Human
Lifespan
When reviewing the last edition of this textbook, a colleague of ours said that ending with the depressing topic of “degeneration associated with aging” was not appropriate to the overall upbeat tone of our book. At first we thought our ending was better than the most obvious and technically accurate ending:
“then you die.” But it occurred to us that we could take this opportunity to point out one of the most remarkable and important areas of achievement in modern medical research—extending the length and improving the quality of life.
In the past few decades, the increased availability of better food, safer surroundings, and advanced medical care has extended quality living for many around the world. But even simple changes in lifestyle, regardless of modern medical wonders, can keep the effects of aging from creeping up too soon.

Perhaps the three most important “low-tech” methods for improving the quality of life as you age are healthful diet, exercise, and stress management. A healthful diet is not available to some individuals, but it is available to most of us. We are learning more every day about what kind of diet is best, even to the point of being able to manage specific diseases through diet. Exercise performed on a regular basis, even if light or moderate, can not only keep our skeletal and muscular systems more fit but also decrease aging’s effects on the nervous system, endocrine system, digestive system, immune system—the list seems endless. And last, even ancient and simple techniques of stress management such as meditation have been shown to help reduce the effects of aging and the diseases that often accompany aging such as heart disease and strokes.
So to end this chapter, and this book, we say: you can stay young much longer if you eat right, exercise, and relax. And keep studying human structure and function so you’ll know that you are doing it right!

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Chapter 21 Growth and Development

OUTLINE SUMMARY
PRENATAL PERIOD
A. Prenatal period begins at conception and continues until birth (about 39 weeks)
B. Science of fetal growth and development called embryology
C. Fertilization to implantation requires about
10 days
1. Fertilization normally occurs in outer third of oviduct (Figure 21-2)
2. Fertilized ovum called a zygote; zygote is genetically complete—all that is needed for expression of hereditary traits is time and nourishment
3. After 3 days of cell division, the zygote has developed into a solid cell mass called a morula
4. Continued cell divisions of the morula produce a hollow ball of cells called a blastocyst 5. Blastocyst implants in the uterine wall about 10 days after fertilization
6. Blastocyst forms the amniotic cavity and chorion of the placenta (Figure 21-4)
7. Placenta provides for exchange of nutrients between the mother and fetus
D. Periods of development
1. Length of pregnancy, or gestation period, is about 39 weeks
2. Embryonic phase extends from the third week after fertilization to the end of week 8 of gestation
3. Fetal phase extends from week 8 to week 39 of gestation
4. All organ systems are formed and functioning by month 4 of gestation
(Figure 21-6)
E. Three primary germ layers appear in the developing embryo after implantation of the blastocyst (Table 21-1):
1. Endoderm—inside layer
2. Ectoderm—outside layer
3. Mesoderm—middle layer

Histogenesis and organogenesis
1. Formation of new organs and tissues occurs from specific development of the primary germ layers
2. Each primary germ layer gives rise to definite structures such as the skin and muscles 3. Growth processes include cell differentiation, multiplication, growth, and rearrangement
4. From 4 months of gestation until delivery, the development of the baby is mainly a matter of growth
G. Birth defects
1. Any structural or functional abnormality present at birth
2. May be caused by genetic factors a . Abnormal genes
b. Abnormal number of chromosomes
3. May be caused by environmental factors a . Environmental factors are called teratogens b. Include radiation, chemicals, and infections c . Especially harmful during the first trimester (Figure 21-7)
F.

BIRTH, OR PARTURITION
A. Process of birth called parturition
(Figure 21-8)
1. At the end of week 39 of gestation, the uterus becomes “irritable”
2. Fetus takes head-down position against the cervix
3. Muscular contractions begin, and labor is initiated
4. Amniotic sac (“bag of waters”) ruptures
5. Cervix dilates
6. Fetus moves through vagina to exterior Chapter 21 Growth and Development

541

O UTLINE SUMMARY—cont’d
B. Stages of labor
1. Stage one—period from onset of uterine contractions until dilation of the cervix is complete
2. Stage two—period from the time of maximal cervical dilation until the baby exits through the vagina
3. Stage three—process of expulsion of the placenta through the vagina
POSTNATAL PERIOD
A. Postnatal period begins at birth and lasts until death
B. Divisions of postnatal period into isolated timeframes can be misleading; life is a continuous process; growth and development are continuous
C. Obvious changes in the physical appearance of the body—in whole and in proportion—occur between birth and maturity (Figure 21-9)
D. Divisions of postnatal period
1. Infancy
2. Childhood
3. Adolescence and adulthood
4. Older adulthood
E. Infancy
1. First 4 weeks called neonatal period
(Figure 21-10)
2. Neonatology—medical and nursing specialty concerned with the diagnosis and treatment of disorders of the newborn 3. Many cardiovascular changes occur at the time of birth; fetus is totally dependent on mother, whereas the newborn must immediately become totally selfsupporting (in respect to respiration and circulation)
4. Respiratory changes at birth include a deep and forceful first breath

5. Developmental changes between the neonatal period and 18 months include:
a. Doubling of birth weight by
4 months and tripling by 1 year
b. 50% increase in body length by
12 months c . Development of normal spinal curvature by 15 months (Figure 21-11)
d. Ability to raise head by 3 months
e. Ability to crawl by 10 months f . Ability to stand alone by 12 months
g. Ability to run by 18 months
F. Childhood
1. Extends from end of infancy to puberty—
13 years in girls and 15 in boys
2. Overall rate of growth remains rapid but decelerates
3. Continuing development of motor and coordination skills
4. Loss of deciduous (baby) teeth and eruption of permanent teeth
G. Adolescence and adulthood
1. Average age range of adolescence varies; usually considered to be from
13 to 19 years
2. Period of rapid growth resulting in sexual maturity (adolescence)
3. Appearance of secondary sex characteristics regulated by secretion of sex hormones 4. Growth spurt typical of adolescence; begins in girls at about 10 and in boys at about 12 (Figure 21-12)
5. Growth plates fully close in adult; other structures such as the sinuses assume adult placement
6. Adulthood characterized by maintenance of existing body tissues
7. Degeneration of body tissue begins in adulthood Continued

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Chapter 21 Growth and Development

OUTLINE SUMMARY—cont’d
H. Older adulthood (Figure 21-13)
1. Degenerative changes characterize older adulthood (also called senescence)
2. Every organ system of the body undergoes degenerative changes
3. Senescence culminates in death
EFFECTS OF AGING
A. Skeletal system
1. Aging causes changes in the texture, calcification, and shape of bones
2. Bone spurs develop around joints
3. Bones become porous and fracture easily
4. Degenerative joint diseases such as osteoarthritis are common
B. Integumentary system (skin)
1. With age, skin “sags” and becomes thin, dry, wrinkled
2. Pigmentation problems are common
3. Frequently thinning or loss of hair occurs C. Urinary system
1. Nephron units decrease in number by
50% between ages 30 and 75
2. Blood flow to kidney decreases and therefore ability to form urine decreases
3. Bladder problems such as inability to void completely are caused by muscle wasting in the bladder wall
D. Respiratory system
1. Calcification of costal cartilages causes rib cage to remain in expanded position—barrel chest
2. Wasting of respiratory muscles decreases respiratory efficiency
3. Respiratory membrane thickens; movement of oxygen from alveoli to blood is slowed E. Cardiovascular system
1. Degenerative heart and blood vessel disease is among the most common and serious effects of aging
2. Fat deposits in blood vessels (atherosclerosis) decrease blood flow to the heart and may cause complete blockage of the coronary arteries
3. Hardening of arteries (arteriosclerosis) may result in rupture of blood vessels, especially in the brain (stroke)
4. Hypertension or high blood pressure is common in older adulthood
F. Special senses
1. All sense organs show a gradual decline in performance with age
2. Eye lenses become hard and cannot accommodate for near vision; result is farsightedness in many people by age 45
(presbyopia, or “old eye”)
3. Loss of transparency of lens or cornea is common (cataract)
4. Glaucoma (increase in pressure in eyeball) is often the cause of blindness in older adulthood
5. Loss of hair cells in inner ear produces frequency deafness in many older people 6. Decreased transmission of sound waves caused by loss of elasticity of eardrum and fixing of the bony ear ossicles is common in older adulthood
7. Some degree of hearing impairment is universally present in the aged
8. Only about 40% of the taste buds present at age 30 remain at age 75

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543

N EW WORDS amniotic cavity antenatal medicine arteriosclerosis atherosclerosis blastocyst chorion ectoderm embryology

endoderm fertilization fetal alcohol syndrome
(FAS)
gerontology gestation histogenesis implantation mesoderm microcephaly morula neonate neonatology organogenesis osteoarthritis parturition primary germ layers quickening senescence teratogen ultrasonogram yolk sac

REVIEW QUESTIONS
1. Explain what occurs between ovulation and the implantation of the fertilized egg into the uterus.
2. Explain the function of the chorion and placenta. 3. Name the three primary germ layers, and name three structures that develop from each layer.
4. Define histogenesis and organogenesis.
5. Describe and give the approximate length of the three stages of labor.
6. What is the stimulus for the baby’s first breath? 7. Name three developmental changes that occur during infancy.
8. Briefly explain what developmental changes occur during childhood.
9. Briefly explain what developmental changes occur during adolescence.
10. Briefly explain what developmental changes occur during adulthood.

11. Explain the effects of aging on the skeletal system. 12. Explain the effects of aging on the respiratory system.
13. Explain the effects of aging on the cardiovascular system.
14. Explain the effects of aging on vision.

CRITICAL THINKING
15. Where do the nutrients used by the zygote from fertilization to implantation come from? 16. Explain the evolution of the function of the yolk sac.
17. What hormones are produced by the placenta? What is their function?

Continued

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Chapter 21 Growth and Development

CHAPTER TEST
1.
2.

3.

4.

5.
6.

7.

The fertilized ovum is called a __________.
After about 3 days of mitosis, the fertilized ovum forms a solid mass of cells called the __________.
Mitosis continues, and by the time the developing egg reaches the uterus, it has become a solid ball of cells called the
__________.
The __________ anchors the developing fetus to the uterus and provides a bridge for exchanging of substances between mother and baby.
The __________ period lasts about 39 weeks and is divided into trimesters.
The three primary germ layers are the
__________, __________, and the
__________.
The process by which the primary germ layers develop into tissues is called
__________.

8.
9.
10.
11.
12.
13.
14.

15.

The process by which tissues develop into organs is called __________.
The process of birth is called __________.
The first 4 weeks of infancy is referred to as the __________ period.
__________ is a degenerative joint disease that is common in older adults.
__________ is another name for “hardening of the arteries.”
__________ means “old eye” and causes older adults to be farsighted.
If the lens of the eye becomes cloudy and impairs vision, the condition is called a __________.
__________ causes an increase in pressure within the eyeball.

Match each phrase in Column B with the correct corresponding term in Column A.
COLUMN A
16. ______ infancy
17. ______ childhood
18. ______ adolescence
19. ______ adulthood
20. ______ older adulthood

COLUMN B a . period in which the deciduous teeth are lost
b. period in which closure of the bone growth plates occurs c . period that begins at birth
d. senescence e . period during which the secondary sex characteristics usually begin to develop

Chapter 21 Growth and Development

545

S TUDY TIPS—cont’d
Continued from page 521

6. The stages of labor, the important events in the postnatal periods, and the effects of aging on various organ systems can be put on flash cards also. Review them in your study group.

7. Study the questions at the end of the chapter and discuss possible test questions.