HUMAN BIOLOGY

Development anD aging 347

What are the stages of human development?

• Following birth, development proceeds through childhood
  and adolescence, which includes the arrival of sexual maturity
  at puberty.


• Puberty is the gateway to the adult phase of life, including
  changes associated with aging.

taKe-Home message

puberty Onset of function-

ing of reproductive organs.

senescence Aging.

1 7. 1 2

cartilaginous plates near the ends

of long bones to harden into bone.

Humans stop growing by their early

twenties.

Adulthood is also a time of bodily change

Although in the United States the average life expectancy

is 74 years for males and 79 years for females, we reach

the peak of our physical potential in adolescence and early

adulthood. A healthy diet, regular exercise, and other good

lifestyle habits can help keep people vigorous for decades

of adult life. Even so, after about age 40, body parts and

their functioning begin to deteriorate. This process, called

senescence or aging, is a natural part of the life cycle of

all organ isms that have highly specialized cells. We take a

brief look at the possible causes and well-known effects of

aging in the following section.

n    A gene-dictated course of growth and development leads

to adulthood.

There are many transitions from birth to adulthood

Table 17.2 summarizes the prenatal (before birth) and

postnatal (after birth) stages of life. A newborn is called a

neonate. During infancy, which lasts until about 15 months

of age, the child’s nervous and sensory systems mature

rapidly, and a series of growth spurts makes its body

longer. Figure 17.23 shows how body proportions change

during childhood and adolescence. Puberty marks the

arrival of sexual maturity as a person’s reproductive organs

begin to function. Sex hormones trigger the appearance of

secondary sex char acteristics, such as pubic and underarm

hair and behavior changes. A mix of hormones triggers

another growth spurt at this time. Boys usually grow most

rapidly between the ages of 12 and 15, whereas girls tend

to grow most rapidly between the ages of 10 and 13. After

several years, the influence of sex hormones causes the

From Birth to adulthood

Prenatal Period

1. Zygote Single cell resulting from fertilization


2. Morula Ball of cells produced by cleavage


3. Blastocyst Ball of cells with surface layer and inner
   cell mass


4. Embryo All developmental stages from 2 weeks
   after fertilization until end of eighth week


5. Fetus All developmental stages from the ninth
   week until birth (about 39 weeks after
   fertilization)
   Postnatal Period


6. Newborn The first 2 weeks after birth
   (neonate)


7. Infancy From 2 weeks to about 15 months
   after birth


8. Childhood From infancy to about 12 or 13 years


9. Pubescence Puberty, when secondary sexual traits
   develop; girls between 10 and 16 years,
   boys between 13 and 16 years


10. Adolescence From puberty until about 3 or 4 years later;
    physical, mental, emotional maturation
    occur


11. Adulthood Early adulthood (between 18 and
    25 years); bone formation and growth
    completed; changes proceed very
    slowly afterward


12. Old age Aging culminates in general body
    deterioration

Table 17.2 Stages of Human development

2 months 3 months newborn2 513 22 years

© MitarArt/Shutterstock.com

Figure 17.23 Body proportions change during prenatal and

postnatal growth. (© Cengage Learning)

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