This chapter describes how the human organism grows and develops from fertilization through death. The following stages of life are described in detail: germinal stage, embryonic stage, fetal stage, infancy, childhood, adolescence, and adulthood.
- 23.1: Case Study: How Our Bodies Change Throughout Life
- Paul and Vanessa are shocked to discover that their toddler Lucas' blood lead level is 10 µg/dL, which is considered high. Since Vanessa is three months pregnant, they are worried about whether Vanessa was also exposed to lead. If so, what effects could it have on the developing baby?
- 23.2: Germinal Stage
- The germinal stage of development is the first and shortest of the stages of the human lifespan. The main events in this stage of development are illustrated in the figure below and described in detail in the rest of this concept. The germinal stage lasts a total of eight to nine days. It begins in a Fallopian tube when an ovum is fertilized by a sperm to form a zygote (day 0). The germinal stage continues as the zygote undergoes several initial cell divisions to a morula.
- 23.3: Embryonic Stage
- In many cultures, marriage - along with birth and death - is considered the most pivotal life event. For pioneering developmental biologist Lewis Wolpert, however, these life events are overrated. According to Wolpert, "It is not birth, marriage, or death, but gastrulation, which is truly the most important time in your life." Gastrulation is a major biological event that occurs early in the embryonic stage of human development.
- 23.4: Fetal Stage
- This mother-to-be is holding an ultrasound image of her fetus. She is nearly nine months pregnant, so the fetus is fully developed and almost ready to be born. The fetus has grown tremendously and changed in many other ways since it was a tiny embryo seven months previously.
- 23.5: Infancy
- Infancy refers to the first year of life after birth, and an infant is defined as a human being between birth and the first birthday. The term baby is usually considered synonymous with infant, although it is commonly applied to the young of other animals, as well as humans. Human infants seem weak and helpless at birth, but they are actually born with a surprising range of abilities. Most of their senses are quite well developed, and they can also communicate their needs by crying.
- 23.6: Childhood
- Legally, childhood is defined as the period of minority, which lasts from birth until adulthood (majority). The age of majority varies by place and purpose. For example, in the United States, at age 18, you are considered an adult for military service, but a minor for buying alcohol. Biologically, childhood is defined as the stage of a human organism between birth and adolescence.
- 23.7: Adolescence and Puberty
- Adolescence is the period of transition between childhood and adulthood. It is generally considered to start with puberty, during which sexual maturation occurs and adolescents go through a spurt in growth. In many children, however, puberty actually begins during the stage called pre-adolescence, which covers the ages 11 to 12 years. Puberty may begin before adolescence, but it usually continues for several years, well into the adolescent stage, which ends during the late teens.
- 23.8: Adulthood
- This family image includes an elderly woman and her young-adult daughters and granddaughters from the Hmong ethnic group in Laos. Grandmother and daughters are adults, but they are obviously far apart in age. What ages define the beginning and end of adulthood?
- 23.9: Case Study Conclusion: Lead Danger and Chapter Summary
- Earlier in this chapter, you met Paul, Vanessa, and Lucas, who were concerned by elevated levels of lead in Lucas' blood. Many experts agree that preventing lead exposure and more widespread blood lead level screening is critical to prevent permanent damage to children’s health. Infancy and early childhood is a wonderful time of tremendous growth and change in a person’s lifespan, but it is also a time that is highly vulnerable to damage—with potential lifelong consequences.
Thumbnail: Father and son on a beach via Pixabay