Fertilization and Preembryonic Development

Objectives

When you have completed this section, you should be able to

• describe sperm migration, capacitation, fertilization, and the egg's barriers to additional sperm;
• describe the major events that transform a fertilized egg into an embryo; and
• describe the implantation of the preembryo in the uterine wall.

Think About It

What similarity can you see between the slow block to polyspermy and the release of acetylcholine from synaptic vesicles of a neuron? (Compare p.450.)

Key Point Review

1. How soon can a sperm reach an egg after ejaculation? How soon can it fertilize an egg? What accounts for the difference?

2. Describe two ways a fertilized egg prevents the entry of excess sperm.

3. In the blastocyst, what are the cells called that eventually give rise to the embryo? What are the cells that are responsible for implantation?

4. What major characteristic distinguishes an embryo from a preembryo?

Embryonic and Fetal Development

Objectives

When you have completed this section, you should be able to

• describe the formation and functions of the placenta;
• explain how the conceptus is nourished before the placenta takes over this function;
• describe the four embryonic membranes and their functions;
• itemize the major tissues derived from the primary germ layers;
• describe the major events of fetal development; and
• explain how blood circulation differs before and after birth.

Think About It

List the four primary tissue types of the adult body (see chapter 6) and identify which of the three primary germ layers of the embryo predominantly gives rise to each.

Key Point Review

5. Distinguish between trophoblastic and placental nutrition.

6. Identify the two sources of blood to the placenta. Where do these two bloodstreams come closest to each other? What keeps them separated?

7. State the functions of the placenta, amnion, chorion, yolk sac, and allantois.

8. What developmental characteristic distinguishes a fetus from an embryo? At what gestational age is this attained?

9. Identify the three circulatory shunts of the fetus. Why does the blood take these "shortcuts" before birth?

The Neonate

Objectives

When you have completed this section, you should be able to

• describe how and why the circulatory system changes at birth;
• explain why the first breaths of air are relatively difficult for a neonate;
• describe the major physiological problems of a premature infant; and
• discuss some common causes of birth defects.

Key Point Review

10. How does inflation of the lungs at birth affect the route of blood flow through the heart?

11. Why is respiratory distress syndrome common in premature infants?

12. Define nondisjunction and explain how it causes aneuploidy. Name two syndromes resulting from aneuploidy.

Aging and Senescence

Objectives

When you have completed this section, you should be able to

• define senescence and distinguish it from aging;
• describe the major senescent changes that occur in each organ system;
• summarize some current theories of senescence; and
• be able to explain how exercise and other factors can slow the rate of senescence.

Think About It

Draw a positive feedback loop showing how anemia and senescence of the kidneys could affect each other.

Key Point Review

13. Define aging and senescence.

14. List some tissues or organs in which changes in collagenous and elastic connective tissues lead to senescence.

15. Many older people have difficulty with mobility and simple self-maintenance tasks such as dressing and cooking. Name some organ systems whose senescence is most relevant to these limitations.

16. Explain why both endurance and resistance exercises are important in old age.

17. Summarize any five cellular mechanisms that may be responsible for senescence.

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