Figure 23.15

The developing placenta, composed of chorionic and endometrial capillaries, as it appears during the seventh week of development.

because during it, the embryo implants within the uterine wall, and all the essential external and internal body parts form. Disturbances to development during the embryonic stage can cause major malformations or malfunctions. This is why early prenatal care is very important.

Factors that cause congenital malformations by affecting an embryo during its period of rapid growth and development are called teratogens. Such agents include drugs, viruses, radiation, and even large amounts of otherwise healthful substances, such as fat-soluble vitamins. Each prenatal structure has a time in development, called its critical period, when it is sensitive to teratogens (fig. 23.17).

A critical period may extend over many months or be just a day or two. Neural tube defects, for example, are traced to day 28 in development, when a sheet of ectoderm called the neural tube normally folds into a tube, which then develops into the central nervous system. When this process is disrupted, an opening remains in the spine (spina bifida) or in the brain (anencephaly).

In contrast, the critical period for the brain begins when the anterior neural tube begins to swell into a brain, and continues throughout gestation. This is why so many teratogens affect the brain. Clinical Application 23.2 discusses some teratogens and their effects.

> Reconnect to chapter 11, Brain Development, page 411.

O Describe the development of the amnion.

^9 Which blood vessels are in the umbilical cord?

^9 What is the function of amniotic fluid?

Q What types of cells and other structures are derived from the yolk sac?

Q How do teratogens cause birth defects?


By the beginning of the eighth week of development, the embryonic body is recognizable as a human.

Fetal Stage

The fetal stage begins at the end of the eighth week of development and lasts until birth. During this period, growth is rapid, and body proportions change considerably. At the beginning of the fetal stage, the head is disproportionately large, and the lower limbs are relatively short (fig. 23.18). Gradually, proportions come to more closely resemble those of a child.

During the third month, body lengthening accelerates, but growth of the head slows. The upper limbs of the fetus (fe'tus) achieve the relative length they will maintain throughout development, and ossification centers appear in most of the bones. By the twelfth week, the external reproductive organs are distinguishable as male or female. Figure 23.19 illustrates how these external reproductive organs of the male and female differentiate from precursor structures.

In the fourth month, the body grows very rapidly and reaches a length of up to 20 centimeters and weighs about 170 grams. The lower limbs lengthen considerably, and the skeleton continues to ossify. The fetus has hair, nipples, and nails, and may even scratch itself.

In the fifth month, growth slows. The lower limbs achieve their final relative proportions. Skeletal muscles contract, and the pregnant woman may feel fetal movements for the first time. Some hair grows on the fetal head, and fine, downy hair called lanugo covers the skin. A cheesy mixture of sebum from the sebaceous glands and dead epidermal cells (vernix caseosa) also coats the skin. The fetus, weighing about 450 grams and about 30 centimeters long, curls into the fetal position.

By the beginning of the eighth week of development, the embryonic body is recognizable as a human.

(a) When physical structures develop

Essentials of Human Physiology

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