892
UNIT SIX
PRACTICE
16
Describe the development of the amnion.
17
What is the function of amniotic F
uid?
18
Which blood vessels are in the umbilical cord?
19
What types of cells and other structures are derived from the
yolk sac?
20
How do teratogens cause birth defects?
Fetal Stage
The
fetal stage
begins at the end of the eighth week of pre-
natal 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 dispropor-
tionately large, and the lower limbs are relatively short.
Gradually, proportions come to more closely resemble those
of a child
(f
g. 23.21)
.
During the third month, body lengthening acceler-
ates, but growth of the head slows. The upper limbs of the
fetus
(fe
tus) achieve the relative length they will maintain
throughout development, and ossiF
cation centers appear in
most bones. By the twelfth week, the external reproductive
organs are distinguishable as male or female.
Figure 23.22
illustrates how these external reproductive organs of the
male and female differentiate from precursor structures.
The
embryonic stage
concludes at the end of the eighth
week. It is the most critical period of development, because
during it, the embryo implants in 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 important.
±actors that cause congenital malformations by affect-
ing an embryo during its period of rapid growth and devel-
opment are called
teratogens.
Such agents include drugs,
viruses, radiation, and even large amounts of otherwise
healthful substances, such as fat-soluble vitamins. Each pre-
natal structure has a time in development, called its
critical
period,
when it is sensitive to teratogens
(f
g. 23.20)
.
A critical period may extend over many months or be just
a day or two. Neural tube defects, for example, are traced to
day twenty-eight in development, when a sheet of ectoderm
folds into the neural tube. When this process is disrupted, an
opening may remain in the spine (spina biF da). In contrast,
the critical period for the developing brain begins when the
anterior neural tube begins to swell into a brain, and contin-
ues throughout gestation. This is why so many teratogens
affect the brain. Clinical Application 23.1 discusses some
teratogens and their effects.
RECONNECT
To Chapter 11, Brain Development, page 398.
FIGURE 23.19
By the beginning of the eighth week of
development, the embryonic body is recognizable as a human.
FIGURE 23.20
Critical periods. (
a
) Structures in the developing
embryo and fetus (
b
) are sensitive to speci±
c teratogens at di²
erent
times in gestation.
0
Accutane
123456789
Diethylstilbestrol
Thalidomide
Month
(b)
When different teratogens disrupt development
0
Upper and lower limbs
123456789
Eyes
Ears
Reproductive system
Heart
Central nervous system
Month
(a)
When physical structures develop
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