Describe the development of the amnion.
What is the function of amniotic F
Which blood vessels are in the umbilical cord?
What types of cells and other structures are derived from the
How do teratogens cause birth defects?
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
During the third month, body lengthening acceler-
ates, but growth of the head slows. The upper limbs of the
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.
illustrates how these external reproductive organs of the
male and female differentiate from precursor structures.
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
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
when it is sensitive to teratogens
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.
To Chapter 11, Brain Development, page 398.
By the beginning of the eighth week of
development, the embryonic body is recognizable as a human.
Critical periods. (
) Structures in the developing
embryo and fetus (
) are sensitive to speci±
c teratogens at di²
times in gestation.
When different teratogens disrupt development
Upper and lower limbs
Central nervous system
When physical structures develop