beating, and limb buds are visible. The period of
embryonic growth continues until the 8th week. At
this time, all of the organ systems have been estab-
lished. They will continue to grow and mature until
the end of gestation. The period of fetal growth
extends from the 9th through the 40th week. Table
21–2 lists some of the major aspects of development in
the growth of the embryo-fetus. The fetus at 16 weeks
is depicted in Fig. 21–4 (see Box 21–3: Congenital
Fetal Infections, and Box 21–4: Fetal Alcohol
Syndrome). Maternal changes during pregnancy are
summarized in Table 21–3.
PLACENTA AND UMBILICAL CORD
The placentais made of both fetal and maternal tis-
sue. The chorion of the embryo and the endometrium
of the uterus contribute, and the placenta is formed
by the third month of gestation (12 weeks). The
mature placenta is a flat disc about 7 inches (17 cm) in
diameter.
The structure of a small portion of the placenta is
shown in Fig. 21–5. Notice that the fetal blood vessels
are within maternal blood sinuses, but there is no
direct connection between fetal and maternal vessels.
Normally, the blood of the fetus does not mix with
that of the mother. The placenta has two functions: to
be the site of exchanges between maternal and fetal
blood and to produce hormones to maintain preg-
nancy. We will consider the exchanges first.
The fetus is dependent upon the mother for oxygen
and nutrients and for the removal of waste products.
The umbilical cordconnects the fetus to the pla-
centa. Within the cord are two umbilical arteries that
carry blood from the fetus to the placenta and one
umbilical vein that returns blood from the placenta to
the fetus.
When blood in the umbilical arteries enters the
placenta, CO 2 and waste products in the fetal capillar-
ies diffuse into the maternal blood sinuses. Oxygen
diffuses from the maternal blood sinuses into the fetal
capillaries; nutrients enter the fetal blood by diffusion
and active transport mechanisms. This oxygen- and
nutrient-rich blood then flows through the umbilical
vein back to the fetus. Circulation within the fetus was
described in Chapter 13.
When the baby is delivered at the end of gestation,
the umbilical cord is cut. The placenta then detaches
from the uterine wall and is delivered, with the rest of
the umbilical cord, as the afterbirth.
The blood in the remnant of the umbilical cord
contains abundant stem cells, and their potential to
become different kinds of cells is between that of
embryonic stem cells (which may become any human
cell) and adult stem cells (which are more limited).
Umbilical cord stem cells have already been used to
treat newborns with infantile Krabbe’s disease, a neu-
rological disorder that is fatal by the age of 2 years.
Such treatment depends on genetic testing and having
suitable donated cord stem cells available at birth.
Banks of cord blood are being established by some
medical centers (because such donated blood can be
frozen and stored), and the use of cord stem cells may
someday decrease the need for bone marrow trans-
plants.Placental Hormones
The first hormone secreted by the placenta is human
chorionic gonadotropin(hCG), which is produced
by the chorion of the early embryo. The function of480 Human Development and Genetics
Table 21–1 STRUCTURES DERIVED FROM
THE PRIMARY GERM LAYERSLayer Structures Derived*
EctodermMesodermEndoderm*These are representative lists, not all-inclusive ones. Keep in
mind also that most organs are combinations of tissues from
each of the three germ layers. Related structures are
grouped together.- Epidermis; hair and nail follicles; sweat
glands - Nervous system; pituitary gland; adre-
nal medulla - Lens and cornea; internal ear
- Mucosa of oral and nasal cavities; sali-
vary glands - Dermis; bone and cartilage
- Skeletal muscles; cardiac muscle; most
smooth muscle - Kidneys and adrenal cortex
- Bone marrow and blood; lymphatic tis-
sue; lining of blood vessels - Mucosa of esophagus, stomach, and
intestines - Epithelium of respiratory tract, includ-
ing lungs - Liver and mucosa of gallbladder
- Thyroid gland; pancreas