By the end of the second week,
The embryonic disc initially consists of two distinct
layers: an outer
and an inner
time later, through a process called gastrulation, a third
layer of cells, the
forms between the ectoderm
and endoderm. These three layers of cells are called the
primary germ layers
ma-re jerm la
erz) of the primor-
dial embryo. They are the primitive tissues from which all
organs form. At this point, the embryo is termed a
troo-lah). Also during this time, a structure called a con-
necting stalk appears. It attaches the embryo to the develop-
summarizes the stages
of early human prenatal development.
Gastrulation is an important process in prenatal develop-
ment because a cell’s fate is determined by which layer it is
in. The cells of the ectoderm and endoderm are epithelia. The
mesoderm is loosely organized connective tissue.
give rise to the nervous system, parts of special sensory
organs, the epidermis, hair, nails, glands of the skin, and lin-
ings of the mouth and anal canal.
types of muscle tissue, bone tissue, bone marrow, blood, blood
vessels, lymphatic vessels, internal reproductive organs, kid-
neys, and the mesothelium of the body cavities.
produce the epithelial linings of the digestive tract, respi-
ratory tract, urinary bladder, and urethra
(f g. 23.11)
. The pri-
mary germ layers retain stem cells, a few of which persist in
the adult, enabling tissues to grow and repair damage.
As the embryo implants in the uterus, proteolytic
enzymes from the trophoblast break down endometrial
tissue, providing nutrients for the developing embryo.
A second layer of cells begins to line the trophoblast, and
together these two layers form a structure called the
re-on). Soon, slender projections grow out from the tro-
phoblast, including the new cell layer, eroding their way into
the surrounding endometrium by continuing to secrete prote-
olytic enzymes. These projections become increasingly com-
plex and form the highly branched
well established by the end of the fourth week. Figure 24.13
depicts prenatal genetic tests performed on chorionic villi or
shed fetal skin cells.
Continued secretion of proteolytic enzymes forms
irregular spaces called
in the endometrium
around and between the chorionic villi. These spaces
fill with maternal blood that escapes from endometrial
blood vessels eroded by enzyme action. At the same time,
embryonic blood vessels carrying blood to and from the
embryo extend through the connecting stalk and establish
capillary networks in the developing chorionic villi. These
embryonic vessels allow nutrient exchange with blood in
the lacunae, meeting the increased nutrient demands of
the growing embryo.
During the fourth week of development, the ﬂ at embry-
onic disc becomes cylindrical, and the precursor of the cen-
tral nervous system, called the neural tube, forms. By the end
of week four, the head and jaws appear, the heart beats and
Nausea and vomiting in pregnancy may shield a fetus from foods that
might contain toxins or pathogens. The condition aF
ects two in three
pregnancies and coincides with the time in gestation when a woman’s
immune system is at its weakest. An analysis of more than 80,000 preg-
nant women found that they avoid foods that spoil easily, such as eggs
and meats, as well as coF
ee and alcohol. Yet many pregnant women
eat more fruits and vegetables than usual. In societies where the diet
is mostly grains with little if any meat, incidence of morning sickness is
much lower than in groups with more varied, and possibly dangerous,
diets. Rates of morning sickness are highest in Japan, where raw ±
a dietary staple, and European countries, where undercooked meat is
often eaten. Evolution has likely selected for “morning sickness” where
it correlates to, and possibly contributes to, better birth outcomes.
extends from the beginning of the
second week through the eighth week of prenatal develop-
ment. During this time, the placenta forms, the main inter-
nal organs develop, and the major external body structures
During the second week of prenatal development, the
blastocyst completes implantation, and the inner cell mass
changes. A space, the
forms between the
inner cell mass and the part of the trophoblast that “invades”
the endometrium. The inner cell mass then flattens and is
1. ²ollowing implantation, cells of the trophoblast begin to secrete hCG.
2. hCG maintains the corpus luteum, which continues to secrete
estrogens and progesterone.
3. As the placenta develops, it secretes abundant estrogens and
4. Placental estrogens and progesterone
a. stimulate the uterine lining to continue development.
b. maintain the uterine lining.
c. inhibit secretion of ²SH and LH from the anterior pituitary gland.
d. stimulate development of the mammary glands.
e. inhibit uterine contractions (progesterone).
f. enlarge the reproductive organs (estrogens).
5. Relaxin from the corpus luteum also inhibits uterine contractions and
relaxes the pelvic ligaments.
6. The placenta secretes placental lactogen that stimulates breast
7. Aldosterone from the adrenal cortex promotes reabsorption of sodium.
8. Parathyroid hormone from the parathyroid glands helps maintain a
high concentration of maternal blood calcium.