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Part 1
General Embryology
Problems to Solve
1.
A 22-year-old woman consumes large
quantities of alcohol at a party and loses
consciousness; 3 weeks later, she misses her
second consecutive period.A pregnancy test
is positive. Should she be concerned about
the effects of her binge-drinking episode on
her baby?
2.
An ultrasound scan detects a large mass near
the sacrum of a 28-week female fetus.What
might the origin of such a mass be, and what
type of tissue might it contain?
3.
On ultrasound examination, it was deter-
mined that a fetus had well-developed facial
and thoracic regions, but caudal structures
were abnormal. Kidneys were absent, lumbar
and sacral vertebrae were missing, and the
hindlimbs were fused.What process may have
been disturbed to cause such defects?
4.
A child has polysplenia and abnormal posi-
tioning of the heart. How might these two
abnormalities be linked developmentally, and
when would they have originated? Should
you be concerned that other defects might
be present?What genes might have caused
this event, and when during embryogenesis
would it have been initiated?
by cells in the node and streak, induces
Nodal
and
LEFTY-2
expression on the left side and these
genes upregulate
PITX2
, a transcription factor
and master gene for left-sidedness (Fig. 5.6).The
neurotransmitter serotonin (5HT)
also plays
a role as a signal molecule upstream from FGF8.
Disruption of 5HT levels or misexpression of
PITX2
results in laterality defects, such as dextro-
cardia, situs inversus, and cardiac abnormalities.
Epiblast cells moving through the node and
streak are predetermined by their position to
become specific types of mesoderm and endo-
derm.Thus, it is possible to construct a fate map
of the epiblast showing this pattern (Fig. 5.7).
By the end of the third week,three basic
germ
layers
, consisting of
ectoderm
,
mesoderm
, and
endoderm
, are established in the head region,
and the process continues to produce these germ
layers for more caudal areas of the embryo until
the end of the fourth week.Tissue and organ dif-
ferentiation has begun, and it occurs in a cepha-
locaudal direction as gastrulation continues.
In the meantime, the trophoblast progresses
rapidly.
Primary villi
obtain a mesenchymal
core in which small capillaries arise (Fig. 5.12).
When these villous capillaries make contact with
capillaries in the chorionic plate and connect-
ing stalk, the villous system is ready to supply the
embryo with its nutrients and oxygen (Fig. 5.13).
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