19.The answer is c.(Sadler, pp 112–117. Moore and Persaud, Developing,
pp 171–185.)Exposure of the embryo to harmful environmental factors
(teratogens), such as chemicals, viruses, and/or radiation, can occur at any
time. During the third through eighth weeks of embryonic life, organ sys-
tems are developing and are most susceptible to teratogens. During that
time, each organ system has its own specific period of peak susceptibility.
Exposure of the embryo to teratogens during the first 2 weeks of fetal life
(answers a and b)generally induces spontaneous abortion and is, there-
fore, lethal. After the eighth week of intrauterine development (answers d
and e),teratogenic exposure generally results in retardation of organ
growth rather than in new structural or functional changes.
20.The answer is e.(Sadler, pp 67, 75, 135, 136, 305. Moore and Persaud,
Developing, pp 171–172, 178, 439.)Vitamin A is a member of the retinoic acid
family. Retinoic acid directs the polarity of development in the central nervous
system, the axial skeleton (vertebral column), and probably the appendicular
skeleton. Retinoic acid induces transcription of various combinations of
homeobox genes, depending on tissue type and location (distance and direc-
tion from the source of retinoic acid). Exogenous sources of retinoic acid may
induce the wrong sequence or combination of homeobox genes, leading to
structural abnormalities in nervous and skeletal systems. The other organ sys-
tems listed are not as susceptible to vitamin A (answers a, b, c, and d).
21.The answer is a.(Sadler, pp 104, 195, 206. Moore and Persaud, Devel-
oping, pp 139–141, 143, 158, 184, 196, 251, 295, 421.)An amniotic fluid
index of 27 cm is indicative of polyhydramnios. Duodenal and/or
esophageal atresia result in an inability of the fetus to swallow amniotic
fluid(answer c).The result is that normal recirculation of amniotic fluid
through the embryo is greatly reduced or eliminated, causing an excess of
amniotic fluid. Excess amniotic fluid is defined as greater than 2000 mL in
the third trimester. Low volumes of amniotic fluid (oligohydramnios) are
caused by rupture of the fetal membranes, bilateral agenesis of the kidneys
(answer b),or obstructive uropathy (answer e,blockage of the calyces or
ureters), which prevents urine from being added to the amniotic fluid.
Hypoplasia of the lungs (answer d)and compression of the umbilical cord
are associated with oligohydramnios, but do not cause it. The presence of
adequate fluid in the uninflated lungs is essential for lung maturation, and
growth factors in the amniotic fluid may also be important. Low levels of
90 Anatomy, Histology, and Cell Biology