Reproduction
631
The epithelial layer lining the cavity is derived from the inner cell
mass and is called the
amnion,
or
amniotic sac.
It eventually
fuses with the inner surface of the chorion so that only a single
combined membrane surrounds the fetus. The fl uid in the amni-
otic cavity, the
amniotic fl
uid,
resembles the fetal extracellular
uid, and it buffers mechanical disturbances and temperature
variations.
The fetus, fl oating in the amniotic cavity and attached
by the umbilical cord to the placenta, develops into a viable
infant during the next eight months. Note in Figure 17–28
that eventually only the amniotic sac separates the fetus from
the uterine lumen.
Amniotic fl uid can be sampled by
amniocentesis
as early
as the sixteenth week of pregnancy. This is done by insert-
ing a needle into the amniotic cavity. Some genetic diseases
can be diagnosed by the fi nding of certain chemicals either
in the fl uid or in sloughed fetal cells suspended in the fl
uid.
The chromosomes of these fetal cells can also be examined for
diagnosis of certain disorders as well as to determine the sex
of the fetus. Another technique for fetal diagnosis is
chorionic
villus sampling
.
This technique, which can be performed as
early as 9 to 12 weeks of pregnancy, involves obtaining tis-
sue from a chorionic villus of the placenta. This technique,
however, carries a higher risk of inducing the loss of the fetus
Gland in
endometrium
Endometrium
Myometrium
Uterine vein
and artery
Branch of
umbilical
artery and vein
Umbilical
vein (to fetus)
Umbilical
arteries (from fetus)
Umbilical cord
to fetus
Main stem of
chorionic villus
Chorionic villi
Chorion
Pool of
maternal blood
Placenta
Amniotic
cavity
Figure 17–27
Interrelations of fetal and maternal tissues in the formation of the
placenta. See Figure 17–28 for the orientation of the placenta.
From B. M. Carlson,
Patten’s Foundations of Embryology,
5th ed., McGraw-Hill, New York, 1988.
Myometrium
Endometrium
Cervix
Embryo
Embryo
(a)
Chorion
Amnion
Amniotic
cavity
Cervical
glands
Yolk sac
Cervical
canal
Placenta
Chorion
(c)
(b)
Figure 17–28
The uterus at (a) 3, (b) 5, and (c) 8 weeks after fertilization.
Embryos and their membranes are drawn to actual size. Uterus
is within actual size range. The yolk sac is formed from the
trophoblast. It has no nutritional function in humans but is
important in embryonic development.
From B. M. Carlson,
Patten’s Foundations of Embryology,
5th ed., McGraw-Hill, New York, 1988.
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