Chapter 28
Pregnancy and Human Development
1073
28
Te placenta secretes hCG from the beginning, but the abil-
ity of its syncytiotrophoblast cells (the “hormone manufactur-
ers”) to produce the estrogens and progesterone of pregnancy
matures much more slowly. If, for some reason, placental hor-
mones are inadequate when hCG levels wane, the endometrium
degenerates and the pregnancy is aborted. Troughout preg-
nancy, blood levels of estrogens and progesterone continue to
increase (see Figure 28.6). Tey encourage growth and further
month of pregnancy. However, well before this time, oxygen
and nutrients are diffusing from maternal to embryonic blood,
and embryonic metabolic wastes are passing in the opposite di-
rection. Te barriers to free passage of substances between the
two blood supplies are embryonic barriers—the membranes of
the chorionic villi and the endothelium of embryonic capillar-
ies. Although the maternal and embryonic blood supplies are
very close, they normally do not intermix
(Figure 28.8)
.
4
-week embryo.
The decidua capsularis, decidua basalis, amnion, and yolk
sac are well formed. The chorionic villi lie in blood-filled intervillous spaces
within the endometrium. The embryo is nourished via the umbilical vessels that
connect it (through the umbilical cord) to the placenta.
Decidua basalis
Maternal blood
Chorionic villus
Umbilical blood
vessels in
umbilical cord
Amnion
Amniotic cavity
Yolk sac
Extraembryonic
coelom
Chorion
Decidua
capsularis
Lumen
of uterus
(d)
1
/
2
Placenta
Decidua basalis
Chorionic villi
Yolk sac
Amnion
Amniotic
cavity
Umbilical
cord
Uterus
Lumen of
uterus
Decidua
capsularis
Extraembryonic
coelom
(e) 13-week fetus.
Maternal
blood vessels
Lacuna (intervillous
space) containing
maternal blood
Endometrium
Amniotic
cavity
Primary
germ layers
• Ect
oderm
Chorionic villus
Chorion
Amnion
Yolk sac
Extraembryonic
mesoderm
Chorion
being formed
Lumen of uterus
Proliferating
syncytiotrophoblast
Cytotrophoblast
Amniotic cavity
Bilayered
embryonic disc
• Epib
last
• Hypob
last
Endometrial
epithelium
Implanting 7
-day blastocyst.
The
syncytiotrophoblast is eroding the
endometrium. Cells of the embryonic
disc are now separated from the
amnion by a fluid-filled space.
(a)
(b)
(c)
• Mesoder
m
• Endoder
m
Forming
umbilical
cord
Allantois
Extraembryonic
coelom
1
/
2
12-day blastocyst.
Implantation is
complete. Extraembryonic mesoderm
is forming a discrete layer beneath the
cytotrophoblast.
16-day embryo.
Cytotrophoblast and associated
mesoderm have become the chorion, and chorionic
villi are elaborating. The embryo exhibits all three
germ layers, a yolk sac, and an allantois, which
forms the basis of the umbilical cord.
Figure 28.7
Events of placentation, early embryonic development, and extraembryonic
membrane formation.
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