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Chapter 28 – Section 2
Hormonal Changes During Pregnancy
• Human chorionic gonadotropin (hCG)
• Secreted by trophoblast cells, later the chorion
• Prompts corpus luteum to continue secretion
of progesterone and estrogen
• hCG levels rise until the end of the second
month, then decline as the placenta begins to
secrete progesterone and estrogen
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Human chorionic
gonadotropin
Estrogens
Progesterone
Gestation (weeks)
Ovulation
and fertilization
Birth
Figure 28.6
Placentation
• Formation of the placenta from embryonic and
maternal tissues
1. Embryonic tissues
• Mesoderm cells develop from the inner cell
mass and line the trophoblast
• Together these form the chorion and
chorionic villi
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Placentation
2. Maternal tissues
• Decidua basalis (stratum functionalis
between chorionic villi and stratum basalis
of endometrium) develops blood-filled
lacunae
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Placentation
• The chorionic villi
• Grow into blood-filled lacunae (intervillous
spaces)
• Vascularized by umbilical arteries and veins
• Lie immersed in maternal blood
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Endometrium
Lacuna (intervillous
space) containing
maternal blood
Maternal
blood vessels
Proliferating
syncytiotrophoblast
Chorionic villus
• Ectoderm
Chorion
• Mesoderm
Amnion
• Endoderm
Forming
body stalk
Cytotrophoblast
Amniotic cavity
Bilayered
embryonic disc
• Epiblast
• Hypoblast
Endometrial
epithelium
Amniotic
cavity
Primary
germ layers
Yolk sac
Allantois
Extraembryonic
mesoderm
Chorion
being formed
Lumen of uterus
(a) Implanting 71/2-day blastocyst. (b) 12-day blastocyst. Implantation
The syncytiotrophoblast is eroding
is complete. Extraembryonic
the endometrium. Cells of the
mesoderm is forming a discrete
embryonic disc are now separated
layer beneath the cytotrophoblast.
from the amnion by a fluid-filled
space.
Extraembryonic
coelom
(c) 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 (a-c)
Placentation
• Placenta is fully formed and functional by the
end of the third month
• Placenta also secretes human placental
lactogen, human chorionic thyrotropin, and
relaxin
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Decidua basalis
Maternal blood
Chorionic villus
Umbilical blood
vessels in
umbilical cord
Amnion
Amniotic cavity
Yolk sac
Extraembryonic
coelom
Lumen
of uterus
Chorion
Decidua
capsularis
(d) 41/2-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 now receiving its nutrition via the umbilical vessels
that connect it (through the umbilical cord) to the placenta.
Figure 28.7d
Placenta
Decidua basalis
Chorionic villi
Yolk sac
Amnion
Amniotic
cavity
Umbilical
cord
Decidua
capsularis
Uterus
Extraembryonic
coelom
Lumen of
uterus
(e) 13-week fetus.
Figure 28.7e
Placentation
• Maternal and embryonic blood supplies do not
intermix
• Embryonic placental barriers include:
• Membranes of the chorionic villi
• Endothelium of embryonic capillaries
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