The Placenta and the Embryo
Per Brodal
Institutt for medisinske basalfag
Universitetet i Oslo
9th Semester 2008
Week 12
Approx. 6 cm Crown
Rump (CR) length
England 1983
P. Brodal 2008
2
Week 12 (approx.)
Placenta
Fetal
membranes
England 1983
P. Brodal 2008
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Week 7
Placental villi
Amnion
P. Brodal 2008
England 1983
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Placenta
Larsen, 3d Edition
P. Brodal 2008
Placental villi
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The First Week
Day 4:
Morula
Shedding of the
zona pellucida
Day 5:
Blastocyst
Day 6:
Implantation
Fertilization
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Implantation – 6th day
Endometrium
Inner cell
mass: gives
rise to the
embryo
Trophoblast start to
invade the
endometrium
Outer cell mass
(trophoblast):
gives rise to the
placenta
Langman, 8 Ed.
P. Brodal 2008
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Chorion and Decidua
(End of Second Month)
Decidua
parietalis
Decidua
capsularis
Decidua basalis
Chorion
frondosum (villi)
Amniotic cavity
Chorion laeve
Chorion: forms fetal
part of the placenta
Langman, 8 Ed.
P. Brodal 2008
Decidua: forms
maternal part of the
placenta
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Decidua – Transformed Cells of the
Endometrial Stroma
Decidua parietalis
Decidua capsularis
 large, round, epithelial-like
cells
 secrete nutrients and
regulatory substances
 Important for successful
placentation
P. Brodal 2008
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7th day
Endometrium
changes: Decidual
reaction
Syncytiotrophoblast
Cytotrophoblast
Amnioblasts
Ectodermal cells
Langman, 8 Ed.
P. Brodal 2008
Entodermal cells
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9th day: Further Development of the
Placenta
Maternal blood vessels
Lacunae in the
syncytiotrophopblast
Cytotrophoblast
P. Brodal 2008
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12 days
Maternal sinusoids
connect with
lacunae in the
trophoblast
Primitive
yolk sac
Endometrial
epithelium
Langman, 8 Ed.
P. Brodal 2008
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13 days
Amniotic cavity
Trophoblastic lacunae
present all around
the embryo
Yolk sac
Extra-embryonal
coelom (chorionic
cavity)
Sometimes a small
bleeding occurs at
this stage
Langman, 8 Ed.
P. Brodal 2008
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End of Third Week
Cytotrophoblast
Syncytiotrophoblast
The cytotrophoblast
grows through the
syncytiotrophoblast
to form an external
shell
Langman, 8 Ed.
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The Cytotrophoblastic Shell (End of 3d
Week)
Cytotrophoblasts
Syncytiotrophoblast
Umbilical cord
Langman, 8 Ed.
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Placental Villi – at Fourth Week
and Fourth Month
Maternal blood vessels
Cytotrophoblast
shell
Syncytiotrophoblast
Decidua
(endometrium)
Intervillous
space
(maternal
blood)
Villous blood vessels
(fetal)
P. Brodal 2008
Villus with extra-embryonal
mesoderm
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Development of the Villi
Primary villus
Secondary villus
Tertiary villus (end of third week)
Diffusion barriere
Langman, 8 Ed.
Early
P. Brodal 2008
Late
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Foeto-Placental Circulation (End of 4th
Week)
Cardinal veins
(anterior and
posterior)
Dorsal aorta
Umbilical artery (2)
Placental
villi
Vitelline artery
Yolk sac
P. Brodal 2008
Umbilical vein (1)
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The Placenta at Term
Decidual plate
Amnion
P. Brodal 2008
Decidual septum
Spiral artery
Chorionic plate
Umbilical cord
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The Placenta at Term
15-25 cm diameter, 500-600 g, 3 cm thick
Exchange area: ~ 10 m2
Blood flow: 600 ml/min
Cotyledon
Amnion
P. Brodal 2008
Umbilical cord
Decidual layer
removed
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Extravillous Trophoblast
 Trophoblastic cells growing as
columns into the decidua,
forming anchoring villi
 Invading also the uterine arteries
– eroding the media and forming
the intima
 Important for maintaining
perfusion of the placenta
 Inadequate formation: related to
preeclampsia
 Ecxessive: invasive growth, e.g.
choriocarcinoma
P. Brodal 2008
Langman, 8 Ed.
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The Fetal Membranes (End of 4th Month)
Fusion of amnion,
chorion laeve,
decidua capsularis
and decidua parietalis
Fusion of amnion,
chorion laeve and
decidua capsularis
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Fetal Membranes
Amnion
(vannhinnen)
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Fetal membranes – Dizygotic twins
2 cell zygote
Fused chorions
Langman
P. Brodal 2008
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Fetal membranes - Monozygotic Twins
2-cell
zygote
Split at 2-cell
stage;
separate
chorion and
amnion
Split of inner
cell mass
early; share
chorion,
separate
amnion
Split of inner
cell mass
late; share
chorion and
amnion
Langman
P. Brodal 2008
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Formation of The Umbilical Cord
Amnion
Yolk sac
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Bilaminar germ disk (2d week)
Amnion
Ectoderm
Entoderm
Langman, 8 Ed.
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Formation of the Mesoderm
(2d-3d Week)
Ectoderm
Entoderm
Mesoderm
Week 3
Langman, 8 Ed.
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Third week – Trilaminated Disc
Amniotic cavity
Amnion
Ectoderm
Intra-embryonic
coelom (starting
to form)
Mesoderm
(paraxial)
Entoderm
Langman
P. Brodal 2008
Yolk sac
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Foldings Determine the Shape of the
Embryo (Fourth week)
Neural tube
Amniotic cavity
Intermediate
mesoderm
(nephrotomes)
Paraxial mesoderm
(somit)
Intraembryonic
coelom
Midgut
Yolk sac
P.Langman
Brodal 2008
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Some Landmarks in Embryonic
Development
 2d-3d week: The trilaminar germ disc formed
 3d week: The heart develops, links up with
the vascular system
 4th week: Formation of the gut
 6th week: Urogenital sinus formed
 7th week: most organs have formed
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Some Landmarks in Fetal Development (weeks
after fertilization)
 6 weeks ~ 2 cm CR-length; nose, ears, fingers
identifiable
 10 weeks ~ 6 cm; external genitals formed but
undifferentiated
 14 weeks ~ 12 cm; external genitals can be
differentiated, skin transparent red
 18 weeks ~ 15 cm; fine hairs (lanugo) covers the
body, skin opaque
 22 weeks ~ 21 cm; eyebrows and fingernails
present
 26 weeks ~ 26 cm; eyes open, scalp hair growing
Llewellyn-Jones 7th Ed.
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