week of twos, formation of two layers 1. Epiblast 2. Hypoblast two cavities 1. amniotic cavity 2. exocoelomic cavity two trophoblast divides into 1. cytotrophoblast 2. syncytiotrophoblast

1. Second week of intrauterine life
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2. Learning objectives
• Enlist the events of 2nd week of development
• Define ectopic pregnancy
• Enumerate the different sites of ectopic
pregnancy
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4. Bilaminar embryonic disc
• Epiblast
• Hypoblast
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6. Trophoblast
• Cytotrophoblast
• Syncytiotrophoblast
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7. hCG
• Syncytiotrophoblast produces this hormone
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8. Amniotic cavity
• A small cavity in the embryoblast
• Amnioblasts- amnion forming cells
• Amnion- thin membrane enclosing amniotic
cavity
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9. Exocoelomic cavity
• Exocoelomic membrane
• It lines the inner surface of the cytotrophoblast
• Membrane and cavity modify to form the
primary umbilical vesicle
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12. Extraembryonic mesoderm
• The outer layer of the cells from the umbilical
vesicle endoderm form a layer of loosely
arranged CT, the extraembryonic mesoderm,
which surrounds the amnion and the umbilical
vesicle (yolk sac)
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13. Lacunae
• Isolated cavities appear in the syncytiotrophoblast-
the lacunae
• Fluid in the lacunae called embryotroph
• Passes to the embryonic disc by diffusion
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15. Uteroplacental circulation
• Communication of the eroded uterine vessels
with the lacunae
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16. Extraembryonic coelom
• As changes occur in the trophoblast and the
endometrium the extraembryonic mesoderm
increases and isolated extraembryonic
coelomic spaces appear within it
• These spaces rapidly fuse to form a large,
isolated cavity, the extraembryonic coelom
• Fluid filled cavity
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17. Connecting stalk
• Implanted embryo is attached to the chorion
by connecting stalk
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20. Secondary umbilical vesicle
• It may have a role in the transfer of nutritive
materials to the embryonic disc
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21. Development of the chorionic sac
• Extraembryonic somatic mesoderm and the
two layers of trophoblast form the chorion
• The embryo and its amniotic sac and umbilical
vesicle are suspended in the chorionic cavity
by the connecting stalk
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