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The audio narrations of these slides have been transcribed verbatim and can be downloaded from
http://www.anat.sunysb.edu/HBA531/Embryology/
Early Embryogenesis - Week 3
2 weeks old
extraembryoniccoelom
amnioticcavity
connectingstalk
remants ofprimary yolksac
yolk sac
amnion
prochordalplate
primitivestreak
Jack Stern 2003
cranial (head)
caudal (tail)
left right
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Jack Stern 2003
amnioticcavity
yolk sac
amnionprimitivestreak
connectingstalk
extraembryoniccoelom
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extraembryoniccoelom
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primitivegroove
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intraembryonicmesoderm extraembryonic
mesoderm
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nonproliferatingepiblastectoderm
amnioblast
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endoderm
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Jack Stern 2003
prochordal plate(oropharyngeal membrane)
primitivepit
primitivegroove
primitive(Hensen’s)node
primitivestreak
ectoderm
cloacal membrane Jack Stern 2003
Migration of Intraembryonic Mesoderm
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endoderm
intraembryonicmesoderm ectoderm
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endoderm ofcloacal membrane
endoderm oforophrayngealmembrane
intraembryonicmesoderm
prechordalmesoderm notochord
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Jack Stern 2003
primitivenode
primitivepit
prechordalmesoderm
notochord intraembryonicmesoderm coming from primitive streak
cloacalmembrane
orophrayngealmembrane
connectingstalk
15-16 days
amnion
yolk sac
amnioticcavity
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connectingstalk
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allantoicdiverticulum
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grow
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grow
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notochord
intraembryonicmesoderm from primitive streak
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notochord
intraembryonicmesoderm from primitive streak
endodermof prochordalplate
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amniotic cavity
yolk sac
extraembryonicmesodermnotochord
intraembryonicmesoderm
15-16 days
mesenchyme
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paraxial
lateralplate
intermediate
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neural folds
neural groove
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neural plate
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grow
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grow
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Changes in Intraembryonic
Mesoderm During Third
Week
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amniotic cavity
yolk sac
15-16 days
paraxial
lateralplate
intermediate
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lateral platemesoderm
paraxialmesoderm
notochord
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lateral platemesoderm
paraxialmesoderm
notochord
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lateral platemesoderm
paraxialmesoderm
notochord
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20 days
somites
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somite
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somiteamniotic cavity
yolk sac
notochord
lateralplate
microdropletsof intercellularfluid
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microdropletsof intercellularfluid
microdropletsof intercellularfluid
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intraembryonic coelom
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left channel ofthe intraembryoniccoelom
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right channel ofthe intraembryoniccoelom
mesothelium mesothelium
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mesothelium
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notochord allantoicdiverticulum
neural fold
bend at the frontof the intraembryoniccoelom
cardiogenic mesoderm
amniotic acvity
yolk sac
primitivestreak
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3D View of Mesothelial Wall of Coelom
oropharyngeal membrane
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heart tubes(form at 18-19 days)
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bend at thefront of theintraembryoniccoelom
amniotic cavity
yolk sacheart tubes
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oropharyngealmembrane
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aorta (left)
aortic arch (left)umbilical arteries
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amniotic cavity
yolk sacaortae
intraembryoniccoelom
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Two Days After Heart Tubes Form
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bend at thefront of theintraembryoniccoelom
amniotic cavity
yolk sacheart tube
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A Little Later
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amniotic cavity
yolk sacaorta
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vitelline vein
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umbilical vein
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cardinal vein
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21 days
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cardinal vv.
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Changes in Neural Folds During Third
Week
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crests of neural folds
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neural tubeneural crest
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Neural tube closure begins on the first day of the 4th week (i.e., day 22). It starts in the middle of the embryo and sweeps forward toward the head and backward toward the tail. It takes 2 –3 days to reach the front of the brain and another couple of days to reach the tip of the tail, so it is completed by the end of the 4th week.
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One More Change in the Coelom
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amniotic cavity
yolk sacintraembryoniccoelom
?
?
?
?
?
?
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extraembryoniccoelom
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3D View of of Coelom
oropharyngeal membrane
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Bottom LineAt the end of the 3rd Week
• embryonic disc is still flat, but now it is much longer than wide, and it has three layers rather than two
• fundamental segmentation of the embryo has begun, as seen in somites and neural crest clumps
• circulation of blood has begun
• a U-shaped intraembryonic coelom has formed and its side arms open to the extraembryonic coelom in the region caudal to the oropharyngeal membrane
• neural folds are just beginning to touch in the vicinity of what will become the neck
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