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

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cranial (head)

caudal (tail)

left right

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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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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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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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