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Devo - Lecture 5 Embryogenesis Week 3


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What 3 major events occur during the 3rd week of embryogenesis?
1. Trilaminar disc formation
2. Primary germ layer formation
3. Beginning of organogenesis
What is the process of forming a three-layered embryo called?
What is the first step in gastrulation?
Appearance of a primitive streak
What causes the primitive streak to form?
Converging epiblast cells
Where does the primitive streak form?
At the caudal end of the epiblast, at the midline of the embryo.
What transition occurs in the converging epiblast cells?
Epithelial-mesenchymal transformation.
So now how is the embryo oriented?
Lying on its belly, with a caudal and cranial end.
How does the embryo shape change during formation of the Primitive streak?
Changes from round -> oval
What is the 2nd step in gastrulation?
Appearance of the primitive NODE.
What is the primitive node?
An inducer of development of additional embryonic structures.
What is another name for the primitive node?
Signaling center/Organizer
What tissue type is the primitive node?
Mesenchymal (transformed from epithelial like the primitive streak)
What happens after the Primitive Node appears?
Primite streak regresses.
What happens to the primitive streak as it regresses?
It regresses to the caudal end of the embryo and is called the Caudal Eminence.
What is the name for a tumor arising from persistent primitive streak tissue?
Where are teratomas commonly located, and in whom?
Sacrococcygeal location, 4x more common in women.
Where was the primitive node formed in relation to the primitive streak?
At the cranial end of the streak.
What is the prechordal plate?
An important HEAD organizing and signaling center
Where is the prechordal plate located in relation to the primitive streak and node?
At the cranial end of the embryo - opposite of the primitive streak.
What are the 2 possible fates of the transformed mesenchymal cells at the primitive streak?
1. Become Definitive Endoderm

2. Become Intraembryonic mesoderm
Recap: after migrating through the streak/node, what can happen to mesenchyme cells? (2 options)
1. REmain as mesenchyme

2. Transform into an epithelium
What are the terms for the 2 tissue options available to mesenchyme after streak/node migration?
1. Definitive endoderm

2. Intraembryonic Mesoblast
What is Definitive Endoderm?
How does it travel?
-The first cells to ingress through the primitive streak;
-Pass through at prim. node
-Cells spread out and form an epithelial roof over yolk sac; displaces hypoblast.
What is Intraembryonic Mesoblast?
-The remaining cells that ingress at the prim. streak.
-Cells spread out in different directions, form layer btwn endoderm/epiblast.
What happens after differentiation of mesenchyme at the primitive node?
What is the first step in forming the notochord?
Cells at primitive node ingress, migrate cranially in embryo's midline.
What are these cells called? What happens right after their migration?
Notochordal process; forms a lumen
What is the mesoderm in front of the notochordal process called?
Prechordal plate
What will the notochord become?
The central axis of the embryo
Why is the Notochord important?
It is an important Signaling center for dorsal-ventral patterning of the neural tube and somites.
What happens to the notochord process as it forms?
Fuses with the endoderm under it.
What is the function of the lumen that forms in the Notochordal process?
Functions as a temporary communication between amniotic cavity and 2' yolk sac
What is the name for this temporary communication?
Neurenteric CAnal
What is the fate of the notochordal process roof?
It becomes the notochordal plate
What happens to the Notochordal plate?
It infolds on itself to become the DEFINITIVE NOTOCHORD!
What happens to the primitive streak as the notochord extends?
Becomes the cranial bud - caudal eminence.
What is probably the most important thing that happens in gastrulation?
Cell migration.
What are the 3 results of gastrulation cell migration?
1. Separation of Endoderm/mesoderm from epiblast
2. Formation of 3 Primary Germ Layers
3. New spatial relationships
What are the 3 primary germ layers formed in gastrulation?
1. Ectoderm
2. Mesoderm
3. Endoderm
What is the consequence of setting up new spatial relationships?
Cell-cell and tissue interactions will occur between cell groups in the 3 new germ layers.
What 3 other significant things form around the time of notochord formation?
1. Cloacal plate
2. Oral plate
3. Allantois
What are the cloacal and oral plates?
Areas where there is ectoderm-endoderm fusion, but no mesoderm.
What is the cloacal plate?
A membrane that is the future site of the anus
What is the oral plate?
A membrane that is the future site of the mouth.
when does each form:
-cloacal plate
-oral plate
Cloacal = Day 16
Oral = Day 22
What is the allantois?
A diverticulum of the yolk sac that extends into the body stalk
What type of tissue is allantois? What will it form?
Endoderm - will form blood vessels.
So what 3 germ layers form and where?
1. Ectoderm -> original epiblast
2. Mesoderm -> middle
3. Endoderm -> replaces yolk sac covering (hypoblast)
What germ layer does neural tissue come from?

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