Devo - Lecture 5 Embryogenesis Week 3
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- What 3 major events occur during the 3rd week of embryogenesis?
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1. Trilaminar disc formation
2. Primary germ layer formation
3. Beginning of organogenesis - What is the process of forming a three-layered embryo called?
- Gastrulation
- 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?
- Teratoma
- 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?
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1. Become Definitive Endoderm
2. Become Intraembryonic mesoderm - Recap: after migrating through the streak/node, what can happen to mesenchyme cells? (2 options)
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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?
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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?
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-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?
- FORMATION OF THE NOTOCHORD.
- 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?
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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?
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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?
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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.
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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?
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1. Ectoderm -> original epiblast
2. Mesoderm -> middle
3. Endoderm -> replaces yolk sac covering (hypoblast) - What germ layer does neural tissue come from?
- Ectoderm