Unit 3: Gastrulation

Question 1

Define gastrulation

Answer 1

A process where a single layer gives rise to 3 distinct layers

Question 2

What 3 germ layers do vertebrates have ?

Answer 2

1) Ectoderm
2) Mesoderm
3) Endoderm

Question 3

What is the ectoderm ?

Answer 3

The outside layer

Question 4

What is the mesoderm ?

Answer 4

The middle layer

Question 5

What is the endoderm ?

Answer 5

The inner layer

Question 6

How many layers does the blastula embryo have and what are they ?

Answer 6

2 layers
- Epiblast
- Hypoblast

Question 7

Does the epiblast give rise to any germ layers, if so how many ?

Answer 7

Epiblast gives rise to all 3 germ layers

Question 8

Does the hypoblast give rise to any germ layers, if so how many ?

Answer 8

Hypoblast does not contribute to germ layers

Question 9

How does the blastula form a gastrula (2) ?

Answer 9

• 2 New layers develop from the epiblast (Mesoderm and Endoderm)
• Cells remaining in epiblast become ectoderm

Question 10

How is the primitive streak formed (3) ?

Answer 10

• Cells in the middle of the epiblast converge to the middle of the disk
• Epiblast cells ingress as individual cells
• Moving from the edge to the midline

Question 11

What are the first cells to ingress ?

Answer 11

Endoderm cells

Question 12

How is the endoderm formed ?

Answer 12

• Posterior cells drop down to contact the hypoblast
• This forms a sheet of cells

Question 13

What are the second cells to ingress ?

Answer 13

Mesoderm cells

Question 14

How is the mesoderm formed ?

Answer 14

• Mostly anterior cells sit between the epiblast and endoderm
• Cells have to undergo EMT

Question 15

How is the primitive streak formed ?

Answer 15

• Cells found in the middle of the epiblast
• Undergo EMT
• Ingress (go under) and then undergo MET
• ‘Gap’ in the middle forms

Question 16

What is the node ?

Answer 16

Knot of diving stem cells

Question 17

How is the node formed ?

Answer 17

• Cells condense at the anterior end of streak to form the node
• Epiblast cells that ingress through the node migrate anteriorly to form parts of the head

Question 18

When the node starts progressing posteriorly, what cell type does it leave behind and what does this form ?

Answer 18

• Leaves behind mesoderm cells
• This forms the notochord and somites

Question 19

What is the tailbud ?

Answer 19

A knot of stem cells in the posterior of the streak

Question 20

What is primary gastrulation ?

Answer 20

Formation of primitive streak and hensens node

Question 21

Describe the steps of secondary gastrulation (3)

Answer 21

1) The tailbud moves posteriorly leaving cells behind
2) This forms mesoderm cells of the posterior notochord and somites
3) It never forms endoderm