1.2 Gastrulation

Question 1

What is gastrulation?

Answer 1

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

Question 2

What are the three germ layers that vertebrates have?

Answer 2

Ectoderm - outer layer - skin
Mesoderm - middle layer - bone, muscles and organs
Endoderm - inner layer - gut
- all three are organised by the epiblast

Question 3

Which two layers develop from epiblast?

Answer 3

Mesoderm and endoderm

Question 4

What happens in the beginning of gastrulation?

Answer 4

primitive streak forms by cells in the epiblast layer moving to the tail end (convergence)
- continously gastrating, moving through the primative streak, single cells dropping off and spreading out across hypoblast

Question 5

How does the gastrula form the blastula?

Answer 5

Cells in middle of epiblast converge to middle of disk to form, the primitive streak
Gastrulation starts in the tail
Epiblast cells are ingressing/moving in, not attached to each other so not epithelial cells
- 1st wave of cells are forming the endoderm layer of cells. Drop into the hypoblast layer, connect back up - become epithelium

Question 6

How is mesoderm formed?

Answer 6

Primitive streak extends further away from tail
next cells in ingress through the streak (more anterior) sit between epiblast + endoderm become mesoderm
- not a single sheet

Question 7

What happens as the streak extends towards the head?

Answer 7

Hypoblast becomes epiblast
Node begins to form (Henson’s node in chick embryo)
- primitive streak elongation stops about 75% of length of axis i.e. tail - clavicle
- epithelial cells that condense at anterior end to form node = knot of diving stem cells
epiblast cells that ingress anteriorly through node form parts of head
– 1st cells form head endoderm
– next cells form head mesoderm

Question 8

What is the role of the regressing node?

Answer 8

Node regresses posteriorly (detaches), leaves behind cells that are the mesoderm - form notochord and somites
whilst node is moving cells are gastrulating through it, being pushed into mesoderm layer
Extra cells only ever form mesodermal cells

Question 9

How does the tail bud form through gastrulation?

Answer 9

Secondary gastrulation
Tailbud = knot of stem cells in posterior of streak - moves posteriorly leaving cells behind
Only forms the mesoderm cells of the posterior notochord and somites - tail vertebrate formation
- never forms endoderm