L8 - Germ Layer and Gastrulation

Question 1

What is cell differentiation?

Answer 1

Process by which cells become different from each other and acquire specialized properties

Question 2

What is cell differentiation governed by?

Answer 2

Governed by changes in gene expression, which dictate the repertoire of protein synthesized

Question 3

Gene expression in a cell can be governed by?

Answer 3

Extrinsic factors – morphogens

Intrinsic factors – transcription factors

Question 4

What are the steps to transform a single cell into an embryo?

Answer 4

1. Single fertilised cell divides into 2 then 4
2. Next cleavage happens around the equator, separating top 4 cells from bottom 4 cells
3. Bottom cells signal to top cells so that the top cells start to differentiate, forming 3 germ layers

Question 5

Where are vegetal cells found?

Answer 5

Bottom

Question 6

Where are animal cells found?

Answer 6

Top

Question 7

What are the steps to establish the differences in a Xenopus embryo?

Answer 7

1. Initiate early differences - oocyte polarised even before fertilization
2. Due to gravity different cytoplasmic determinants sink to one part of the egg
3. Early cleavage separates vegetal expressing cells from non-expressing animal cells
4. Cytoplasmic factors that were restricted to vegetal part are now restricted to vegetal hemisphere cells
5. These factors act on the DNA to regulate gene expression in vegetal hemisphere cells
6. Particular transcription factors are turned on in vegetal hemisphere cells
7. Post-fertilisation Xenopus divides into a morula
8. All the progeny of these two groups will be different

Question 8

What hemisphere do all the germ layers come from?

Answer 8

The animal hemisphere - non Vg1 expressing part

Question 9

What is the process through which the germ layers are formed?

Answer 9

Mesoderm induction

Vegetal hemisphere signals to animal hemisphere to induce mesoderm and endoderm

Question 10

What are the steps involved in germ layer formation?

Answer 10

1. Transcription factor, VgT, is localised to nucleus in vegetal hemisphere cells
2. It binds to the promoter and activates transcription of Nodal – autonomously
3. Codes for a secreted morphogen – diffuses out of vegetal cells into animal cells
4. In animal cells that contain receptors for Nodal - Nodal signal transduction pathway activated
5. Causes that cell to change its behaviour

Question 11

In Xenopus cells with highest levels of nodal signalling form?

Answer 11

Endoderm

Question 12

In Xenopus cells with mid-levels of nodal signalling form?

Answer 12

Mesoderm

Question 13

In Xenopus cells with no nodal signalling form?

Answer 13

Ectoderm

Question 14

When is the Wnt signalling pathway activated?

Answer 14

Wnt signalling pathway is activated on dorsal side of embryo

• Marks the site where gastrulation movements begin
• Marks the future posterior dorsal part of the body axis

Question 15

Why is the Wnt signalling pathway activated?

Answer 15

Sperm always fertilises the animal side of the egg
When the sperm enters the egg, it initiates a rotation of the cortex
Leads to activation of Wnt signalling pathway on one side of the embryo
- Opposite the site where the sperm entered

Question 16

If Wnt signalling is activated what are the downstream events?

Answer 16

The key event in responding cells is that

B-catenin is stabilised and enters nucleus

Question 17

What is the Nieuwkoop centre?

Answer 17

Region of vegetal hemisphere where beta-catenin and Nodal overlap - Nieuwkoop Centre
How you get a difference in cells in the vegetal hemisphere
Get very high levels of Nodal

Question 18

What is Nodal activated by?

Answer 18

VegT or B-catenin

Question 19

B-catenin causes Nodal activity to be transformed into?

Answer 19

A gradient
High Nodal activity - organiser (N.centre)
Low Nodal activity - ventral mesoderm

Question 20

Why does high Nodal and active Wnt signaling induce the organiser?

Answer 20

Genes like goosecoid are only transcriptionally activated via both a

• Nodal downstream effector - Smad2/4
• Wnt/B-catenin downstream effector

Question 21

What causes different types of mesoderm to be induced?

Answer 21

Different levels of Nodal

Question 22

Which genes are induced by low levels of Nodal?

Answer 22

Brachyury

Question 23

Which genes are induced by high levels of Nodal?

Answer 23

Goosecoid
Chordin
Xnot
Xlim

Question 24

What causes differentiation of the organiser?

Answer 24

Transcription factors expressed in the nucleus of cells in the organiser (siamois, goosecoid) begin to bind to promoters to alter the behaviour of the cells
Organiser cells begin to differentiate into axial mesoderm
Siamois and goosecoid are expressed at different levels in different cells in the organiser so different types of axial mesoderm cells form

Question 25

After the organiser has differentiated into axial mesoderm what happens?

Answer 25

Axial mesoderm cells migrate collectively - convergent extension

Question 26

What is gastrulation?

Answer 26

Driven by the movement of cells
Movement begins at one end so defines the posterior
It then moves anteriorly

Question 27

Overview of how the A/P axis if formed

Answer 27

Organiser undergoes self-differentiation
Organiser is composed of a mix of progenitor cells which together form the axial mesoderm
- Prechordal mesendoderm
- Prechordal mesoderm
- Notochord
Organiser then undergoes convergent extension