Term
| How do dorsalising factors determine the distribution of β-catenin in the early embryo? |
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Definition
- After fertilization the cortical layer just under the membrane rotates
- This results in vegetal-cortex and associated factors (like Wnt-2 mRNA) moving to the point opposit sperm entry.
- The Wnt pathway is activated where these factors are found (opposite sperm entry)
- This results in an accumulation on β-catenin on the future dorsal side of the blastula.
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Term
| What cellular processes result in the localisation of β-catenin? |
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Definition
It is originally spread throughout the embryo.
- The WNT signal is not bound to the cell surface.
- Glycogen synthase kinase 3 (GSK-3) and adenomatous polyposis coli (APC) form an enzyme that phosporylates β-catenin.
- The phosporylation leads to proteolysis of the protein.
- When WNT is present GSK-3 binding protein (GBP) and disheveled (DSH) inhibit the GSK-3 and ACP activity.
- β-catenin can therefore go on to affect transcription in the cells.
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Term
| How does the localisation on β-catenin affect the cells? |
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Definition
| the future dorsal side has larger cells due to the accumulation of β-catenin. |
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Term
How do we know that β-catenin affects the D/V axis?
(experiment) |
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Definition
β-catenin is injected into ventral vegetal cells of a Xenopus embryo.
This results in the formation of a twinned embryo with a duplicated axis. |
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Term
| How is disheveled (DSH) and GSK-3 binding protein (GBP) moved to the new dorsal side? |
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Definition
| They are bound to kinesin that moves along microtubules near the outside of the embryo- this is a fast process. |
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Term
| How do we know that GSK-3 is involved in the dorsalisation of the embryo? |
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Definition
We can silence the function of GSK-3 by dominant-negative point mutation.
If we then inject the non-functional GSK-3 into cells on the prospective ventral side of the embryo it leads to the formation of a duplicated D/V axis.
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Term
| What signalling pathways are involved in organising the prospective organiser region? |
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Definition
The Wnt pathway informs the cells that they are dorsal
The TGF-β pathway informs the cells that they are mesodermal |
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Term
| Explain the Nieuwkoop experiment. |
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Definition
Dissected embryo sections form different tissue cultures. Aminal cap cells (from the top of the embryo) go on to form the ectoderm.
Marginal cells (in the middle of the embryo) form the mesoderm.
Vegetal cells (from the bottom) form the endoderm.
When the animal cap cells and vegetal cells are placed together without any marginal cells (like a sandwich with no filling) the animal cap transforms to mesoderm instead of ectoderm.
This means there must be signalling from the vegetal cells directing mesoderm development. |
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Term
| What is the Nieuwkoop centre? |
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Definition
The dorsalmost vegetal cells of the blastula, which contain both the Wnt (dorsal) pathway and the TGF-β (mesoderm) pathway. These have the power of inducing the organiser.
(It's found just below the organiser) |
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Term
| Overall what does the Wnt pathway and the TGF-β pathway induce? |
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Definition
Wnt pathway induces the D/V axis and the position of the organiser.
TGF-β pathway induces the formation of the mesoderm and the position of the organiser along the D/V axis. |
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Term
| What are the fuctions of the β-catenin proteins? |
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Definition
They activate the siamois gene which causes the transcription of the siamois protein.
This together with Smad2/4 (from the TGF-β (mesoderm)) pathway activate the transcription of the goosegoid gene.
The goosecoid protein goes on to initiate organiser functions within the cell. |
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Term
| What are the functions of the organiser? |
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Definition
- Self-differentiate into dorsal mesoderm
- Dorsalise the ectoderm and induce the formation of the neural tube
- Ability to dorsalise the surrounding mesoderm into paraxial mesoderm (somite forming) when it would otherwise form ventral mesodem (blood, kidney)
- Ability to initiate the movements of gastrulation
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