Segregation of TE from ICM Flashcards
Introduce segregation of TE from ICM.
TE forms from cells on outside of morula in contact with extra embryonic fluid. Express cdx2. Later gives rise to trophoblast stem cells, giant tophoblast cells and placenta. Need to segregate from ICM, which expresses Oct4 and downregulates Cdx2.
Therefor segregation of TE from ICM depends on the expression of Cdx2 and Oct4.
What are the 2 models for the establishment of this cell fate?
Polarisation model and Inside-Out model.
What is the polarisation model?
The polarisation model states the the expression of Cdx2 or Oct4 depends on their distribution (cytoplasmic determinants), altered by polarisation.
What is the experimental support for the polarisation model?
Track cell divisions between the 2-32 cell stage. See consistent asymmetric cleavages, therefore distribution of cytoplasmic determinants uneven so different determinants in different daughter cells.
Is the polarisation model evidence good evidence?
Not particularly, it doesn’t prove that altered distribution of cytoplasmic determinants is necessary, it just shows a possibility.
Also haven’t identified cytoplasmic determinants, just have candidates.
What is more recent evidence for polarisation model?
Upregulation of Cdx2 promotes symmetrical, down regulation of Cdx2 promotes asymmetrical. Shows difference and importance.
Can also see that Cdx2 transcripts are polarised but protein isn’t.
What is the inside-out model?
Compaction creates different microenvironments for inside and outside cells. This was the original model, believed to be correct.
What is the experimental evidence for the inside-out model?
Making chimeras.
- Take a cell from the inside of a morula and transplant to outside of another. 97% became trophectoderm.
- Take a cell from outside of a morula and transplant to inside. 60% became ICM. (Less of a majority because arguably more difficult transplant to get right)
Therefore this suggests microenvironment plays a role. But still need molecular mechanism to prove it isn’t due to other factors.
What is the molecular evidence for the inside-out model?
Set of experiments using mutant mice identified genes important for regulating Cdx2 expression required for TE cells.
TEAD4 (transcription enhance family of TF) mutants lack Cdx2 expression therefore fail to develop TE. TEAD4 activation of Cdx2 requires YAP cofactor to translocate to nucleus. This is part of the known Hippo signalling pathway (role in tumour suppression). Can now link its role to embryology.
Describe the hippo pathway
Hippo is a transmembrane receptor. It activates Lats which phosphorylates YAP. YAP gets degraded so doesn’t go to nucleus. TEAD4 (TEF TF)unable to activate Cdx2.
However, if Cdx2 is expressed, the hippo receptor is inactive so YAP can go to the nucleus and activate TEAD4 expression of Cdx2. Show using Hippo mutants - all cells become TE.
ICM - active hippo due to cell-cell contact, Cdx2 off
TE - inactive hippo, Cdx2 on
So which model is it?
Clearly inside-out model has good molecular mechanism to base model on and so is generally accepted. However, it hasn’t had a final definitive effect over polarisation model. Still unanswered questions. Maybe a combination of both? Keep exploring.
