1- Concepts and Language of Development

Question 1

list 5 factors that affect the first cell fate decision

Answer 1

1. presence of localised maternal determinants + early polarisation
2. Cdx2 mRNA localisation
3. Tead4 and Hippo signalling
4. Oct4 kinetics between fated TE vs ICM cells
5. epigenetic regulation of cell fate

Question 2

what role does the presence/absence of localised maternal determinants play in early cell fate decisions?

Answer 2

presence of localised maternal determinants = when they’re unequally distributed in the oocyte, it leads to asymmetrical inheritance and allow cells to adopt different fates

no localised maternal determinants = external influences like sperm entry can determine cell polarity and fate

Question 3

how does cell division at the 8-16 cell stage contribute to cell fate?

Answer 3

outer cells are more likely to undergo symmetrical division = become trophectoderm (TE) cells

inner cells often undergo asymmetrical division = form the inner cell mass (ICM)

Question 4

how does Cdx2 mRNA localization affect cell fate?

Answer 4

Cdx2 mRNA is more localized in outer cells = leads to the inheritance of TE characteristics

Inner cells receive less Cdx2 due to asymmetric division (one daughter cell inherits more) = inner cells with less Cdx2 tend to develop into ICM cells

Cdx2 represses ICM pluripotency TFs (Nanog, Oct4) via mutual inhibition and promotes TE cell fate

Question 5

what is the role of Tead4 and Hippo signalling in trophectoderm (TE) regulation?

Answer 5

polarised outer cells
- Hippo signalling pathway suppressed & inactivates kinase complexes upstream
- no Yap protein phosphorylation
- Yap protein translocates from cytoplasm to nucleus and partners with Tead4
- Tead4 induces expression of TE-specific TFs (Gata4, Cdx2) = mutual inhibition of ICM-specific genes to reinforce TE fate

Question 6

how does cell polarity influence Yap localization in the early embryo?

Answer 6

in polar cells - Yap protein moves from cytoplasm to nucleus to interact with Tead4 and promote expression of TE-specific genes for TE fate

in non-polar cells - Yap remains in the cytoplasm, prevents TE-specific gene expression and favouring ICM differentiation

Question 7

how do Oct4 kinetics influence cell fate?

Answer 7

slower Oct4 binding kinetics due to a more open chromatin structure & more accessible Oct4 binding sites
= cells favour asymmetrical division & ICM differentiation

faster Oct4 kinetics due to less accessible binding sites & chromatin structure
= cells favour symmetrical division & TE differentiation

Question 8

what is the impact of histone modification on Oct4 kinetics and cell fate?

Answer 8

higher levels of H3R26me (histone 3 arginine 26 methylation) increase accessibility to Oct4 binding sites = slow Oct4 kinetics = favour ICM differentiation

Question 9

how does DNA methylation regulate TE and ICM fate during the morula to blastocyst transition?

Answer 9

DNA methylation silences TE-specific genes in ICM cells

TE cells maintain the expression of TE-specific genes like Cdx2, through mechanisms like the positive feedback loop with Elf5 when Elf5 is unmethylated

Question 10

what effect does Elf5 methylation have on TE cell fate?

Answer 10

Elf5 methylated = Elf5 expression is repressed = prevents activation of TE-specific genes (Cdx2)

this stops the positive feedback loop between Elf5 and Cdx2, leading to ICM fate instead of TE fate

Question 11

how do histone modifications differ between TE and ICM cells?

Answer 11

TE cells have histone modifications (e.g. H3K9me2) = promote TE-specific gene expression and repress ICM genes

ICM cells have histone modifications (e.g. H3K9me3) = promote pluripotency and repress TE-specific genes