Origin and specification of the germ layer Flashcards
Start of development
cleavage division. The embryo does not grow in the beginning, but the cells cleave. A type of embryonic cell division, increase number of cells without increase of cytoplasmic volume
Maternal to zygotic transition (MZT)
The early stages are largely driven by maternal gene products (RNAs, proteins, etc already present in the oocyte). Oocyte is quite large thus quite a lot potential for the maternal products to contribute to the earliest stages of development.
Overall rule to development seems to be remarkable similar between species: at the earliest cleavages division/earliest phase of development after fertilisation, the embryonic/zygotic genome is not transcriptionally active. The embryo does not transcribe its own genes! It is completely dependent on maternal products
After a while the genes of the embryo itself are being described, this way it is taking control over its own genetic program:
o Maternal RNA degradation
o Zygotic genome activation (ZGA)
why MZT in flies, fish and frogs it is called MBT (mid-blastula transition)?
because it happens at the mid-blastula stage:
o New (embryonic) transcription
o Loss of cell cycle synchrony (waves of cell division going through the embryo)
o Cells become more motile → gastrulation about to happen
o Maternal RNA starts to be degraded
Multiple layers of regulation in MBT:
There is a repressor of transcription present in the early zygote: a repressor of the events of MBT. As development starts, the nucleus-to-cytoplasm ratio changes → increase in the amount of zygotic DNA present. So, there is a change in the amount of repressor compared to the amount of DNA. If the repressor is diluted enough, the zygotic genome can start to be described.
Partial construction experiment
Loss of cell cycle synchrony earlier in life results in half with many nuclei: you deform the normally round egg in such a way you almost pinch it in two different parts, but there still is a small connection. One half of this partially constructed embryo has a nucleus, the other doesn’t. It starts to divide and eventually one of the nuclei will be able to make it through the small opening and get in to the other site. Here it will start
to divide as well but the nucleus to cytoplasm ratio here is quite different. This results in a delayed division
Premature onset of (tRNA, RNAPIII) transcription in polyspermic embryo
fertilisation by more than one sperm, more DNA in the embryo. You also provide a radiolabel to the ribonucleotides. You can extract the RNA and run it on a gel and expose it to film. Each lane in the gel are different time points. By having polyspermy the embryo starts its own transcription earlier than the normal embryo.
Limiting DNA replication factors slow cell cycle
Overexpression of replication factors causes additional rapid cell divisions and delay in transcription (repressor titrated by genomic DNA:
replication factors). This leads to smaller cells. At the MBT there is the onset of zygotic transcription, but also loss of cell cycle synchrony → slowing down of the cell cycle → especially the earliest DNA replications are very rapid
Specific mRNAs are transported to the vegetal pole before fertilization
- gdf1 mRNA (Vg-1, TGFβ family)
- Wnt11b mRNA (Xwnt-11, Wnt family)
- Vegt mRNA (T-box transcription factor)
Breaking radial symmetry by fertilization in Xenopus
Sperm entry, cortical rotation and dorso-ventral (D-V) axis formation:
1. Fertilization of egg: Sperm entry anywhere in the animal hemisphere causing a symmetry breaking event. It causes the outer layer of the cytoplasm, the cortex, to loosen from the inner dense cytoplasm so that it is able to move independently. (cortex = a gel-like layer rich in actin filaments and associated material).
2. The site opposite to sperm entry will start to shift, taking the whole cortex with it → cortical rotation mediated by microtubules. There is a ray of pigment left behind → the grey crescent.
3. Dorsalizing factors are before fertilization present at the vegetal pole, but they rotate and induce the dorsal side at one side of the embryo → relocation of vegetal RNAs mediated by microtubules. → the symmetry breaking sperm entry causes D-V axis formation
*Gdf1 mRNA (β signalling molecule) & wnt11b mRNA (wnt signaling molecule, is the earliest signal that breaks the symmetry) are important in this progress (= dorsalizing factors).
*sperm entry → cortical rotation by microtubules → relocation dorsalizing factors at the vegetal pole opposite to the side of sperm entry. Wnt11 is one of these dorsalizing factors: on the dorsal side more Wnt11. Wnt signalling on the future dorsal side of the embryo → accumulation of B-catenin in the nucleus → influencing transcription
Wnt pathway
- Wnt absent: β-catenin is degraded by the GSK-3 destruction complex: kinase GSK-3 phosphorylates β-catenin → targeted for degradation by the proteosome complex.
- Wnt present: part of GSK-3 is relocated to the receptor: β-catenin is no longer phosphorylated by GSK-3 → accumulation in the cell → translocation to the nucleus → β-catenin will influence transcription
Wnt signalling essential for dorsal structures
You can inject synthetically made mRNA encoding for β-catenin it into a blastomere
at the future ventral-side → induction of a second dorsal axis at the ventral side → a
partial duplication the axis with a double head develops = a Siamese twin embryo.
β-catenin can induce (second) dorsal axis at the ventral side
Knockdown of fzd7 (Xfz7, Wnt receptor) → loss of dorsal structures.
Wnt inhibitors are expressed ventrally (pre-MBT)
Nieuwkoop Center
Dorsal β-catenin accumulation is important for establishing the dorsal axis. The area containing this dorsalizing activity at the blastula stage. A related concept, but present in the early gastrula stage is called the Organizer (the Spemann-Mangold Organizer). The Nieuwkoop centre gives rise to the Organizer.
- The Nieuwkoop centre is present in the dorsal-vegetal quadrant.
- The Organizer is present at the dorsal blastopore. ➔ Both important for the dorsal axis.
- Goosecoid is an organizer gene.
what happens to the germ layers (mesoderm and endoderm) during gastrulation?
the endoderm and mesoderm move inside via the blastopore at the dorsal side.
→ blastopore = the vegetal mass of cells that forms the endoderm, this will be pushes inside the
embryo.
3 protein signals that are important for mesoderm induction
- VegT (T box transcription factor) specifies endoderm
- Ectodermin (trim33) speciefs ectoderm: E3 ubiquitin ligase →
ubiquitinates Smad4 → atagonizes mesoderm to equatorial zone - Foxi1e (foxi1): zygotic transcription factor, maintains ectodermal fate
what could happen during mesoderm induction when the signals are overexpressed from the vegetal pole?
you’d run the risk of turing
all of the animal cap into the mesoderm. This would lead us without a skin or brain. → Animal pole: antagonism of vegetal pole signals limits mesoderm induction to equatorial region
