Mid-blastula transition and Spemann's organiser Flashcards
What happens at the mid-blastula transition?
Embryos start to have G1/G2 phases and genomic transcription starts. Division becomes asynchronous.
What type of genes do the animal and vegetal cells produce?
Animal cap: ectodermic genes
Vegetal cells: endodermic genes.
How does the mesoderm come about?
The vegetal pole can instruct cells from the animal cap to become mesodermic.
The animal cap cells have the capacity to become mesodermic as their fate is not determined yet.
How do we know what type of molecule induces the mesoderm?
If you put animal cap and vegetal pole together mesodermic marker genes are express.
If you put a membrane between the two then mesodermic marker genes are still expressed which suggests that the vegetal mass secretes a diffusible substance which can induce the mesoderm in the animal cap.
Where is vg1 expressed?
Dorsal side of the embryo
Where is vegT expressed and what does it induce?
Ventral side of the embryo. Induces zygotic expression of Nodal ligands.
What does RT-PCR tell us?
When a specific gene is expressed and can be quantitative.
What is the evidence that Nodal signalling is required for mesodermic specification?
Inhibiting Nodal signalling causes a loss of mesoderm in Xenopus embryos.
Knockout of Nodal in mouse causes loss of mesodermic structure.
What are the properties of the Spemann organiser?
- It has the ability to become dorsal mesoderm
- It induces surrounding mesoderm to become lateral mesoderm.
- It can induce ectoderm to become neural ectoderm
- It can initiate the movement of blastomeres for gastrulation.
- It induces the neural plate to become neural tube.
What are some of the genes in the 4-signal model?
Signal 1 (maternal)- VegT, Vg1 Signal 2- B-catenin Signal 1 (zygotic)- Xnr Signal 3 (dorsalising factors) Signal 4 (ventralising factors)- BMP4, Xwnt8
4 differences between Nieuwkoop centre and Spemann organiser
Nieuwkoop centre
- maternally established
- localised in the future endoderm
- high B-catenin activity
- induces the Spemann organiser
Spemann organiser
- zygotically established
- localised in the mesoderm
- expressed inhibitors of Wnt/BMP
- induced by the Nieuwkoop centre
What are some of the processes that the embryo goes through to reorganise from the gastrula stage to the neural stage?
- Ingression
- Intercalation
- Invagination
- Convergent extension
- Delamination
- Epiboly
- Involution
What does ingression and delamination do?
Allow 2 tissues to segregate without breaking the integrity of the tissue.
What is gastrulation initiated by and where do they form?
Initiated by bottle cells. Form at the border of mesoderm and endoderm and on the dorsal side of the embryo.
What is involution?
Rolling in of endoderm and mesoderm.
Steps of the involution of mesoderm.
- Animal pole epithelium expands
- Mesodermal cells migrate over fibronectin
- Bottle cells help force curvature of invagination epithelium.
How is fibronectin secreted and what does it do?
Fibronectin is secreted by cells from the blastocoel roof.
Fibronectin forms an extracellular matrix.
The mesodermic cells express integrins that bind to FN.
What are the two areas important for gastrulation?
Hensen’s node and primitive streak
What do the first cells to invaginate do?
- Displace the cells from the hypoblast
- Will form the definitive endoderm
Why do mesodermal cells ingress?
- To become more motile
- Need to stop expressing epithelial markers and start expressing mesenchymal markers
Process of gastrulation in chick embryos
- Polonaise movements of epiblast
- Formation of the primitive streak
- 1st cell to invaginate will become endoderm
- The mesodermic cells ingress and undergo EMT (become mesenchymal).
