Germ layer formation Flashcards
(19 cards)
At what stage of a chick embryo’s development is the primitive streak developed
Stage 3
When does hensons node form at the primitive streaks most anterior end?
Stage 4
At what stage does the notochord extend forward from the node forming the prechordal mesoderm most anteriorly and notochord behind it
Stage 5-6
What crucial similarity is there between chicks and humans development making them a good model for comparison
The morula has an epiblast and a hypoblast, similar to stage 2 of the chick embryo
Where does germ layer formation/ gastrulation begin
At the posterior of the developing embryo
As germ layers form cells move into a mound that elongates anteriorly to form the primitive streak, which has Henson’s node at its most anterior
How are the three germ layers formed?
Cells from the epiblast migrate toward the midline and delaminate before moving back out sideways
The cells that remain as they are are the ectoderm
The first cells to delaminate move down and physically displace the hypoblast to form the endoderm layer
Second layer of cells that move from the midline will form the mesoderm layer
How do cells know whether to become mesoderm or endoderm
Due to signals from the hypoblast
What animal model is used for the molecular details of gastrulation
Xenopus
Is the oocyte of the xenopus polarised
Yes, even before fertilisation - particular determinants are located in the vegetal hemisphere - as mitosis proceeds an early cleavage separates vegetal and animal hemispheres. Cytoplasmic factors that were restricted to the vegetal part of the oocyte are now restricted to the vegetal hemisphere cells. In turn, particular transcription factors are expressed only in the vegetal hemisphere cells
What does in situ hybridisation reveal about mRNA in the developing oocyte
mRNA is only being transcribed in the vegeta; hemisphere cells = VgT
What does VgT act on
Acts in nucleus - binds to promoter of genes causing the activation of the gene nodal
Nodal protein is a morphogen
Some cells in the animal hemisphere have a nodal receptor - the nodal signal transduction pathway is activated in those cells
Where does the sperm always fertilise the xenopus oocyte
On the animal side - initiating a rotation of the cortex - activating the Wnt signalling pathway
Where is the Wnt pathway activated and what effect does this have
Activated at the future dorsal side of the embryo
Creates a difference between the cells in the vegetal hemisphere as in the dorsal vegetal cells there is an overlap in Nodal and nuclear beta catenin
What is the area of overlap between nuclear beta catenin and Nodal called
The Nieuwkoop centre - has important properties
How does beta catenin affect Nodal
Increases nodal activity creating a gradient of activity with highest nodal found dorsally
How does nodal affect the surrounding area mesoderm
Acts as a morphogen - High region of activity dorsally will become the organiser/Hensons node
Rest of the mesoderm will become either ventral or lateral mesoderm
What environment is required for Goosecoid to become transcriptionally activated
Nodal downstream effector Smad 2/4 and a Wnt/B catenin downstream effector are both present at distal and proximal elements of the gene.
Therefore goosecoid is only found in the organiser
How does goosecoid start the AP axis formation
Begins to bind promoters in the organiser causing production of antagonists of BMP/Wnt (Noggin) which alter the cells behaviour causing changes in migration and differentiation
How is the AP axis refined
Signals that were close together become spatially separated
High FGF in the posterior and RA give a posterior fate to cells whereas antiWnts maintain anterior identities
