Lecture 8 Flashcards
What is differentiation and how does it occur?
A gradual process by which cell fate becomes restricted. Begin as three cell types whose potency decreases as they differentiate. Occurs via gene expression which dictates the repertoire of proteins synthesised. They are influenced by extrinsic and intrinsic factors
What occurs at eh morula stage?
Cells begin to arrange themselves in a defined axis- takes 4 weeks in humans and 24hours in chicks
Describe the formation of the epiblast and hypoblast
At the 4 cell stage, cleavage occurs along the equator creating the epiblast (animal) and hypoblast (vegetal) cells. The hypoblasts then signal to the epiblast cells to form the 3 germ layers- effects proliferation and migration
How are early differences established in the early embryo?
The oocyte has polarised cytoplasm- is highly ordered and not distributed evenly. Yolk sinks to the vegetal pole and sperm enters in animal pole.
How do vegetal cells become different to animal cells upon the 3rd cleavage?
Because the cytoplasmic components (yolk) are in the vegetal pole- the cells have inherited them and by the third cleavage they enter the nucleus to act as TFs and begin to autonomously upregulate genes
How are the 3 germ layers formed?
The TF vgTBOX activates nodal in vegetal cells which is synthesised and secreted to act as a morphogen. High= endoderm, low= mesoderm, none= ectoderm
What determines where the D/V axis forms?
Sperm enters the A side at an angle of 30 degrees which shifts the cytoplasmic components to one side of the embryo. The cells that inherit these components activate the wnt signalling pathway which marks where gastrulation movements begin and the dorsal side.
How does the organiser form?
The Wnt signalling on the dorsal side causes the accumulation of beta catenin in the nuclei which act as co-activators to turn on nodal expression- this results in high levels of nodal being produced in the dorsal/vegetal side of the embryo called the nieukwoop centre which upregulates specialised mesoderm and ectoderm called the organiser.
How does the organiser differentiate?
Downstream of wnt is xtwn and downstream of nodal is SMAD2/4 which both act to upregulate gsc and siamois in the organiser cells. The fractionally different levels of these genes expressed by the cells causes them to differentiate into the prechordal mesoderm and the notochord.
Describe gastrulation
The pouring of cells from the posterior dorsal side which move up anteriorly to define the A/P axis.
How does the A/P axis form?
As the organiser differentiates, it undergoes convergent extension movements to form a rod under the ectoderm midline to drive cell movement.
How does a blastocyst form?
Due to changes in osmolality, a blastocoel forms (fluid filled cavity), with the hypoblast beneath it (vg1 cells)
