Mesoderm Segmentation Flashcards
Where does mesoderm form?
At the primitive streak during gastrulation
What are the different types of mesoderm along the primitive streak?
Axial mesoder
Paraxial mesoderm
Intermediate mesoderm
Lateral mesoderm
What does paraxial mesoderm contribute to?
Somite formation
What does lateral mesoderm contribute to?
The formation of the limb and heart
What are the somites preceded by?
A row of mesenchyme tissue which did not appear to show any signs of segentation
= pre-somatic mesoderm
List characteristics of somites
Units have a reproducible size depfined by the AP boubndary
Perfect symetry between left and right
Conserved through evolution
How many vertebrae do humans have at birth and in adult hood?
Birth = 33 Adulthood = 24 and 9 fused ones
How many somites do humans, chicks, mice and zebrafish have?
Humans = 38-44 Chicks = 55 Mice = 65 Fish = 33
When is the primitive streak no longer present?
Until somites no longer form
In the chick embryo how often do somites form?
Every 90 minutes
Who proposed the clock and wavefront model?
Cooke and Zeeman 1976
What does the clock and wavefront model predict?
A clock ticks in the posterior pre somatic mesoderm and drives a molecular oscillator that dictates the periodicity of somites
Which gene controls the oscillation?
C-hairy
How was it shown that c-hairy controls the development of somites
In situ hybridisation in the embryo showed it was not expressed in the same pattern eg bands differed in size
Bissected embryos showed that levels of hairy changed over 90 minute periods
Give two properties of c-hairy that means it can oscillate
Short half life of mRNA
Protein is fairly unstable
What happens if a prospective somite boundary region into a non-boundary region of a somite?
A new boundary is created
Which genes are expressed at the somite boundary?
Notch family genes
What is the lunatic fringe?
Glycosyltransferase inhibits the notch singalling pathway
What happens if lunatic fringe is elctroporated in to the centre of a prospective somite?
Notch is inhibited
2 new boundaries form = 2 new somites
How is the determination front established?
RA is expressed in the anterior
FGF8 is expressed in the posterior
The level at which these two gradients come close to each other is precisely at the level of the determination front
What synthesises RA?
RAldh2
How does FGF8 prevent further division?
Drives the expression of Cyp26 which inhibits RA synthesis
The determination front is molecularly defined by what?
MesP2
FGF8 can drive the expression of which gene?
Tbx6
Tbx6 + notch =
MesP2
What is Mesp2 restricted by?
Ripply 2- acts on neigbouring cells to prevent the expression of further MesP2
