Lecture 11 Flashcards
How does AP axis formation emerge in chick embryos
Gastrulation occurs in the primitive streak. EPiblast cells undergo etm- inward migration into the primitive streak forming endoderm and most of the mesoderm
What are the origins of mesoderm tissue in the chick and how can lineage tracing be used to determine this?
Can do fate mapping between quail and chick- fluorescently label the cell and follow fate. Discovered that the point of entry of cells (from A to P) in primitive streak determines what tissue they are fated to contribute to
1) Henson’s node= axial mesoderm
2) paraxial mesoderm
3) Intermediate mesoderm
4) lateral mesoderm
What do each of the types of mesoderm in chicks give rise to?
1) axial= prechordal mesoderm and anterior endoderm
2) paraxial= head and somites
3) intermediate= kidney and gonads
4) lateral= splanchnic, extra-embryonic and somatic
What are somites?
Segmented paraxial mesoderm that emerge at stage 10- have clear clefts between them. No somites most posteriorly- mesenchymal cells
What is the evolution of segmentation?
Highly conserved mainly in vertebrates. Use ISH to see the highly organised segmentation pattern of genes- Fish swimming pattern can be defined by the number of segmented units
What does somite number determine?
How many vertebrae each species has- is species specific e.g. humans are born with 33 vertebrae and have 38-44 somites
What determines somite formation?
paraxial mesoderm is produced in a continuous manner form the P part of the embryo until the required amount of somites have formed. The number of somites is predetermined in the unsegmented paraxial mesoderm e.g. chicks have 12 somites present but have not yet formed
Briefly describe how somites form
The paraxial mesoderm buds from anterior to posterior every 90 minutes.
What are the 4 things that paraxial mesoderm cells need to respond to?
1) positional information= dictates if they are ready to form new somite pairs
2) Left/right coordination= the 2 bands of p mesoderm are separated by the spinal cord- must form at same time and manner
3) mechanism that determines A/P boundary
4) formation of cleft
What is the clock and wavefront model?
Dictates that the posterior part ticks and drives the molecular oscillator that dictates the periodicity of the somites. The travelling wavefront= when encountering the molecular oscillator the cells will undergo and abrupt change which will be translated molecularly to form somites and their boundaries
How were oscillations discovered in the chick embryo?
A student carried out ISH against C-hairy1= saw oscillatory pattern which came under 3 categories= 0, 30 and 60- 90 minute duration for somite formation.
How is oscillation created in the paraxial mesoderm?
Notch controls Hairy/Hes/Her transcription. They code for TF repressors- repress their own transcription to act in a negative feedback loop. Have a very short half lives (protein is quickly ubiquitinated). Many genes controlled by Fgf and wnt show same pattern- cells must undergo 12 cycles of oscillations to under go segmentation- P cells don’t actively move
What are the boundaries of the wavefront?
S2 marks the boundary of formation
The determination front travels from A to P and becomes a wave front when it encounters the molecular oscillators which causes somite formation
What is the determination front and how is it determined?
High FGF posteriorly, high RA anteriorly- where they meet is the determination front. Is created by 2 antagonising gradients. Raldh2 activates RA which is inhibited by FGF. FGF activates cyp26 which codes for an RA degrading enzyme. RA activates mesp2 which is inhibited by FGF
How is mesp2 expressed in somites?
FGF will activate tbx6 which will act with notch signalling to activate mesp2. Initially it is expressed throughout the developing somite but it then activates ripply 2 which represses mesp2 and causes its expression to be restricted in a compartment in s-1
