Mesoderm A-P Patterning- Somites Flashcards
Explain mesoderm formation in the chick
- First evidence of A-P axis forming is a ‘groove’ called the primitive streak which elongates anteriorly
- Primitive streak is where gastrulation takes place
- The cells migrate from the future ‘ectoderm’ to the centre where the PS is and fold inwards to the groove
- The cells that migrate laterally rather than deeply will form the mesoderm
What is the primitive streak?
- linear thickening on the surface of the epiblast (outer layer of the blastula)
- crucial for gastrulation where cells migrate inward to form the 3 germ layers
What tissues are the mesoderm subdivided into?
- Axial mesoderm
- Paraxial mesoderm
- Intermediate mesoderm
- Lateral mesoderm
How did the subdivided mesoderm come about?
- The paraxial mesoderm is subsequent to the axial mesoderm and forms from the cells that go into the primitive streak next to the Hensons node (relatively anterior)
- Intermediate mesoderm is a thin line which runs A-P
- Cells that enter the primitive streak most posteriorly form the lateral mesoderm
What does the axial mesoderm give rise to?
Anteriorly - Pre-caudal mesoderm
Posteriorly - notochord
Where do somites originate from and what is their role?
- Originate from the segmented posterior end of the paraxial mesoderm
- Contribute to formation of tendons, epidermis, endothelial cells and skeletal muscle
What do the intermediate mesoderm and the lateral plate mesoderm contribute to the formation of?
intermediate mesoderm = kidneys and gonads
lateral plate mesoderm = circulatory systems, pelvis and limbs
How is the paraxial mesoderm segmented?
- Paraxial mesoderm is initially born in posterior part of the mesoderm and is initially non segmented
- At some point we start seeing evidence of segmentation which proceeds posterior to anterior
- Highly conserved evolutionary process
- Forms ‘balls’ alongside the neural tube
How do somite’s contribute to the formation of vertebrae?
- Diff. vertebrates have diff. number of somites and diff. number of vertebrae
- THE REPEATED NUMBER OF SOMITES IS SPECIES DEPENDENT
- The somite’s form during embryonic development but the timing of formation is also species dependent
How do somite’s form and what is occurring at the same time as somite formation?
- Form in a sequential, continuous manner until species specific number is reached
- At the same time of formation, gastrulation is happening in the posterior part of the embryo
- New cells form in the paraxial mesoderm as the primitive streak elongates posteriorly
- As the primitive streak elongates, somites are sequentially forming at the anterior part towards the new cells
- Primitive streak is to be present until somites no longer form
What is the unsegmented pre-somitic mesoderm’s role in somtie formation?
Pre-figures the future segmentation of somites
What needs to be achieved in the non-segmented mesoderm for somite formation to occur?
Positional info and intrinsic awareness
- is the somite on the left or right for e.g.
How is the periodicity of somite formation established?
The ‘clock’ theory
- - something located in the P part of embryo which sets timing so cells that enter paraxial mesoderm understand how long they have to stay unsegmented in the axial mesoderm and when to form a somite
- Where cells meet the travelling ‘wavefront’ an abrupt change of property occurs to intiate somite formation
Explain c-Hairy oscillation signalling
- Part of molecular clock theory via gene expression
- c-Hairy is activated + transcribed in on phase of oscillation via Wnt and FGF8 signalling and subsequent Notch signalling
- Negative regulation as the mRNA is not stable and the protein represses its own transcription
- Results in the ‘off’ period of oscillation
- Because the protein isnt very stable itself it cant halt transcription for long so the whole process begins again
How many cycles of oscillations do presomitic cells undergo before forming somites?
12
