L11 - Mesoderm Segmentation Flashcards
Where does gastrulation take place?
Within the primitive streak
Where does the mesoderm form?
Within the primitive streak
What do cells arising from the primitive streak contribute to?
Not all cells arising from primitive streak contribute to the same tissues within the mesoderm
What experiments did hey use to determine what cells of the primitive streak became?
Using lineage fate map studies or directly labelling cells it was discovered that the point of entry of cells within the primitive streak dictates the tissues the cell will
What does axial mesoderm form?
Forms notochord and pre-chordal mesoderm
Comes from cells from Hendersons node
What does paraxial mesoderm form?
Forms head and somites – anterior
What does intermediate mesoderm form?
Forms kidneys and gonads
What does lateral mesoderm form?
Forms circulatory system, somatic tissue and extra embryonic tissue
What do somites form from?
Somites are segmented paraxial mesoderm tissue
Somites are evidence of?
Somites are the earliest evidence of segmentation in vertebrates
What part of the paraxial mesoderm is segmented?
Most posterior paraxial mesoderm is not segmented
Uniform band of tissues made of mesenchymal cells
Segmentation occurs as you move more anterior
What does somite number dictate?
The number of vertebrae
How many vertebrae do humans have?
Humans are born with 33 vertebrae
By adulthood - 24 vertebrae and 9 fused ones
How many somites do human embryos have?
38-44
How many somites do chick embryos have?
55
How many somites do mouse embryos have?
65
How many somites do zebrafish embryos have?
33
How do somites form?
Form in pairs from the paraxial mesoderm
They bud off the pre-somitic mesoderm
- Form every 90 min
What is the role of paraxial mesoderm in somite formation?
Paraxial mesoderm forms in a continuous manner until proper somite number is reached
Number of somites is fixed for a given species
Timing of somite formation remains constant within species
What is the role of pre-somatic mesoderm in somite formation?
Pre-somitic mesoderm, although not segmented, pre-figures the future segmentation of somites
- Potential for 12 somite pairs
How do you explain the periodicity and reproducibility of somite formation?
Cells within pre-somitic mesoderm must respond to
- Positional information
- Mechanism that coordinates left and right somites
- Mechanism that generates anterior boundary
- Mechanism that generates posterior boundary
- Formation of cleft
How is the periodicity of somite formation established?
Clock and wavefront model
Cooke and Zeeman - 1976
What does the clock and wavefront model predict?
Predicts that a clock ticks in the posterior pre-somitic mesoderm and drives a molecular oscillator that dictates the periodicity of somites
Where cells hit the travelling wavefront an abrupt change of property occurs leading to the decision to form somites
How was the oscillation of C-hairy expression in the pre-somitic mesoderm shows?
A series of in situ hybridisations
What were the expression patterns of the gene when looking at the oscillation of C-hairy expression?
Expression patterns of the gene tended to fall into 3 classes
- Large band of expression
- Shorter band of expression
- Shortest band of expression
What pattern of gene expression did C-hairy follow?
Patterns expressed over a period of 90 minutes
Gene expression followed an oscillation pattern
After 90 min, a new somite pair had formed
What is hair/hes/her protein useful for?
Their half life and function is critical for the oscillatory pattern of its mRNA
What are hair/hes/her genes targets for?
They are target genes of notch signalling
- When you have notch signalling
- Genes are transcribed forming Hes1 proteins
- Proteins repressors its own transcription
- However, protein is unstable so quickly degraded
Hair/hes/her proteins are?
bHLH transcriptional repressors
The proteins synthesised by hair/hes/her have?
A very short half life
The oscillatory patterns of hair/hes/her occurs via?
A negative feedback loop and an unstable proteins
What are other examples of clock genes?
All belong to the Wnt, Notch and FGF signalling pathways
They display the same characteristics and expression patterns
How many cycles of oscillations are there?
Pre-somitic mesoderm cells undergo 12 cycles of oscillations before forming somites
What happens when cells encounter the travelling wavefront?
When cells encounter wavefront travelling in opposite direction oscillations stop
The determination front is positioned where?
Determination front is positioned where the two opposing gradients meet
- Retinoic acid
- High anteriorly
- Gradient high where somites are found
- FGF8
- High posteriorly
What does the determination front determine?
Determines the timing of somite formation?
What direction does the wavefront move?
Posteriorly
How is the relationship between retinoic acid and FGF8 maintained?
Negative feedback regulations
Retinoic acid
- Blocks transcription of FGF8
FGF8
- Controls expression of Cyp26 which blocks transcription of retinoic acid
- Blocks the transcription of Raldh2 which promotes transcription of retinoic acid
What does FGF8 promote transcription of?
Tbx6 which along with notch signalling promotes transcription of Mesp2
What expression is maintained in somite -1?
Initially Mesp2 is expressed throughout
Mesp2 transcription generates protein which drives expression of Ripply2
Ripply2 is a negative regulator of Mesp2
Expression of Mesp2 restricted to anterior part of somite -1
What expression is maintained in somite 0
Ripply2 also represses Mesp2
Very narrow band of expression of Mesp2
How can boundary cells induce somite boundary formations?
Transplantation of a prospective somite boundary region into a non-boundary region creates a new boundary
Shows that boundary cells instruct cells that are anterior to form a boundary
Where are notch family genes expressed?
Expressed at the somite boundary
Notch signalling involved in somite boundary formation
What gene construct was created to show the role of notch signalling in somite boundary formation?
Generated a gene construct - lunatic fringe
Codes for a protein which inhibits notch signalling
Introduced construct into chick embryo
- Forced the expression of this gene into central aspect of somite
- Created a new boundary
- Notch signalling is sufficient for boundary formation
How are somite derivatives affected in the absence of Notch signalling?
Delta-like 3 mutant - ossification centres do not align
In humans a similar mutation causes spondylocostal dysplasia
Summary of segmentation
Oscillation slows down as cells mature
Acquisition of anterior and posterior identity
Notch1 and Dl1 in posterior
Notch2 and Dl3 in anterior
An example of a model of somite formation?
- Molecular oscillator
- C-hairy
- Lunatic fringe
- Delta 1 or Notch 1
- If notch1 - ephrins
- Cell adhesion changes
- Somite formation
