L11 - Mesoderm Segmentation

Question 1

Where does gastrulation take place?

Answer 1

Within the primitive streak

Question 2

Where does the mesoderm form?

Answer 2

Within the primitive streak

Question 3

What do cells arising from the primitive streak contribute to?

Answer 3

Not all cells arising from primitive streak contribute to the same tissues within the mesoderm

Question 4

What experiments did hey use to determine what cells of the primitive streak became?

Answer 4

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

Question 5

What does axial mesoderm form?

Answer 5

Forms notochord and pre-chordal mesoderm

Comes from cells from Hendersons node

Question 6

What does paraxial mesoderm form?

Answer 6

Forms head and somites – anterior

Question 7

What does intermediate mesoderm form?

Answer 7

Forms kidneys and gonads

Question 8

What does lateral mesoderm form?

Answer 8

Forms circulatory system, somatic tissue and extra embryonic tissue

Question 9

What do somites form from?

Answer 9

Somites are segmented paraxial mesoderm tissue

Question 10

Somites are evidence of?

Answer 10

Somites are the earliest evidence of segmentation in vertebrates

Question 11

What part of the paraxial mesoderm is segmented?

Answer 11

Most posterior paraxial mesoderm is not segmented
Uniform band of tissues made of mesenchymal cells
Segmentation occurs as you move more anterior

Question 12

What does somite number dictate?

Answer 12

The number of vertebrae

Question 13

How many vertebrae do humans have?

Answer 13

Humans are born with 33 vertebrae

By adulthood - 24 vertebrae and 9 fused ones

Question 14

How many somites do human embryos have?

Answer 14

38-44

Question 15

How many somites do chick embryos have?

Answer 15

55

Question 16

How many somites do mouse embryos have?

Answer 16

65

Question 17

How many somites do zebrafish embryos have?

Answer 17

33

Question 18

How do somites form?

Answer 18

Form in pairs from the paraxial mesoderm
They bud off the pre-somitic mesoderm
- Form every 90 min

Question 19

What is the role of paraxial mesoderm in somite formation?

Answer 19

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

Question 20

What is the role of pre-somatic mesoderm in somite formation?

Answer 20

Pre-somitic mesoderm, although not segmented, pre-figures the future segmentation of somites
- Potential for 12 somite pairs

Question 21

How do you explain the periodicity and reproducibility of somite formation?

Answer 21

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

Question 22

How is the periodicity of somite formation established?

Answer 22

Clock and wavefront model

Cooke and Zeeman - 1976

Question 23

What does the clock and wavefront model predict?

Answer 23

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

Question 24

How was the oscillation of C-hairy expression in the pre-somitic mesoderm shows?

Answer 24

A series of in situ hybridisations

Question 25

What were the expression patterns of the gene when looking at the oscillation of C-hairy expression?

Answer 25

Expression patterns of the gene tended to fall into 3 classes - Large band of expression - Shorter band of expression - Shortest band of expression

Question 26

What pattern of gene expression did C-hairy follow?

Answer 26

Patterns expressed over a period of 90 minutes Gene expression followed an oscillation pattern After 90 min, a new somite pair had formed

Question 27

What is hair/hes/her protein useful for?

Answer 27

Their half life and function is critical for the oscillatory pattern of its mRNA

Question 28

What are hair/hes/her genes targets for?

Answer 28

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

Question 29

Hair/hes/her proteins are?

Answer 29

bHLH transcriptional repressors

Question 30

The proteins synthesised by hair/hes/her have?

Answer 30

A very short half life

Question 31

The oscillatory patterns of hair/hes/her occurs via?

Answer 31

A negative feedback loop and an unstable proteins

Question 32

What are other examples of clock genes?

Answer 32

All belong to the Wnt, Notch and FGF signalling pathways | They display the same characteristics and expression patterns

Question 33

How many cycles of oscillations are there?

Answer 33

Pre-somitic mesoderm cells undergo 12 cycles of oscillations before forming somites

Question 34

What happens when cells encounter the travelling wavefront?

Answer 34

When cells encounter wavefront travelling in opposite direction oscillations stop

Question 35

The determination front is positioned where?

Answer 35

Determination front is positioned where the two opposing gradients meet - Retinoic acid - High anteriorly - Gradient high where somites are found - FGF8 - High posteriorly

Question 36

What does the determination front determine?

Answer 36

Determines the timing of somite formation?

Question 37

What direction does the wavefront move?

Answer 37

Posteriorly

Question 38

How is the relationship between retinoic acid and FGF8 maintained?

Answer 38

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

Question 39

What does FGF8 promote transcription of?

Answer 39

Tbx6 which along with notch signalling promotes transcription of Mesp2

Question 40

What expression is maintained in somite -1?

Answer 40

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

Question 41

What expression is maintained in somite 0

Answer 41

Ripply2 also represses Mesp2 | Very narrow band of expression of Mesp2

Question 42

How can boundary cells induce somite boundary formations?

Answer 42

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

Question 43

Where are notch family genes expressed?

Answer 43

Expressed at the somite boundary | Notch signalling involved in somite boundary formation

Question 44

What gene construct was created to show the role of notch signalling in somite boundary formation?

Answer 44

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

Question 45

How are somite derivatives affected in the absence of Notch signalling?

Answer 45

Delta-like 3 mutant - ossification centres do not align | In humans a similar mutation causes spondylocostal dysplasia

Question 46

Summary of segmentation

Answer 46

Oscillation slows down as cells mature Acquisition of anterior and posterior identity Notch1 and Dl1 in posterior Notch2 and Dl3 in anterior

Question 47

An example of a model of somite formation?

Answer 47

1. Molecular oscillator 2. C-hairy 3. Lunatic fringe 4. Delta 1 or Notch 1 5. If notch1 - ephrins 6. Cell adhesion changes 7. Somite formation