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Developmental biology extras > Lecture 11 Mesoderm AP patterning > Flashcards

Lecture 11 Mesoderm AP patterning Flashcards

1
Q

The organiser undergoes… to become…

A

convergent extension

rod of mesoderm in the middle called axial mesoderm, and bands of mesoderm either side called paraxial mesoderm

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2
Q

Which species was used to learn about the formation of paraxial mesoderm

A

Chick embryo

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3
Q

Difference between chick and xenopus embryo paraxial mesoderm

A

Flattened discs vs a fold with animal and vegetal poles

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4
Q

Describe the chick embryo as it is undergoing gastrulation

A

2 sheets of cells
Epiblast layer top
Hypoblast layer bottom

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5
Q

What is equivalent to the Xenopus organiser

A

Hensen’s node

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6
Q

Where does mesoderm form during gastrulation

A

At the primitive streak

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7
Q

At the 2 layer (epiblast/hypoblast stage), what occurs?

A

Signals from hypoblast change the fate of the epiblast

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8
Q

Which cells become endoderm

A

Cells that see a high conc of nodal

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9
Q

Which cells become mesoderm

A

Cells that see a lower conc of nodal

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10
Q

What are the four mesodermal tissues

A

Axial mesoderm
Paraxial mesoderm
Intermediate mesoderm
Lateral mesoderm

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11
Q

Mesoderm that forms immediately adjacent to the axial mesoderm/either side of the NT is..

A

Paraxial mesoderm

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12
Q

What is the mesdoerm which sits under the flat ectoderm i.e. the furthest away

A

Lateral mesoderm

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13
Q

What is the mesoderm which sits between the paraxial mesoderm and the lateral mesoderm

A

Intermediate mesoderm

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14
Q

What is the mesoderm which also makes up the notochord

A

Axial mesoderm

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15
Q

Intermediate mesoderm gives rise to

A

Gonads and kidney

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16
Q

Paraxial mesoderm gives rise to

A

Head anteriorly

Somites at the trunk

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17
Q

Somites give rise to

A
  • Sclerotome/cartilage
  • Syndotome/tendons
  • Myotome/skeletal muscle
  • Dermatome/dermis, skeletal muscle
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18
Q

Lateral plate mesoderm gives rise to

A

Extraembryonic
Splanchnic
Somatic

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19
Q

What are somites

A

Segmented paraxial mesoderm tissues

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20
Q

What is pre-somatic mesoderm

A

Where the somites form, at the posterior

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21
Q

What are somites the earliest evidence of

A

Segmentation in vertebrates

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22
Q

How can meosderm segmentation be visualised

A

Engrailed 1 expression
Can see clear segmentation in the vertebrates and invertebrates as mesodermal segmentation is conserved throughout evolution

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23
Q

What does somite number dictate

A

Number of vertebrae

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24
Q

Somites form in… from the paraxial mesoderm

A

Pairs

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25
Q

Paraxial mesoderm forms in a ….. until ….

A

Paraxial mesoderm forms in a continuous manner until the proper somite number is reached

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26
Q

What is true about somites for a given species

A

Number of somites is fixed

Timing of somite formation is constant

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27
Q

Why can we determine age of embryo

A

Number of somites as time constant for somite formation

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28
Q

Is presomitic mesoderm segmented

A

No

29
Q

In which direction do somites form

A

A –> P

30
Q

Describe series of events leading to epithelialisation of somites

A
  1. Gastrulation: acquisition of paraxial mesoderm identity
  2. Onset of oscillator: activation of c-Hairy1 and Lunatic Fringe
  3. Segmentation: Oscillation slows down as cells mature. Acquistion of A and P identity
    Notch1/ Dl1 in post
    Notch2/Dl3 in ant
  4. Determination front: Forms at interface of 2 opposing gradients
  5. Somitogenesis: epithelialisation of somites
31
Q

Is the PSM composed of uniform cells?

A

No
It is composed of cells that are gradually expressing an oscillating time i.e. the clock which is constantly turned on and off

32
Q

What will the clock hit? After how many cycles?

A

Determination front

12 cycles to form 12 somites

33
Q

Where does this determination front form?

A

Interface of the 2 opposing gradients

34
Q

What does the determination front result in?

A

PSM will undergo somitogenesis to form somites via epithelialisation

35
Q

How often do somites form in chicks?

A

Every 90 minutes

36
Q

How is the periodicity of somite formation established?

A

The clock and wavefront model (Cooke and Zeeman, 1976)

37
Q

What does the clock and wavefront model predict

A

• Predicts a ‘clock’ ticks in the posterior PSM and drives a molecular oscillator that dictates that periodicity of somites

38
Q

What happens to cells in PSM when they hit the travelling wavefront

A

• Where cells hit the travelling wavefront (determination front), an abrupt change of property occurs leading to the decision to form somites

39
Q

Which 2 independent phenomena account for periodic somite formation?

A
  1. an intrinsic biochemical oscillator, clock, by which cells oscillate synchronously between a permissive and a non-permissive state of somite formation
  2. maturation front travelling along the embryonic AP axis, moving posteriorly in concert with the AP differentiation gradient of the embryo, the wavefront
40
Q

What state must be the cells in the PSM be in for a somite to form

A

• For a somite to be formed, a group of PSM cells in the permissive state of the clock must be reached by the wavefront of differentiation

41
Q

Cells within PSM must respond to what x4?

A
  • Positional information
  • Mechanism that coordinates left and right somites
  • Mechanism that generates posterior boundary
  • Formation of cleft between each somite
42
Q

What was the first evidence of a clock model?

A

In situ hybridisation for Notch signalling pathway components

43
Q

Hairy gene
IN CHICK
IN MOUSE
IN ZEBRAFISH

A

c-Hairy-1
Hes
her

44
Q

Describe the expression of c-hairy

A

At 0 mins high
Then falls
Then increases and peaks

45
Q

Hairy/hes/her are targets of

A

Notch signalling

46
Q

What are Hairy/hes/her described as

A

bHLH transcriptional repressors

47
Q

Clock genes are targets of which signalling pathway

A

Wnt
FGF
Notch

48
Q

After how many oscilations will a cell find itself in the anterior part of the presomitic mesoderm

A

12

49
Q

How long will it take to form a somite

A

12x90 min

Approx 18 hours

50
Q

Where are somites specified

A

Determination front

51
Q

S0 marks

A

where the cell has just past the determination front

52
Q

S-1 marks

A

the specification of boundary formation (this cell has gone through 12 oscillations)

53
Q

When do oscillations stop

A

• When cells encounter the wavefront travelling in opposite direction, oscillations stop

54
Q

Where is the determination front positioned?

A

At the interface of 2 opposing gradients i.e. at interface of RA gradient and FGF8 gradient

55
Q

Describe the gradients of FGF8 and retinoic acid

A
  • FGF8 gradient is highest at the posterior end of the paraxial mesoderm. FGF8 is expressed at lowest levels anteriorly
  • Retinoic acid is expressed in early forming somites, but they have to have already formed, highly expressed anteriorly
56
Q

What maintains the reverse relationship between RA and FGF8 gradients?

A

Negative feedback regulations maintain the reverse relationship between RA and FGF8 gradients

57
Q

Draw out the negative feedback regulations to maintain reverse relationship

A

See notes

58
Q

What is expressed at a somite boundary?

A

Notch family genes

59
Q

Describe notch expression at the boundary of somites

A
  • Delta 1/Notch 1 only expressed in P part of the first forming somite
  • Delta 3/Notch 2 only expressed in A part of the first forming somite
60
Q

When are these genes upregulated

A

Only after the determination front when oscillations stop

61
Q

How are the Notch family of genes set up due to the molecular clock

A

• In some way, the molecular clock as it ticks through permissive/non-permissive sets up bands of these different genes

62
Q

How would the Notch family of genes be described in somite boundary formation

A

Instructive

63
Q

How do we know that Notch signalling is important in somite boundary formation?

A

Gain of function

Loss of function

64
Q

Describe gain of function experiment in Notch singallign somite boundary formation

A
  • You electroporate in a component of the notch signalling pathway e.g. lunatic fringe, into an ectopic position
  • Lunatic fringe is not normally expressed inbetween 2 somites
  • Consequences are that you get an extra small somite forming as Lunatic fringe give a 2nd A and P boundary
65
Q

What is lunatic fringe

A

• Lunatic fringe is a glycosyltransferase which is an inhibitor of notch

66
Q

Describe loss of function experiment in Notch signalling somite boundary formation

A
  • Predict fewer somites or somite derived structures
  • In a mouse mutant which lacks Delta-like 3 (normally for formation of A part of somite), then you can see that you have fewer ribs, not proper formation and budding of somites in the anterior portion
67
Q

In human, a mutation in delta 3 causes what

A

• In humans, a similar mutation causes spondylocostal dysplasia (Jarcho Lewin Syndrome) – due to problems with the segmentation of the axial skeleton. This occurs due to a mutation in the delta 3 ligand that alters notch signalling and problems with somite segmentation

68
Q

How do ephrins play a role in control of somite formation?

A
  • Ephrins usually associated with keeping 2 cells separate (mutual repellents)
  • Ephrins cause cell adhesion changes which cause the budding and integrity of the single somite

Developmental biology extras (13 decks)

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