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Development > Patterning in Xenopus > Flashcards

Patterning in Xenopus Flashcards

1
Q

Xenopus gastrulae are ventralised. True or false?

A

True.

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2
Q

At gastrula stage which is the only tissue to have been fully specified?

A

The notochord.

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3
Q

When is the nervous system specified?

A

During gastrulation.

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4
Q

What did Hans Spemann find in his grafting experiments in 1919?

A

In early gastrulation he grafted presumed neural plate onto the ventral side of an embryo and it became epidermis. However by the end of gastrulation it formed ectopic neural plate.

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5
Q

What is sox2?

A

A pan-neural plate marker. It is the first gene to be expressed and is specific to neural plate.

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6
Q

Where is hoxb9 expressed?

A

Only in the spinal cord.

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7
Q

Where is foxg1 expressed?

A

Only in the forebrain.

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8
Q

Where is snai2 expressed?

A

In the neural crest.

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9
Q

What happens to levels of hoxb9, foxg1 and snai2 as gastrulation progresses?

A

They increase.

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10
Q

What tissue does the dorsal blastopore lip become?

A

Notochord.

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11
Q

What experiment did Spemann and Mangold perform in 1924?

A

The Organiser Graft: they transplanted the dorsal blastopore lip of an early gastrula to the ventral side of a host. This resulted in the formation of a conjoined twin.

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12
Q

In the experiment by Spemann and Mangold in 1924, what tissues did a) the graft and b) the host cells always form?

A

a) Notochord

b) Neural tube and somites

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13
Q

How could the graft and host cells be told apart?

A

The graft cells were darkly pigmented.

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14
Q

After his experiments in 1924 Spemann declared the notochord to be the organiser. What does this mean?

A

It is the source of signals that induce neural plate and somite formation.

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15
Q

After his experiments in 1924 Spemann declared the notochord to be the organiser. What does this mean?

A

It is the source of signals that induce neural plate (neural induction) and somite formation (dorsalisation).

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16
Q

The organiser is a different structure in each model organism. What is it in…

a) Fish
b) Chicks
c) Mice

A

a) The shield
b) Hensen’s node
c) The node

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17
Q

Basically what is the organiser?

A

A signalling centre responsible for the formation of other tissues.

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18
Q

Name 3 genes whose transcripts can be localised to the organiser in Xenopus?

A

Chordin, noggin and follistatin.

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19
Q

What do chordin, noggin and follistatin encode?

A

Secreted proteins.

20
Q

Transcripts for chordin and noggin have also been found in which gastrulae? What does this suggest?

A

Zebrafish and amniotes, suggesting a common mechanism.

21
Q

Where is the organiser?

A

In the animal cap on the dorsal side.

22
Q

Put the 3 organiser genes in order of potency, from most to least potent.

A

Noggin is strongest, then chordin then follistatin.

23
Q

Put the 3 organiser genes in order of potency, from most to least potent.

A

Noggin is strongest, then chordin then follistatin.

24
Q

Which of the 3 organiser genes is expressed most highly?

A

Chordin is most strongly expressed, then noggin, with follistatin being weakly expressed.

25
Q

What tissue do C, N and F induce gastrula stage animal caps to form?

A

‘Neural tissue of rostral character’ - basically they cause expression of foxg1 from the forebrain.

26
Q

What happens if you inject chordin or noggin into the ventral blastomeres? Does the same thing happen with follistatin?

A

It causes a conjoined twin to form, with a nervous system, notochord and somites. Follistatin has no effect.

27
Q

If you inject chordin or noggin into the ventral blastomeres, do fore/midbrain structures form?

A

No.

28
Q

How do we know chordin is essential for the organiser?

A

Using antisense oligonucleotides to prevent chordin mRNA translation in graft experiments means that the graft does not form a conjoined twin. Instead it forms ventral tissues.

29
Q

Can noggin and follistatin compensate for the loss of chordin in the graft?

A

No.

30
Q

If antisense (morpholino) oligonucleotides for chordin, noggin and follistatin are injected, what happens? What does this infer?

A

No nervous tissue or muscle forms, plus ventral tissues expand. All 3 proteins are required for normal organiser function.

31
Q

What do C, N and F all inhibit?

A

BMPs (bone morphogenetic proteins). They each prevent BMPs from binding and activating their targets.

32
Q

What is BMP necessary for?

A

Dorsal development.

33
Q

Why is inhibiting BMP necessary?

A

It allows dorsal development.

34
Q

Where is bmp4 expressed?

A

Ventrally.

35
Q

Where is bmp4 never found?

A

In the organiser or the neural plate.

36
Q

What happens if you inject xenopus embryos with bmp4 mRNA?

A

It ventralises the embryo. The notochord, somites and neural plate are not formed.

37
Q

The effect of bmp4 is concentration dependent. What does this mean?

A

It only causes ventralisation in high concentrations.

38
Q

What receptor inhibits bmp4 signalling?

A

dnBMPR1 (dominant-negative BMP receptor 1)

39
Q

What happens if bmp4 is inhibited?

A

The embryo becomes dorsalised.

40
Q

C, N, F and bmp4 could be described as morphogens. Describe their gradients across the embryo. What does this create?

A

C, N, F: high conc. from the organiser on the dorsal side to low ventral conc.

bmp4: high ventral conc. to low dorsal conc.

This creates the dorsal-ventral axis.

41
Q

The organiser produces proteins that inhibit Wnt signalling. They are a) Cerberus, b) Frzb and c) Dkk1. What do they do?

A

a) and b) direcelty bind Wnt molecules.

c) Blocks Wnt receptors.

42
Q

The organiser produces proteins that inhibit Wnt signalling. They are a) Cerberus, b) Frzb and c) Dkk1. What do they do?

A

a) and b) directly bind Wnt molecules.

c) Blocks Wnt receptors.

43
Q

Cerberus, frzb and dkk1 are expressed more anteriorly in the organiser than chordin. True or false?

A

True.

44
Q

Why inhibit Wnt signalling at the anterior of the embryo?

A

It induces head formation.

45
Q

frzb mRNA will cause a head to be formed in the conjoined twin produced by ventrally injecting chordin mRNA. This head is cyclopic. Why?

A

Another signal is required for normal head formation, probably nodal.

46
Q

Wnt patterning is responsible for which axis? How?

A

Wnt signalling forms the anterior-posterior axis. Low Wnt specifies the anterior with high Wnt specifying the posterior.

47
Q

Which germ layers is the organiser responsible for patterning?

A

The meso and ectoderm.

Development (18 decks)

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