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Development > Sea urchins > Flashcards

Sea urchins Flashcards

1
Q

How many eggs do female sea urchins lay?

A

Approx. 1 million

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2
Q

Why do embryos undergo synchronous development?

A

They are all fertilised simultaneously.

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3
Q

Sea urchins are good model organisms. Give 6 reasons why.

A
  1. Embryos are transparent
  2. Cell cycle has been extensively studied
  3. Easy to manipulate
  4. Genome has been fully sequenced
  5. Regulatory networks studied
  6. V. rapid development
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4
Q

How many days does it take for an embryo to become a larva?

A

3 days.

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5
Q

How long is the larval stage?

A

6-8 weeks.

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6
Q

There are 2 forms of larvae, what are they?

A
  1. Prism

2. Pluteus

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7
Q

How does the larva become an adult?

A

It undergoes metamorphosis.

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8
Q

Which larval stage is motile?

A

Both.

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9
Q

At what stage are the cells totipotent?

A

Zygote.

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10
Q

What is the hyaline layer?

A

A membrane around the embryo that elevates after fertilisation.

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11
Q

What is the purpose of the hyaline layer?

A

It prevents polyspermy.

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12
Q

Is the nucleus visible after the elevation of the hyaline layer? Why, why not?

A

Yes - the male and female protonuclei come together to form the zygote nucleus.

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13
Q

What kind of cleavage divisions do sea urchins have?

A

Radial holoblastic cleavage.

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14
Q

What is meant by holoblastic cleavage?

A

The cleavage furrow goes through the whole embryo.

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15
Q

What is meant by radial cleavage?

A

The cells are directly above/below eachother, there is radial symmetry around the pole.

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16
Q

Describe the nature of the first 4 cleavage divisions.

A
  1. Meridional
  2. Meridional but perpendicular to the first
  3. Equatorial - the respective poles are split
  4. Unequal as the resultant hemispheres are different
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17
Q

What is meant be meridional?

A

The cleavage furrow extends through both hemispheres at once.

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18
Q

In the fourth division, 4 animal cells undergo cleavage to form what?

A

8 mesomeres.

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19
Q

In the fourth division, 4 vegetal cells undergo division to form what?

A

Macro and micromeres.

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20
Q

Sea urchins have regulative development. What does this mean?

A

If you split the embryo at the 4 cell stage, each cell will give rise to a normal yet dwarfed adult.

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21
Q

Regulative development is based on a ‘harmonious equipotential system’. What does this mean?

A

Cell interaction is critical for development.

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22
Q

What forms at the 64 cell stage?

A

A cleavage cavity filled with water which then becomes the blastocoel.

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23
Q

What happens during the mesenchyme blastula stage? Give 3 steps.

A
  1. The micromeres begin to ingress
  2. The EMT occurs
  3. The endoskeleton is formed
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24
Q

Where does the ingression of the micromeres begin?

A

At the vegetal pole.

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25
Q

What is the EMT?

A

The endothelium-mesenchyme transition, where the cells lose affinity for the hyaline layer and move inwards towards the basal laminar.

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26
Q

What are the first cells to ingress called?

A

The PMC - primary mesenchyme cells.

27
Q

Where do the PMC come from?

A

They are descended from the micromeres.

28
Q

Why is the formation of the endoskeleton crucial for gastrulation?

A

A structural change within the embryo is needed to support the gut.

29
Q

How do the PMC support the gut?

A

They form a ring of skeletal rods around (what is to be) the archenteron.

30
Q

What cells form the endoskeleton?

A

The PMC.

31
Q

What are the SMC and what do they do in gastrulation?

A

The secondary mesenchyme cells - the undergo convergent extension by extending their filopodia.

32
Q

How does convergent extension help gut formation?

A

It ‘pulls the gut into place’.

33
Q

Which cells are the only group to become autonomously specified?

A

The micromeres - even when split from the embryo they will develop normally.

34
Q

What forms the organiser and what is it?

A

The micromeres (a group of 4), a signalling centre.

35
Q

The vegetal cells have ‘autonomous nuclearisation of beta-catenin’. What happens? Give 2 steps.

A
  1. Dsh inhibits GSK3-beta

2. GSK3-beta can no longer degrade beta-catenin which then moves into the nucleus

36
Q

The nuclearisation of beta-catenin is part of which signalling pathway?

A

Wnt signalling

37
Q

Where is Dsh localised?

A

In the vegetal hemisphere.

38
Q

What is anisotrophy?

A

Directionally dependent expression/signalling etc. It is non-homogenous throughout the embryo.

39
Q

Which proteins cause anisoptrophy, i.e. the nuclearisation of beta-catenin in the vegetal cells?

A

Maternal proteins.

40
Q

What is otx and what does it do?

A

A TF that is only nuclearized in the vegetal cells that form the micromeres.

41
Q

What is soxB1 and where is it excluded from?

A

A TF that is excluded from the micromeres.

42
Q

What is pmar1 and what is it activated by?

A

A TF activated by maternal factors.

43
Q

How are maternal factors expressed?

A

Transiently.

44
Q

When is pmar1 first transcribed?

A

At the beginning of the fourth cleavage division.

45
Q

What does pmar1 do and how does it do this?

A
  1. Activates the entire micromere/skeletogenic programme

2. Via a ‘double-negative gate’, it represses a repressor.

46
Q

What does pmar1 repress?

A

HesC

47
Q

What does HesC do?

A

Represses the skeletal genes.

48
Q

What happens when HesC is repressed?

A

The skeletal genes are activated and initiate skeletogenesis.

49
Q

The maternal genes that initiate the pmar1-hesC complex are transiently expressed. How is skeletogenesis stabilised?

A

By a positive feedback loop of 3 genes; Erg, Hex, Tgif

50
Q

Where are these TFs in the regulatory sequence located in relation to each other?

A

Downstream of each other

51
Q

How is differentiation of the different cell types regulated?

A

By a combination of TFs

52
Q

What is alx1 and what does it do?

A

A TF that represses NSM fate, thus is expressed in the skeletogenic mesoderm.

53
Q

What does NSM stand for?

A

Non-skeletogenic mesoderm.

54
Q

What happens if you remove the micromeres?

A

There are no PMC, thus no endoskeleton or gastrulation.

55
Q

What is the veg2 macromere?

A

A large blastomere in the vegetal hemisphere that gives rise to the endomesoderm.

56
Q

Is there nuclearisation of beta-catenin in the veg2 macromere?

A

Yes.

57
Q

What causes the initiation of gene circuitry in the veg2 macromere?

A

Maternal factors (that initiate Wnt) and early signals from the micromeres.

58
Q

What does Notch/Delta signalling do with the veg2 macromere?

A

Causes a patterning of endoderm and mesoderm by activating NSM genes.

59
Q

What is gcm?

A

An NSM gene expressed in the mesoderm.

60
Q

What does Delta do to gcm?

A

Downregulates it.

61
Q

What is FoxA?

A

An NSM gene expressed in the endoderm.

62
Q

Along which axis do the NSM cells become subdivided?

A

The oral-aboral axis.

63
Q

What is Not?

A

An oral gene that represses gcm in the oral quadrant.

Development (18 decks)

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