Sea urchins Flashcards
How many eggs do female sea urchins lay?
Approx. 1 million
Why do embryos undergo synchronous development?
They are all fertilised simultaneously.
Sea urchins are good model organisms. Give 6 reasons why.
- Embryos are transparent
- Cell cycle has been extensively studied
- Easy to manipulate
- Genome has been fully sequenced
- Regulatory networks studied
- V. rapid development
How many days does it take for an embryo to become a larva?
3 days.
How long is the larval stage?
6-8 weeks.
There are 2 forms of larvae, what are they?
- Prism
2. Pluteus
How does the larva become an adult?
It undergoes metamorphosis.
Which larval stage is motile?
Both.
At what stage are the cells totipotent?
Zygote.
What is the hyaline layer?
A membrane around the embryo that elevates after fertilisation.
What is the purpose of the hyaline layer?
It prevents polyspermy.
Is the nucleus visible after the elevation of the hyaline layer? Why, why not?
Yes - the male and female protonuclei come together to form the zygote nucleus.
What kind of cleavage divisions do sea urchins have?
Radial holoblastic cleavage.
What is meant by holoblastic cleavage?
The cleavage furrow goes through the whole embryo.
What is meant by radial cleavage?
The cells are directly above/below eachother, there is radial symmetry around the pole.
Describe the nature of the first 4 cleavage divisions.
- Meridional
- Meridional but perpendicular to the first
- Equatorial - the respective poles are split
- Unequal as the resultant hemispheres are different
What is meant be meridional?
The cleavage furrow extends through both hemispheres at once.
In the fourth division, 4 animal cells undergo cleavage to form what?
8 mesomeres.
In the fourth division, 4 vegetal cells undergo division to form what?
Macro and micromeres.
Sea urchins have regulative development. What does this mean?
If you split the embryo at the 4 cell stage, each cell will give rise to a normal yet dwarfed adult.
Regulative development is based on a ‘harmonious equipotential system’. What does this mean?
Cell interaction is critical for development.
What forms at the 64 cell stage?
A cleavage cavity filled with water which then becomes the blastocoel.
What happens during the mesenchyme blastula stage? Give 3 steps.
- The micromeres begin to ingress
- The EMT occurs
- The endoskeleton is formed
Where does the ingression of the micromeres begin?
At the vegetal pole.
What is the EMT?
The endothelium-mesenchyme transition, where the cells lose affinity for the hyaline layer and move inwards towards the basal laminar.
What are the first cells to ingress called?
The PMC - primary mesenchyme cells.
Where do the PMC come from?
They are descended from the micromeres.
Why is the formation of the endoskeleton crucial for gastrulation?
A structural change within the embryo is needed to support the gut.
How do the PMC support the gut?
They form a ring of skeletal rods around (what is to be) the archenteron.
What cells form the endoskeleton?
The PMC.
What are the SMC and what do they do in gastrulation?
The secondary mesenchyme cells - the undergo convergent extension by extending their filopodia.
How does convergent extension help gut formation?
It ‘pulls the gut into place’.
Which cells are the only group to become autonomously specified?
The micromeres - even when split from the embryo they will develop normally.
What forms the organiser and what is it?
The micromeres (a group of 4), a signalling centre.
The vegetal cells have ‘autonomous nuclearisation of beta-catenin’. What happens? Give 2 steps.
- Dsh inhibits GSK3-beta
2. GSK3-beta can no longer degrade beta-catenin which then moves into the nucleus
The nuclearisation of beta-catenin is part of which signalling pathway?
Wnt signalling
Where is Dsh localised?
In the vegetal hemisphere.
What is anisotrophy?
Directionally dependent expression/signalling etc. It is non-homogenous throughout the embryo.
Which proteins cause anisoptrophy, i.e. the nuclearisation of beta-catenin in the vegetal cells?
Maternal proteins.
What is otx and what does it do?
A TF that is only nuclearized in the vegetal cells that form the micromeres.
What is soxB1 and where is it excluded from?
A TF that is excluded from the micromeres.
What is pmar1 and what is it activated by?
A TF activated by maternal factors.
How are maternal factors expressed?
Transiently.
When is pmar1 first transcribed?
At the beginning of the fourth cleavage division.
What does pmar1 do and how does it do this?
- Activates the entire micromere/skeletogenic programme
2. Via a ‘double-negative gate’, it represses a repressor.
What does pmar1 repress?
HesC
What does HesC do?
Represses the skeletal genes.
What happens when HesC is repressed?
The skeletal genes are activated and initiate skeletogenesis.
The maternal genes that initiate the pmar1-hesC complex are transiently expressed. How is skeletogenesis stabilised?
By a positive feedback loop of 3 genes; Erg, Hex, Tgif
Where are these TFs in the regulatory sequence located in relation to each other?
Downstream of each other
How is differentiation of the different cell types regulated?
By a combination of TFs
What is alx1 and what does it do?
A TF that represses NSM fate, thus is expressed in the skeletogenic mesoderm.
What does NSM stand for?
Non-skeletogenic mesoderm.
What happens if you remove the micromeres?
There are no PMC, thus no endoskeleton or gastrulation.
What is the veg2 macromere?
A large blastomere in the vegetal hemisphere that gives rise to the endomesoderm.
Is there nuclearisation of beta-catenin in the veg2 macromere?
Yes.
What causes the initiation of gene circuitry in the veg2 macromere?
Maternal factors (that initiate Wnt) and early signals from the micromeres.
What does Notch/Delta signalling do with the veg2 macromere?
Causes a patterning of endoderm and mesoderm by activating NSM genes.
What is gcm?
An NSM gene expressed in the mesoderm.
What does Delta do to gcm?
Downregulates it.
What is FoxA?
An NSM gene expressed in the endoderm.
Along which axis do the NSM cells become subdivided?
The oral-aboral axis.
What is Not?
An oral gene that represses gcm in the oral quadrant.
