Cortical reaction and Nieuwkoop centre

Question 1

What happens during the early development of Xenopus?

Answer 1

1. Contraction of the pigment
2. Pigmented side up
3. Pigment starts to rotate- first division after 90mins
4. White spot= meiotic spindle
5. Vitalin membrane expands to prevent other sperm from entering.
6. Embryo rotates according to gravity.

Question 2

What does the gene coco do?

Answer 2

Inhibit TGFb signalling

Question 3

What is vg1, when is it expressed and where?

Answer 3

TGFb family member, expressed early during oogenesis, localised at vegetal cortex of the oocyte.
Required for mesoderm induction later during development.

Question 4

What is whole mount in situ hybridisation good for?

Answer 4

Finding out where the gene is expressed

Question 5

Process of the cortical reaction

Answer 5

• Fertilistion induces a reorganisation of the microtubule network.
• 90mins after fertilisation the cortex rotates approx. 30deg.
• The vegetal cortex moves towards the animal pole.
• This leads to the formation of a signalling centre opposite the sperm entry site: the Nieuwkoop centre.

Question 6

How does the rearrangement of the microtubule network happen?

Answer 6

Sperm brings a centriole which matures to a centrosome. This is able to nucleate the microtubules and cause a rearrangement of the microtubule network to be more parallel (less messy).

Question 7

What does the Nieuwkoop centre do?

Answer 7

Region of the embryo that defines the dorsal side and influences the development of surrounding cells.

Question 8

What do the 1st and 2nd cleavages separate?

Answer 8

1st cleavage: separates anterior from posterior

2nd cleavage: separates ventral from dorsal (pigment/non-pigment)

Question 9

What does grafting the most dorsal/ventral blastomere on the ventral side of a host embryo cause?

Answer 9

Twinned embryo with two dorsal sides and two heads

Question 10

What type of graft doesn’t cause any effects?

Answer 10

Ventral cells grafted onto the dorsal side of the host embryo.

Question 11

What is a signal from the Nieuwkoop centre required for?

Answer 11

Developing dorsal and anterior structures

Question 12

What happens if you shine UV light on the embryo?

Answer 12

• UV light disrupts the microtubule network blocking the cortical reaction.
• UV light treatment results in a decrease of dorsal and anterior structures.

There is a direct link between cortical reaction and Nieuwkoop centre.

Question 13

How was the molecular nature of the Nieuwkoop centre identified?

Answer 13

A double axis was formed by injecting the most ventral vegetal blastomere with B-catenin.

Question 14

What are the predictions for the importance of canonical Wnt signalling in the establishment of the DV axis?

Answer 14

1. Nuclear B-catenin should be localised in the dorsal side of the embryo.
2. Exposing the egg to UV light should reduce nuclear B-catenin throughout the embryo.
3. Stabilising B-catenin should result in a formalised embryo.