Segment formation

Question 1

What is a pair-rule gene?

Answer 1

Gene involved in the development of the segmented embryos of insects. Expressed as a result of differing concentrations of gap gene proteins, which encode transcription factors controlling pair-rule gene expression.

Question 2

Where do adult vertebrate segments come from?

Answer 2

Somites.

Question 3

What do somites go on to form?

Answer 3

• Vertebrae and ribs
• Skeletal muscle
• Dermis of back

Question 4

What happens at the node?

Answer 4

Cell proliferation.

Question 5

how is constant length of the presomitic mesoderm maintained?

Answer 5

Somites form at a given distance from the node.

Question 6

How is segmentation different in vertebrates to Drosophila?

Answer 6

In vertebrates the segments bud off one by one whereas in Drosophila segments all emerge at the same time by subdivision of existing space.

Question 7

What are the 3 features of somite formation and what questions are associated with them?

Answer 7

1. Temporal: how is a constant interval between somite appearance controlled?
2. Spatial: what controls the distance between the node and the site where somites appear?
3. Physical: how do groups of cells bud off from the presomitic mesoderm to form somites?

Question 8

Where is hairy expressed?

Answer 8

Presomitic mesoderm and somites BUT irregular pattern of expression between different embryos.

Question 9

Why is hairy expression inconsistent between embryos?

Answer 9

Hairy expression is dynamic: it changes rhythmically, with a 90min period.

Question 10

How is hairy activated and deactivated?

Answer 10

Activated by Notch signalling. Hairy inhibits Notch so Hairy is no longer active.

Question 11

How are somites formed at a given distance form the node?

Answer 11

Maturation front coming from the anterior to posterior of the presomitic mesoderm. When oscillation reaches maturation front you got signals telling cells to become somites.
Somites can only form where there is not enough FGF8 to stop them doing so. FGF8 gradient instructs cells on their distance from the node.

Question 12

Where is the expression of FGF8 highest in the embryo?

Answer 12

At the posterior

Question 13

What does FGF8 do?

Answer 13

Inhibits somite formation.

Question 14

What does impaired FGF8 cause?

Answer 14

• More somites
• Premature somites
• Somites form closer to the node

Question 15

What causes cells to stick to one another?

Answer 15

N-Cadherin

Question 16

What causes cells to repel one another?

Answer 16

Eph/Ephrin interaction (RTKs)

Question 17

Comparison of segmentation mechanisms between Drosophila and vertebrates.

Answer 17

Drosophila

• subdivision of existing space
• simultaneous appearance
• maternal, gap and pair-rule genes
• maternal input
• gradient driven, then refined by gap and pair rule genes

Vertebrate

• growth and budding
• sequential appearance
• notch pathway, Hairy, FGF8
• no maternal input
• ‘clock and ruler’ regulated

Question 18

What is spatial collinearity?

Answer 18

Where they are positioned determines order of expression.

Question 19

What is discrete prevalence?

Answer 19

At each point of AP axis have discrete paralogous group of Hox genes in charge of that segment.

Question 20

What is combinatorial prevalence?

Answer 20

Combination of Hox genes which gives a discrete fate.

Question 21

What is posterior prevalence?

Answer 21

Most 5’ expressed gene which is dominant- recognisable structures but anteriorisation.

Question 22

What do 3’ genes affect?

Answer 22

anterior structures

Question 23

What do 5’ genes affect?

Answer 23

Posterior structures

Question 24

What does retinoic acid cause?

Answer 24

A high conc. of retinoic acid administered in utero causes a misexpression of Hox genes- results in developmental transformation such as more posterior patterns.