L6- Somitogenesis

Question 1

What are somites?

Answer 1

• Blocks of mesoderm that form the vertebral column and associated musculature. They order the peripheral nerves and compartmentalise progenitor cells.
• Total number of somites varies between different vertebrates as it relates to the length of backbone.
• Somitic fate is pre-determined, and doesn’t change with location.

Question 2

How do somites form in the chick?

Answer 2

• Mesodermal cells ingress during gastrulation and are then organised into different domains.
• Somites in pairs form on either side of the notochord- paraxial mesoderm.

Question 3

What is somitogenesis?

Answer 3

The process by which somites form- bud off from the anterior part of the pre-somitic mesoderm.

Question 4

What are the steps in somitogenesis?

Answer 4

1. Periodicity- regular blocks of tissue forming accurately
2. Separation of somites
3. Epithelialisation
4. Specification
5. Differentiation

Question 5

How is paraxial and lateral mesoderm differentiated?

Answer 5

High levels of Bmps promote lateral plate mesoderm

Ectopic noggin can promote somite formation laterally

Question 6

How do somites form in regular pairs?

Answer 6

Various mechanisms:
• Notch-delta signalling e.g delta1 in the mouse is expressed in presomitic mesoderm and required for segmentation (but not derivatives)

• Clock and wave front hypothesis- cHairy1 is a BLHH transcription factor which is expressed in waves in somite formation in the PSM.

Question 7

How does somite epithelialisation occur?

Answer 7

• Dependent upon signals from the neural tube and surface ectoderm.
• Paraxis for epithelialisation of somite but not segmentation of PSM.

Question 8

How are spinal nerves and vertebral bones patternedd?

Answer 8

• Somites have rostral (anterior) and caudal (posterior) halves.
• NCC’s and axons are repulsed by the caudal halves.

Question 9

What happens in the regionalisation of each somite?

Answer 9

At each axial level somite will generate:
• Sclerotome- cartilage of vertebrae, PAX1 expression
• Myotome- muscle
• Dermatome- dermis of skin
• Syndetome- forms tendons

Question 10

How does somite differentiation occur?

Answer 10

Sclerotome induced by ectopic notorchord
Shh and noggin impacts PAX1 expression

Signals from neural tube and ectoderm support dermamyotome development- Wnt6

SCX expressed in syndetome, linked to FGF.