Lecture 2 - Regulating the regulators Flashcards
How does the timing and localisation of MyoD and myf5 expression vary among species? What does this indicate?
Avians
MyoD: expressed first in the medial part of the somite adjacent to the neural tube
Followed quickly by myf-5 and then myogenin
Mammals
myf5 expressed first in somites at 8dpc
myoD expression follows later at 10dpc
Frogs
myoD and myf5 are activated in the early mesoderm
Mice
all 4 MRF genes have different expression patterns during early development
Distinct expression patterns of the MRFs indicate that each has a distinct role during skeletal muscle development
Give an example of the different expression patterns of the MRF genes in mice
myf5 is initially expressed in the dorsal medial (epaxial) mytome very early
myoD expressed ventral lateral (hypaxial) myotome later
Why do myf5 (-/-) mice have delayed skeletal muscle formation?
mrf5 is expressed before myoD in mice
What do MRFs bind?
MRF are BHLH proteins that bind E-boxes (simple base pair)
How do somites form?
Condense from the segmental plate mesoderm as balls of columnar epithelial cells
Happens in the anterior to posterior (rostral to caudal) direction
What do the hypaxial cells of the myotome form?
migrate out and form muscles of the limbs
What is the hypaxial myotome otherwise known as?
the ventral lateral myotome
What is the epaxial myotome otherwise known as?
the dorsal medial myotome
How have lineage studies demonstrated the fate of of the medial and lateral halfs of the somites?
Using chick/quail chimeras (quail cells have dense purple nuclei - replaced medial part of the somite with quail somite) of half somite grafts
- the medial part of the early somite gives rise to the myotome and epaxial muscles
- lateral part of the early somite gives rise to cells that migrate to the limb and for hypaxial muscles
When the medial and laterial parts of the early somite are swapped, the somite reorganises and gives rise to normal myotome and limb musculature. Showing the somite cells are still plastic in the early somite and the local environment influences their speciation
How did Powell demonstrate visually that there may be an external signal influencing the speciation of the somites?
- In situ hybridsation
- for MyoD in newly segmented somites: expression is adjacent to the axial structures ) NT/NC
- Embryological manipulations
- If move the somite, respecifies because of the signals around it
What is the environment of the somite?
Medial part of the somite up against the dorsal midline (axial) structures: neural cord tube and notochord
How do somites form?
Condense from the segmental plate mesoderm in balls of columnar epithelial cells
In an anterior to posterior direction (rostral caudal)
What is an inducer and what is the mesoderm induced by?
Mesoderm: FGF, nodals
Inducing: cell signalling tells cells to be a certain part e.g. mesoderm/ectoderm
What did Powell show about MRF expression in the early somite?
Both Myf5 and MyoD on in the early somite
MyoD only expressed in the medial domain
If move the early somite it respecifies because f the signals around it
What did Powell demonstrate about the MRF expression in the somites by embryological manipulations?
Used embryological manipulations to demonstrate that the mideline structures (specifically the notochord) are required for the activation and maintenance of MRF gene expression in the somites