Unit 3: Somitogenesis Flashcards
What are the 3 adult vertebrate derivates of somites ?
- Axial skeleton
- Skeletal muscle
- Dermis
Which germ layer gives rise to somites ?
Paraxial mesoderm
What are the 6 steps in somite generation ?
1) Gastrulation
2) Mesoderm
3) Paraxial mesoderm
4) Presomitic mesoderm
5) Somitomeres
6) Somites
What part of the somite is epithelial ?
The outside
What part of the somite is mesodermal ?
The inside
What processes are important in somitogenesis (3) ?
- Cell division
- Mesenchymal to epithelial transition
- Fate restriction
What can cell division in the presomatic mesoderm provide ?
It provides more tissue that is able to differentiate into somites
Does the length of presomatic mesoderm stay constant during cell division ?
Yes it does
What is Fgf8 ?
A signalling molecule that is expressed in cells around the node
Where are low levels of Fgf8 expressed ?
Rostral/ anterior
Where are high levels of Fgf8 expressed ?
Caudal/ posterior
What would happen if Fgf8 expression was lower ?
The first somites would develop closer to hensens node
Do adjacent tissues influence somitogenesis ?
No, environment is not important
Is the number of cells in the presomitic mesoderm important ?
No, if you remove a slice of presomitic mesoderm somites still form at normal rate- only smaller in size
What evidence shows that somite cell identity is already determined in the presomitic mesoderm ?
An experiment done shows that the identity of somites swapped when pre somitic mesoderm was inverted. e.g. posterior tissues formed posterior vertebrae
What is homeotic transformation ?
The conversion of one body part to another (Usually more anterior)
What controls WHEN somites are formed ?
Cycles of hairy expression
What controls WHERE somites are formed ?
Fgf8
What controls somite identity ?
HOX genes
Are anterior or posterior somites older ?
- Anterior somites are older
- They are more developed and differentiated then posterior somites
(In roman numerals) What is the most newly formed and most posterior site called ?
I
What is a young somite (somites I-III) called ?
Somitocoel cell- An epithelial ball surrounding cells that remain mesenchymal
What portion of somites undergoes EMT to become sclerotome ?
Ventral-medial somites (Somites IV-V)
What forms dermomyotome ?
Dorsal somites- shaped like a fitted sheet
When somites reach VI-X, where do they ingress and what do they form (2) ?
- Dermomyotome cells ingress from the dorsal medial lip and ventral lateral lip
- To form a new myotome layer between dermomyotome and sclerotome
What type of cells do not divide ?
Myotomal cells
What is patterning into somite derivatives driven by ?
Specific signals from the neural tube and notochord
What gene is required for myotome formation ?
Want
What tissue gives rise to vertebrae ?
Sclerotome
