Mesoderm differentiation and Somitogenesis Flashcards
What are 4 types of mesoderm?
Axial, paraxial, lateral plate, intermediate
What does the axial mesoderm become?
Notochord
What does the paraxial mesoderm become?
Somites and head
Which type of mesoderm splits into two plates as development progresses? What are they?
Lateral plate mesoderm splits into the somatic mesoderm and the splanchic mesoderm
What does the lateral plate mesoderm become?
Skeleton, muscle, circulatory system
What does the somatic mesoderm become?
Muscles
What does the splanchic mesoderm become?
Connective tissues of internal organs
What does the intermediate mesoderm become?
Kidneys and gonads
How does the anterior-posterior axis in elongate in vertebrates?
Proliferation of neuromesodermal progenitor cells in the posterior progenitor zone
How does the notochord elongate?
The notochord cells inflate their vacuole and spread out
What are the 3 things somites differentiate into?
Sclerotome, myotome, dermatome
What does the sclerotome become?
Vertebrae and rib cage
What does the myotome become?
Back/rib cage and body wall muscles
What does the dermatome become?
Skeletal muscles and some dermis
What is somitogenesis?
The anterior-posterior division of the paraxial mesoderm that separates it into distinct units
What are the 5 steps in somitogenesis?
- Establish molecular periodicity
- Fissure formation between 2 cell populations
- Epithelialization
- Specification
- Differentiation
What are the two steps that establish periodicity in the presomitic mesoderm?
The wave and the clock
What 3 signals are involved in the wave to cause periodicity in the somites?
Retinoic acid, FGF, and Mesp
Where is retinoic acid expressed in the somites?
Anterior
Where is FGF expressed in somites?
Posterior
Where is Mesp expressed in the somites?
Where RA and FGF are both absent
What does Mesp do?
Activates transcription of Eph A4 - an ephrin receptor
How is the fissure between somites formed?
Cells with EphA4 are localized in the anterior end of the somite, and are repelled by cells with ephrin B2. The EphA4 cells are also repelled by the posterior end of the next somite and causes a fissure between them
What regulates the pattern of Mesp expression?
Delta-Notch signalling. Isolates Mesp to the anterior end of the somite
Why is Mesp not expressed in the posterior end of the somites?
Those cells are receiving FGF signalling which prevents Notch from responding to Delta. Cells get pushed away from the FGF signal towards the anterior end and then Notch can respond to Delta
What are the two transcription factor activated by Notch in the anterior somite?
Mesp and Lunatic Fringe (Lnfg)
What does Lunatic Fringe do?
Inhibits Notch
What is causing the clock that establishes periodicity?
Self inhibition by Notch
What determines the number of somites?
The rate at which periodicity occurs
Why do snakes have more somite divisions?
They have more areas of Lunatic Fringe activity, which creates more somite divisions, but each somite is smaller
