Establishing a body plan II Flashcards
What are the 3 subdivisions of the mesoderm from innermost to outermost? Describe each.
Paraxial- closest to and parallel with the notochord. Becomes segmented with into somites
Intermediate- Lateral to the paraxial mesoderm. Gives rise to the urogenital system.
Lateral Plate- lateral to the intermediate mesoderm. Splits to form the lining of body cavities and of most internal organs/limbs.
Describe the developmental pathway of the mesoderm.
What does the mesoderm arise from, where does it pass and what does it become as it passes, what does it ultimately form?
The mesoderm originates from the epiblast, passes through the primitive streak as bottle cells, then spreads laterally to form a layer between the ectoderm and endoderm.
Somitomeres verses Somites. What is a Somitomere? Where are they added?
They are initial pairs of segments of mesenchyme that begin to develop along either side of the neural plate in the paraxial mesoderm.
New somitomeres are added caudally as the primitive streak regresses.
Somitomeres verses Somites. What is a Somite? What direction are they added?
More dense blocks of mesoderm that form along the notochord.
In the caudal direction, a pair of somitomeres is transformed into a pair of somites. In the anterior direction, pairs of somitomeres are formed.
What are the 2 steps in somitogenesis?
Wavefront
Segmentation Clock
Describe the 2 regulatory factors involved in the stages of somite differentiation, AKA somitogenesis. What do the balance between these two lead to?
Note* What is another molecule that facilitates this balance between these 2 molecules?
An increase in [FGF-8] leads to mitosis in mesenchymal cells in the posterior primitive streak
An increase in [retinoic acid] more anteriorly opposes the action of FGF-8
The balance between these leads to the formation of what is called the Wavefront, which refers to the determination towards somitogenesis.
*Note that Mesp-2 is important in this delicate balance between FGF-8 and retinoic acid.
Describe the Segmentation clock in Somitogenesis. Oscillating molecules in what pathway lead to this. What are the 2 molecules that oscillate? What do they form?
Oscillating molecules in the Notch pathway (Wnt and FGF) stimulate the expression of the lunatic fringe at the anterior and c-hairy at the posterior border of a future somite.
Describe the molecular events involved in the differentiation of somites. (Look at sheet)
Ectoderm dorsal to somites expresses Wnt-6, which stimulates the release of Paraxis in somites.
The release of Paraxis and a decrease in Snail leads to mesenchymal cells to converting to epithelial cells and the formation of a somitocoel.
Notochord releases Shh and Noggin, which stimulates the release of Pax-1 and Pax-9 in the ventral part of somite, which leads to the formation of the sclerotome
Wnt genes in dorsal neural tube leads to the transformation of dorsal half of somite into dermomyotome.
Dermomyotome expresses Pax3, Pax7, and Paraxis which leads to the separation of the dermomyotome into dorsal dermotome and ventral dermotome.
What is a dermomyotome?
This is the dorso-lateral part of a somite
What do the signaling factors Shh and Wnt lead to in the organization of a somite?
They are released from the dorsal neural tube and create a balance that causes the myotome portion to commit to the myogenic lineage.
What is the role of BMP-4 in the organization of a somite?
What does it inhibit and what does it stimulate?
BMP-4 is produced by the lateral plate and inhibits myogenesis in the ventrolateral dermomyotome.
Stimulates cells from this area to migrate from the somite into the limb bud.
What is the role of Noggin in the organization of a somite?
Inhibits BMP-4 from inhibiting myogenesis
What is the role of FGF in the organization of a somite? What does it stimulat the sclerotome to produce and what does the production of that molecule cause?
Released from the myotome and signals the sclerotome to produce scleraxis, which causes the anterior and posterior borders of each somite to form the syndetome, a precursor of tendons!
Describe the Sclerotomal breakup. What does it break into? What 2 parts of the somite join together and what does this allow to pass through?
A sclerotomal portion of each somite breaks up into an anterior and posterior portion.
The posterior portion of one somite joins the anterior portion of another adjacent somite, which creates a gap that allows nerves to grow out from the neural tube into the muscles derived from the myotomes.
What 2 signaling molecules stimulate the formation of the Intermediate Mesoderm and what does that intermediate mesoderm express?
BMP(from lateral ectoderm) and Activin (from paraxial mesoderm)
Intermediate mesoderm expresses Pax-2
