Lecture 11: A-P patterning - somite formation Flashcards
Explain the embryonic origins of somites in the chick embryo.
The check embryo primitive streak along the A-P axis contains Henson’s node, formed from mesoderm during gastrulation. Cells of the blastoderm involute and emerge inside as mesoderm or endoderm. They undergo epithelial to mesenchymal transition and migrate laterally.
Cells in Henson’s node become axial mesoderm, which goes on to form the prechordal mesoderm and notochord.
Then moving posteriorly: paraxial mesoderm, intermediate mesoderm and the lateral plate mesoderm.
The paraxial mesoderm gives rise to the somites. Anteriorly it gives rise to the head, but this isn’t segmented.
How can we show that cells are specified to various regions of the mesoderm?
Inject a fluorescent dye at various levels of the primitive streak.
What are somites?
Somites are transient structures laid down in bilateral pairs along the developing spinal cord and are the earliest evidence of segmentation in vertebrates. Mesodermal segmentation is conserved throughout evolution.
Give some background on somatogenesis.
As the notochord is laid down, mesoderm on each side begins to form the somites. This has been studied in chick embryos mainly because they are easy to observe.
The most anterior paraxial mesoderm doesn’t segment because it forms the head. Posterior to the hindbrain and anterior to the regressing Henson’s node, somite formation begins from anterior to posterior.
Somite formation occurs in the pre-somatic mesoderm which lies between the most recently formed somite and the Henson’s node. In chicks, pairs form every 90 minutes. Somatogenesis involves epithelial-to-mesenchyme transition.
Describe the history of the clock and wave front model, what it proposes and the modern additions.
The model was proposed before molecular biology to explain the periodic and spatial organisation of the somites.
Theories:
- The size of a somite is defined as a distance that a ‘wave’ of determination travels from anterior to posterior during one oscillation of a ‘clock’ that operates in the mesoderm
- When cells in the pre-somatic mesoderm have passed the determination wave front they receive a signal from a clock of oscillatory gene expression that commits them to form a somite
The first molecular evidence for a ‘clock’ was the oscillatory expression of the gene hairy-1 during somatogenesis in chick embryos. 1 oscillation of gene expression means 1 pair of somites formed (90 mins) and therefore it must drive segmentation.
Now, more genes have been found to oscillate such as lunatic fringe.
How is the position of the wave front specified?
By levels of FGF and Wnt which move from the posterior to anterior. Somite formation occurs where FGF and Wnt are below a threshold level.
Another gradient of RA runs from anterior to posterior and antagonises FGF, preventing the pre-somatic mesoderm region from continually getting longer.
When does somatogenesis start?
In each cycle of gene expression, each cell in the pre-somatic mesoderm expresses a given gene for a given period of time. The wave of gene expression meets the determination wave front i.e., where RA and FGF/Wnt expression intersects, and stops at a boundary where somatogenesis starts.
What happens at the determination front?
Cells acquire the specification to form somites and acquire boundaries. Notch genes are expressed in the anterior and posterior boundaries of somites and, with FGF, control ephrins. Ephrins cause changes in cell adhensions in the pre-somatic mesoderm and lead to formation of the somite through a cleft.
Draw the oscillatory gene expression pathway that occurs in section each pre-somatic mesoderm before somatogenesis.
List some of the structures derived from somites.
- Skeletal muscle
- Cartilage
- Tendons
- Endothelial cells
- Dermatome
