In the early mouse embryo, the immediate environment of the somite comprises mul
ID: 31253 • Letter: I
Question
In the early mouse embryo, the immediate environment of the somite comprises multiple tissues secreting different signalling molecules that pattern the somite.
a) Which of these neighbouring embryological structures secretes Sonic hedgehog (Shh)?
b) Outline the Shh signal transduction pathway.
c) Describe the experiments in mice that showed that Shh is essential for the activation of myf5transcription in the early somite.
d) In a somite cell, how does the myf5 genomic locus integrate the cell signalling in its local environment to regulate transcription?
Explanation / Answer
a) notochord and floorplate of neural tube;
b) The shh signalling pathway should be described and/or illustrated to include ptc receptor interacting and inhibiting with membrane bound smo, shh ligand interfering with this interaction. Should say that in the absence of shh, ptc
represses smo and the cos/fu microtubule complex causes the transcription factor Glii to be processed to a smaller form which represses hh target genes. Shh binding ptc, releases smo and results in the stabilisation of full length Gli which can move to the nucleus and function as a transcriptional activator.
c) creating a shh mutant mouse by gene targeting and analysing myf5 expression using in situ hybridisation showed it was absent in epaxial somite.
d) The distinct enhancers of the myf5 locus should be described (maybe with some discussion of how the enhancers elements were identified by deletion mapping and making transgenic embryos). Tthis work defined a specific epaxial enhancer. The epaxial enhancer would be a good example of how shh regulates myf5 expression in the specific cells in the medial somite. Define the requirement of this cis sequence for the epaxial expression of myf5 and the presence of a gli binding site which would be activated by shh signalling coming from the
notochord/floorplate. Finally, generalising about other signals similarly activiting other enhancers to regulate myf5 in other cells or at other times would get full marks.