2. Signaling molecules can regulate transcription and as a result control cell differentiation during development. One signaling molecule, sonic hedgehog (shh), has been shown to regulate the dorsoventral patterning of the neural tube.

a) In what tissue is shh produced and what is the evidence that this tissue regulates the ventral pattern of the neural tube (even before this tissue was known to produce shh)?

b) Describe one experiment that shows that shh is the critical molecule produced by the inducing tissue referred to in (a). 

c) How is the gradient of shh converted into differential transcriptional activation? What I am looking for here is how the investigators determined the stoichiometric relationship between the shh gradient and active Gli transcription factors. Describe the experiment that elucidated this relationship. I do not need to have all the details but I want you to describe the experimental approach that was taken, the expression constructs that were generated, how these constructs were delivered to the neural tube and what the results were.