The chick embryo is a useful experimental system because it can be easily manipulated during early development. Imagine that you injected sonic hedgehog (Shh) into the optic vesicle of a chick embryo just prior to the onset of differentiation in the neural retina, and then you allowed the embryo to develop. How might the mature chick differ from a normal, untreated chick? Why?
Shh
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I believe the mature chick would have a much thicker retina and retinal pigment epithelium. Sonic hedgehog plays a role in development by promoting cell division. This occurs because the Shh pathway eventually leads to expression of cyclins, which when they reach a critical level promote the transition from G1 to S phase of the cell cycle. Expression of Shh prior to differentiation would promote further division of neural progenitor cells in the optic vesicle (symmetric division), leading to a larger volume of cells and a delay of asymmetric division.