The Shh gene is expressed in many different parts of the developing chick embryo (indicated by dark staining), including (a) the zone of polarizing activity in each of the two developing limb buds and the long neural tube and (b) the developing feather buds. Shh mRNA is visualized by in situ hybridization.
[Photomicrographs courtesy of (a) Cliff Tabin and (b) Dr. John Fallon, University of Wisconsin, and Matthew Harris, Harvard Medical School, Department of Genetics.]