Gallus gallus (Chick) Limb Development-Retinoic Acid and Sonic Hedgehog (SHH)

To the left is a chick embryo (sans shell) with normal wing development. Courtesy of Dr. Fallon, Univ. of Wisconsin

Courtesy of Dr. Tickle, Univ. of Dundee

A bead soaked in Retinoic Acid was implanted into the anterior margin of the early wing limb-bud. Note the mirror-image duplication of the digits. IV III II ----------- II III IV

Digit IV represents a posterior limb structure. The ectopic release of Retinoic Acid from the bead leads to ectopic expression of Sonic Hedgehog (SHH), forming a secondary ZPA.