EurekAlert! notes that researchers have "been able to regenerate a wing in a chick embryo - a species not known to be able to regrow limbs - suggesting that the potential for such regeneration exists innately in all vertebrates, including humans. ... vertebrate regeneration is under the control of the powerful Wnt signaling system: Activating it overcomes the mysterious barrier to regeneration in animals like chicks that can't normally replace missing limbs while inactivating it in animals known to be able to regenerate their limbs (frogs, zebrafish, and salamanders) shuts down their ability to replace missing legs and tails. ... In this simple experiment, we removed part of the chick embryo's wing, activated Wnt signaling, and got the whole limb back - a beautiful and perfect wing. By changing the expression of a few genes, you can change the ability of a vertebrate to regenerate their limbs, rebuilding blood vessels, bone, muscles, and skin - everything that is needed."

Link: http://www.eurekalert.org/pub_releases/2006-11/si-sow111706.php