RETINOIC acid is a putative morphogen in limb formation in the chick and other vertebrates1-5. In chick limb formation, it is thought that retinoic acid is released from the zone of polarizing activity (ZPA) and the concentration gradient of retinoic acid formed from the posterior to the anterior provides positional cues for digit formation1-4,6. Implantation of a bead containing retinoic acid at the anterior margin of the limb bud induces a mirror-image symmetrical duplication of the digit pattern similar to that observed when the ZPA is grafted into the anterior margin of the host limb bud7,8. Also, the level of endogenous retinoic acid (25 nM on average) is higher in the posterior one third of the limb bud1,6. We found that when the bead containing either retinoic acid or an analogue but not the ZPA, was implanted in the anterior margin of the chick limb bud, expression of the retinoic acid receptor type-β gene was induced around the bead within 4 h. These results indicate that exogenous retinoic acid is not identical with the ZPA morphogen. As the anterior tissue exposed to retinoic acid has polarizing activity9, we conclude that the primary function of exogenous retinoic acid is to induce polarizing activity in the limb bud.
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