Characterization of retinoid metabolism in the developing chick limb bud

Retinoids (vitamin A derivatives) have been shown to have striking effects on developing and regenerating vertebrate limbs. In the developing chick limb, retinoic acid is a candidate morphogen that may coordinate the pattern of cellular differentiation along the anteroposterior limb axis. We describe a series of investigations of the metabolic pathway of retinoids in the chick limb bud system. To study retinoid metabolism in the bud, all-trans-3H]retinol, all-trans-3H]retinal and all-trans-3H]retinoic acid were released into the posterior region of the limb anlage, the area that contains the zone of polarizing activity, a tissue possibly involved in limb pattern formation. We found that the locally applied 3H]retinol is primarily converted to 3H]retinal, 3H]retinoic acid and a yet unidentified metabolite. When 3H]retinal is locally applied, it is either oxidized to 3H]retinoic acid or reduced to 3H]retinol. In contrast, local delivery of retinoic acid to the bud yields neither retinal nor retinol nor the unknown metabolite. This flow of metabolites agrees with the biochemical pathway of retinoids that has previously been elucidated in a number of other animal systems. To find out whether metabolism takes place directly in the treated limb bud, we have compared the amount of 3H]retinoid present in each of the four limb anlagen following local treatment of the right wing bud. The data suggest that retinoid metabolism takes place mostly in the treated limb bud. This local metabolism could provide a simple mechanism to generate in a controlled fashion the biologically active all-trans-retinoic acid from its abundant biosynthetic precursor retinol. In addition, local metabolism supports the hypothesis that retinoids are local chemical mediators involved in pattern formation.