Serially duplicated regenerates from the anterior half of the axolotl limb after retinoic acid treatment.

Axolotl (Ambystoma mexicanum) forearms were divided, by an incision between the radius and ulna, to produce anterior and posterior halves. These were prevented from fusing together again by a graft of head skin and amputated through the wrist. This procedure enabled independent regeneration from both halves of the stump. Anterior half stumps produced a single digit while the posterior halves mainly regenerated three digits, the two halves together making a single hand. Treatment with retinoic acid, injected intraperitoneally four days after amputation, abolished regeneration from the posterior half stump and produced proximo-distally duplicated regenerates from the anterior half. The duplicated regenerates had in most cases a complete four digit hand and were therefore more than proximalised regenerates from the anterior side of the limb. Replacement of anterior limb skin with head skin had no effect on the response of the regenerating limb to retinoic acid. In species where application of retinoic acid induces anterior-posterior duplications, these are always derived from the anterior side of the limb. The results presented here show that the morphogenic effects of retinoic acid in inducing proximo-distal duplications are also due to its effects on the anterior tissues of the limb.