Retinoic acid (RA) induces pattern duplication in the proximodistal (PD) axis of axolotl limb regenerates. The effect is dose-dependent, with the maximum extent of duplication being evoked at a dose of 150 pg RA/g body weight.
The same dose of RA induces maximum pattern completion in the anteroposterior (AP) axis of regenerating anterior half or double anterior half limbs. RA inhibits the regeneration of posterior half or double posterior half limbs (Kim, W.S., and Stocum, D.L. [1986] Dev Biol 114:170-179). The effects of lower doses of RA on the AP axis of limb regenerates have not been tested and thus it is not known whether the effects of RA on positional identity in this axis are dose-dependent or are allor-none. To answer this question, we examined the effects of a range of doses of RA on AP positional identity in regenerating double anterior and double posterior axolotl forelimbs and hindlimbs constructed by exchanging the anterior and posterior halves of right and left zeugopodia as either autografts or homografts. Ten days after the exchange, the double half zeugopodia were amputated through their distal ends. At 4 days postamputation, the animals were injected intraperitoneally with 20, 50, 75, or 100 pg RA/g body weight and the constructs allowed to regenerate for at least 6 weeks. Control double anterior forelimbs and hindlimbs formed symmetrical regenerates with an average of two and 1.8 anterior digits, respectively. RA treatment induced the blastema of double anterior zeugopodia to regenerate missing posterior structures in mirror-image patterns and to duplicate zeugopodial and stylopodial elements in the PD axis. The extent of posterior regeneration and PD duplication was proportional to dose. Control double posterior forelimbs and hindlimbs produced symmetrical regenerates with an average of 2.4 and 3.3 posterior digits, respectively. Regeneration was inhibited at 100% frequency at doses above 50 pg of RA, but regeneration took place with increasing frequency at the 50 and 20 pg doses. We conclude from these data that RA posteriorizes AP positional identity in a dose-dependent way, analogous to the dose-dependent proximalization of po-