Effects of radius--ulna removal on forelimb regeneration in Xenopus laevis froglets.

Regeneration of boneless amputated forearms of adult newts was found to progress at a rate and to a degree comparable to amputated control limbs in which stump bones were not removed. In contrast, regeneration of boneless amputated Xenopus froglet forearms was significantly delayed and did not occur until two to three weeks following amputation. This is in comparison with the initiation of distal cartilage formation observed one week postamputation in control forelimbs of Xenopus froglets. The regeneration of cartilage in boneless forearms of adult newts was found to occur distal to the amputation level. In contrast, distal as well as proximal (centripetal) regeneration of cartilage was observed in the amputated boneless forearms of Xenopus. In froglets and newts, unamputated forelimbs in which forearm bones were extirpated did not initiate cartilage regeneration. Our findings support the hypothesis that forelimb regeneration in Xenopus froglets is primarily a tissue response. In comparison, limb regeneration in the adult newt is predominantly an epimorphic response.