Intercalary and supernumerary regeneration in the limbs of the frog, Xenopus laevis.

Anuran amphibians, such as Xenopus laevis, can regenerate their limbs only when they are young tadpoles, whereas urodele amphibians have a regenerative ability throughout their lives. It is still unclear whether anuran and urodele use the same mechanism during regeneration. In the present study, we analyzed intercalary and supernumerary regeneration in Xenopus. In contrast to urodele blastema that induces intercalary regeneration along the proximodistal (PD) axis, intercalation did not occur in the Xenopus limb bud when the presumptive zeugopodium (fibula and tibia) was removed. However, when the limb bud tip (presumptive autopodium) was transplanted to the presumptive stylopodium (femur) with a 180-degree rotation at stage 52, the complete zeugopodium was regenerated. These results were similar to the results of urodele mature limbs, suggesting that Xenopus limb buds are equivalent to the urodele mature limbs but not to the urodele blastemas. We hypothesized that the ability for intercalation depends on the expression pattern of fibroblast growth factor (fgf)-8, because the expression of fgf-8 in the urodele spreads over the whole blastema and is close enough to activate the growth of the stump. To test this hypothesis, an FGF-8-soaked bead was implanted at the boundary between the stump and tip of a Xenopus limb bud. Intercalary regeneration was induced at stages 52 and 53. These results suggest that the Xenopus limb bud possesses the potential for intercalation, but endogenous FGF-8 in the apical ectodermal ridge (AER) does not induce intercalation to the stump because of the long distance between the AER and stump. Copyright 2003 Wiley-Liss, Inc.