ACQUISITION OF NERVE DEPENDENCE FOR THE FORMATION OF A REGENERATION BLASTEMA IN AMPUTATED HINDLIMBS OF LARVAL XENOPUS-LAEVIS - THE ROLE OF LIMB INNERVATION AND THAT OF LIMB DIFFERENTIATION

In larval and adult urodeles and late-stage larval anurans, blastema f ormation after limb amputation requires an adequate nerve supply. Expe rimental evidence obtained from aneurogenic limbs indicates that, in u rodeles, the acquisition of nerve dependence during embryonic developm ent is due to the ''addiction'' of limb tissues to factors released by the ingrowing nerves rather than to limb differentiation. The aim of this work was to establish whether, in the toad Xenopus laevis, nerve- dependence for blastema formation after hindlimb amputation, which is acquired gradually during larval development and becomes complete at s tage 57 is due to limb innervation or to limb differentiation. Two ser ies of experiments were carried out. In the first series, limb differe ntiation was inhibited by treating the larvae with an anti-thyroid dru g, and innervation was maintained for an interval much longer than tha t normally required for development from nerve-independent stages to s tage 57. In the second series, the limb was caused to differentiate in the absence of nerves by maintaining the limbs denervated. Limb diffe rentiation was often accelerated by treating early-stage larvae with t hyroxine or by grafting early-stage limbs onto denervated limbs of lat e larvae, which, being near metamorphic climax, possessed high levels of circulating thyroid hormones. Results showed that in the first seri es of experiments the denervated limbs formed regeneration blastemas a fter amputation, but in the second series they did not. It was therefo re concluded that the acquisition of nerve dependence for blastema for mation in larval Xenopus laevis is not directly imposed by factors rel eased by the nerve fibers, but is strongly related to differentiation of limb tissues. (C) 1995 Wiley-Liss, Inc.