Nerve-Independence of Limb Regeneration in Larval Xenopus laevis Is Correlated to the Level of fgf-2 mRNA Expression in Limb Tissues

Abstract

In both larval and adult urodele amphibians, limb blastema formation requires the presence of an adequate nerve supply. In previous research, we demonstrated that the hindlimb of early Xenopus laevis larvae formed a regeneration blastema even when denervated, while the denervated limb of late larvae did not. We hypothesized that the nerve-independence was due to the autonomous synthesis of a mitogenic neurotrophic-like factor by undifferentiated limb bud cells. In this paper, we demonstrate that fgf-2 mRNA is present in larval limb tissues and that its level is correlated to the extent of mesenchymal cells populating the limb: in early limbs, fgf-2 mRNA is present at high levels all over the limb, while, in late limbs, the fgf-2 expression is low and detectable only in the distal autopodium. After denervation, fgf-2 mRNA synthesis increases in amputated early limbs but not in amputated late limbs. The implantation of anti-FGF-2 beads into amputated early limbs hardly lowers the mitotic activity of blastema cells. However, FGF-2 beads implanted into the blastema of late limbs prevent the denervation-induced inhibition of mitosis and oppose blastema regression. Our data indicate that FGF-2 is a good candidate for the endogenous mitogenic factor responsible for blastema formation and growth in amputated and denervated early limbs. However, in amputated late limbs, the very limited fgf-2 expression is not sufficient to promote blastema formation in the absence of nerves.

Beyond early development: Xenopus as an emerging model for the study of regenerative mechanisms

“…These factors are thought to target the wound epidermis and enable blastema maintenance. Nerve dependence has not been found for Xenopus at a stage when limbs do regenerate successfully (Filoni and Paglialunga, 1990;Cannata et al, 2001). In contrast, the regeneration of a hypomorphic spike in postmetamorphic animals does appear to be nerve dependent.…”

Fetal Skin Possesses the Ability to Regenerate Completely: Complete Regeneration of Skin

“…8 The nerve dependence of fetal wound healing has also been reported. In lamb fetal limb experiments, the presence of scarring was reported on the denervated side, whereas the skin was regenerated without scarring on the opposite enervated side.…”

Molecular and Cellular Basis of Regeneration and Tissue Repair

“…Studies on Xenopus limb regeneration have shown that the early buds will regenerate without nerves, but the later buds do need them; otherwise the loss of regenerative ability occurs about two developmental stages earlier than usual [84]. The synthesis of FGF2 is found in early but not later mesenchyme, suggesting that FGFlike factors may underlie the neurotrophic effect [85].…”