Ultrastructure of Xenopus laevis forelimbs: effects of denervation and delayed amputation

Ultrastructural studies were carried out on left forelimbs of postmetamorphic Xenopus laevis froglets which had been denervated 1 week prior to limb amputation. Limbs were fixed at intervals of 3, 9, and 15 days postamputation. In control and sham-denervated series limbs in which peripheral (brachiospinal) nerves remained intact, tissue regeneration of the amputated forelimb ensued. Extracellular matrix deposition was observed to be minimal among the cells that accumulate subjacent to the wound epithelium of the control and sham cases, not unlike that observed in intact limbs. However, regeneration was not observed in denervated limbs. In denervated forelimbs 9 and 15 days postamputation, deposition of extracellular matrix surrounding the limited fibroblast or fibroblast-like cellular accumulation in the amputated limb stumps was considerably greater than was observed in normally innervated cases. Presumably, extracellular matrix binds fibroblasts together to form scar tissue (cicatrix) which leads to stumping of a denervated, amputated limb. The fibroblasts become narrower, fusiform, and more elongate in denervated nonregenerating forelimbs by 15 days postamputation compared with controls. In all cases fixed 3 days postamputation, epithelial wound healing appeared to proceed normally in the absence of peripheral innervation; also, no ultrastructural difference was observed among amputated control and sham-control limbs that had intact nerves. This investigation is the first to provide a fine-structure analysis of the effects of denervation and delayed amputation on the earlier stages of tissue regeneration in X. laevis froglets.