Fine structure of the forelimb regenerate of the African clawed toad, Xenopus laevis

Forelimb regenerates from postmetamorphic Xenopus froglets were examined at various stages postamputation by light microscopy and scanning and transmission electron microscopy. The outside surface of the wound epithelium was found to exhibit progressive changes following amputation with a distinct difference in appearance between stump epidermis and wound epithelium at all stages examined. The internal structure of the wound epithelium is characterized by loosely arranged cells with numerous cell junctions and abundant intracellular filaments. The wound epithelium is separated at an early stage from the underlying cells by a thick band of extracellular matrix. Cells accumulating beneath the wound epithelium were morphologically similar to blastemal cells from completely regenerating limbs in other species but no evidence of myogenesis or abortive myogenesis was seen. Blastemal cells from the central portion of the regenerate were observed at varius stages of chrondrogenesis with those immediately beneath the wound epithelium least advanced in this respect. Those located more laterally appear not to be directly involved in chondrogenesis. Although the usual explanation for lack of complete regeneration in this species is inadequate innervation of the regenerate, the fine structure of the regenerating spikes noted here is markedly different than that of denervated, amputated newt limbs.