No abstract

The present study directly compares the response of amputated forelimbs in Xenopus laevis froglets with epimorphic regeneration of forelimbs in adult newts (Notophthalmus viridescens). Epidermal wound healing was initiated following limb amputation in both froglets and newts. Dedifferentiation of mesodermal stump tissues was comparatively extensive in the regenerating newt limb, whereas Xenopus forelimbs underwent little if any tissue dedifferentiation. Blastema accumulation and growth are prominent features of newt forelimb regeneration. In contrast, only a small fibroblastlike cell accumulation was observed at the distal tip of the Xenopus limbs. Differentiation of blastemal cells in the urodele limb was delayed until extensive blastema growth was achieved, whereas differentiation was immediate in Xenopus. Morphogenesis of the regenerative outgrowth in Xenopus was limited to the differentiation of connective tissue elements, primarily cartilage. In contrast, complete regeneration of all limb tissues occurred in newt forelimbs. We conclude from our findings that a dominant tissue regenerative response exists following forelimb amputation in Xenopus froglets, whereas epimorphic regeneration prevails in the amputated adult newt limb.

A morphological and histological analysis of the response of forelimb regenerates of Xenopus laevis to denervation is presented. Following forelimb amputation, young Xenopus froglets (3 months postmetamorphosis) consistently regenerated cartilagenous, heteromorphic spikes under conditions of normal innervation. Denervation of forelimbs at 1 or 2 weeks prior to amputation had little effect on the progress of regeneration. Denervation of regenerates at various weekly stages resulted in the inhibition or delay of further regenerative responses and in the regression of any regenerate tissues present at the time of the ablation. Specifically, the formation of the blastema, the initiation of differentiation, and the maintenance of the differentiated state in Xenopus forelimb regeneration were found to require nerves. Following reinnervation some of the denervated forelimb regenerates subsequently reinitiated regeneration, whereas others remained stumped. Stumping was observed histologically to be a function of dermis intervention. These results with Xenopus are discussed and contrasted with previous well-documented observations of the influence of nerves on appendage regeneration in urodeles.

The 1Vorop/zrlralrnus viridescerrs radicalfringe cDNA was isolatsd, cloned and sequenced. It codes for a 396 amino acid protein with a predicted N-terminal signal sequence. N e w r-aciical frirrge shows high homology to the radical frirrge genes of Gallus gall~is (chick). .Yrrrup~~ 1aei.i.s (tkog) and Mus ntiwciiks (mouse).There is 77% identity between newt and chick genes in the C-terminal 270 arnino acids of the protein (89% when conserved changes are considered). During forelimb development. newt radicalfrirge is first expressed in the lirnb field, with intense expression as the forelimb bud develops. Homogeneous rudical j,+ge gene expression spaming both the dorsal and ventral halves of the developing forelimb is detscted at this stage. radical fringe is exclusively expressed in the mesenchyme regions of the developing forelimb bud.Gene expression diminishes with advanced digit development. rudical frirtge is re-expressed in the mesenchyrnal tissues during epimorphic regeneration of the adult newt forelimb. Weak radical fringe expression is first seen in the tw.0-week forelimb regenerate. The expression signal intensifies in subsequent weeks as the regeneration bud (blastema) increases in size. A very intense signal is seen in the region immediately flanking the differentiating cartilagenous tissues. This gene appears to be highly expressed in regions of active ce11 division. The highest expression pattern is seen at the early four digit stage where the signal seems to be localised in the interdigital mesenchyme of the reçenerating farelimb. As the regenerate progresses past the advanced digit stage, newt rudicul frirrge expression diminishes and eventualIy disappears. Overall. the results obtained suggest that radical fringe might be involved in both hrelirnb development as well as in adult forelimb regeneration in the new. Notopiitlialmtcs viridescerts.