Can differences in limb regeneration ability between individuals within certain amphibian species be explained by differences in the quantity of innervation?

Abstract

In some species of amphibians, the regenerative response to limb amputation is not uniform. For example, regenerative ability varies from one individual to another within the species Xenopus laevis, Ambystoma maculatum, and Amphiuma tridactylum. In many species, limb regenerative ability declines with increasing age. The objective of this investigation was to determine if this variation in regenerative ability can be explained by variation in the quantity of innervation. In each of the above-named species, limbs were amputated and allowed to regenerate. The quantity of innervation (percentage of cross-sectional area of amputated surface occupied by nerves) was measured in the amputated part of the limb as close as possible to the amputation surface. The presence, rate, or extent of limb regeneration in that specific limb was then compared to the quantity of innervation at the amputation surface. The results indicated that there was no correlation between innervation and the rate or extent of regeneration in Xenopus laevis or Ambystoma maculatum. On the other hand, in Amphiuma tridactylum, those limbs that regenerated were the ones that had the highest levels of innervation. In Ambystoma maculatum, the quantity of innervation was measured at five different stages, from early larvae to mature adult. The results showed that the quantity of innervation was highest during the larval period, reaching a maximum at the late larval stage before declining greatly to the adult level. It appears that the decline in regenerative ability in the adult cannot be attributed solely to a reduction in innervation to below the threshold level.