Motoneuron and muscle fibre counts in normal and bilaterally innervated Xenopus hindlimbs.

Motoneurons of the Lumbar Lateral Motor Column (LMC) and muscle fibres of gastrocnemius and tibialis anterior were counted in juvenile Xenopus frogs, including normal animals and those reared with a single bilaterally innervated hindlimb (monopodal frogs). In many monopodal frogs, the single hind limb becomes hyperinnervated by a large number of motoneurons on the contralateral side in addition to the normal ipsilateral number, even after the completion of cell death. In frogs with hyper-innervated limbs, muscle fibre number was evaluated beyond that expected for a normal population, but this increase was not commensurate with the quantity of extra innervation. When taken together with previous findings which showed that the supporting capacity of individual fibres was not elevated, the conclusion is that the ability of the single limb to support extra motoneurons cannot be completely explained by a commensurate increased proliferation of muscle fibres resulting from the operation or the bilateral innervation. The results give further evidence against the hypothesis that motoneuron numbers are controlled solely by peripheral competition. The study also provides evidence that muscle fibre numbers are regulated in part by the quantity of motor innervation received by the limb.