Innervation and behaviour of ectopic limbs in Xenopus.

The hindlimb bud of Xenopus tadpoles was replaced with a forelimb bud, or vice versa, prior to axon invasion of the limb. Ectopic hindlimbs supported 94% as many brachial motoneurons as the remaining forelimb, and ectopic forelimbs supported 46% as many lumbar motoneurons as the remaining hindlimbs, on average, after the period of motoneuron death. The patterns of movement of ectopic limbs were characteristic of the innervating spinal cord segments, and not the limbs. The anatomical patterns of nerve trunks were characteristic of the ectopic limbs, and not the sources of innervation. HRP transport studies showed a resemblance between the locations of lumbar motoneurons supplying ectopic forelimb muscles and those supplying the homologous muscles of the hindlimb. It was concluded that motoneurons could survive the period of cell death following connection to muscles for which their patterns of activity were inappropriate, and the projections to ectopic limb muscles were specific although the nerve paths within the limbs were different.