Axon and neuron numbers after forelimb amputation in neonatal rats

It seems a paradox that more primary sensory neurons are lost but recovery is better after peripheral nerve injury in neonates as compared to adult mammals. A possible explanation is that surviving neurons sprout in the neonate. To test this, forelimbs in neonatal rats were amputated, which caused the death of many primary sensory neurons. The number of neurons in the dorsal root ganglia, and the number of myelinated and unmyelinated fibers in the dorsal and ventral roots were determined on the amputated and contralateral normal sides. On the amputated side, soma loss in the ganglia was 30%, and the fiber numbers were decreased by 16% in the dorsal root and increased by 20% in the ventral root. These data are compatible with the hypothesis that there is axonal branching or sprouting from surviving sensory neurons. In addition, morphometric analyses showed a changed myelin-axon relationship for central processes of sensory cells whose distal processes have been cut.