End structure and neuropeptide immunoreactivity of axons in sciatic neuromas following nerve section in neonatal rats

The formation of neuromas after neonatal nerve injury was studied in rats. In neonatal pups, the sciatic nerve was cut and tightly ligated, and a portion of the distal stump was removed. After 6-10 weeks, a nerve-end neuroma had formed in about 70% of the animals. In the remaining animals the nerve had grown on the side of the ligature. The end structure of the neuroma axons was studied using anterogradely transported WGA-HRP injected into the L5 dorsal root ganglion. HRP labeling occurred in the entire proximal sciatic nerve. In the neuroma, labeled fibers branched profusely and either terminated with minor end swellings or turned in the retrograde direction. Immunohistochemistry showed that the fibers which projected into the neuroma presented a moderate immunoreactivity to substance P and neuropeptide Y and a strong reactivity to calcitonin gene-related peptide. The results show that many sensory and sympathetic sciatic nerve fibers survive chronic axotomy in the newborn and contribute to the formation of nerve-end neuromas. There are, however, important structural differences between adult and neonatally induced neuromas.