Invasion of the L7 ventral root and spinal pia mater by new axons after sciatic nerve division in kittens.

In young kittens the left sciatic nerve was divided followed by nerve suture, distal stump resection, or resection combined with proximal stump capping. The kittens were killed 2 to 3 months postoperatively. In other kittens bilateral L7 ventral rhizotomy or neuroma excision was carried out 80 to 90 days after nerve resection, followed by killing a few days later. Thin sections from the ventral root L7 were examined in the electron microscope. In all animals the average proportion of ventral root unmyelinated axon profiles (UAP) was above normal on the operated side, being highest in capped and resected animals and lower in sutured cases. The proportion of UAP was inversely related to the extent of functional calf muscle reinnervation. In cases with high proportions of UAP, numerous bundles of unmyelinated and small myelinated axons were present in juxtamedullary root fascicles and surrounding pia mater. After L7 ventral rhizotomy in resected animals few intact UAP occurred together with numerous signs of axon degeneration in the proximal root stump on the lesion side. After neuroma excision the proportion of ventral root UAP decreased. Therefore some proportion of the new axons should represent recurrent sprouts from peripheral sources.