Collateral sprouting of sensory axons in the glabrous skin of the hindpaw after chronic sciatic nerve lesion in adult and neonatal rats: a morphological study.

The anterograde transport of wheat germ agglutinin-horseradish peroxidase conjugate was used to study the normal distribution of sensory nerve axons in the plantar skin of the rat hindlimb and at various times after chronic sciatic nerve injury in adult and neonatal rats. In adults, thin saphenous nerve axons were found in a small area laterally to the normal saphenous nerve territory 2-24 weeks after sciatic nerve lesion. In neonatal rats, at 6 and 10 weeks after sciatic nerve injury thin saphenous nerve axons were found almost or all over the sole of the foot, respectively, and in all 5 toes. At longer survival times, the area innervated by saphenous nerve axons became smaller. However, this area was now occupied by thin, as well as coarse axons. When adult animals were subjected to saphenous nerve crush simultaneously with the sciatic nerve lesion, thin, as well as coarse, nerve axons were found laterally to the normal saphenous nerve territory. The findings indicate that thin cutaneous sensory axons of adult mammals can extend collateral sprouts in glabrous skin for a short distance. This capacity appears to be greater in neonatally lesioned animals, where it is present for coarse cutaneous sensory axons as well. However, after neonatal nerve injury collateral sprouts seem to disappear from the initially most distally reinnervated area. Regenerating sensory axons in adult rats seem to have a greater capacity for collateral sprouting than intact axons. Coarse and thin cutaneous sensory axons could be found in this instance. In all instances a great part of the plantar skin remained denervated, suggesting that there is an upper limit for the territory which can be maintained by cutaneous sensory neurons reinnervating glabrous skin.