Gradual stretching of the proximal nerve stump induces the growth of regenerating sprouts in rats.

We investigated the effect of direct gradual stretching on the proximal nerve stump morphologically. A 10-mm-long nerve segment was resected from the sciatic nerve of a rat. The end of the proximal nerve stump was fixed to a small ring and the marking suture was placed at a point 1 mm proximal to the ring. Then, the nerve stump was lengthened at a rate of 1 mm/day via a stretching of the ring using an original external device. After a stretching of 20 days, the distance from the ring to the marking suture became 12 mm. Whereas large mature myelinated axons were observed in the proximal part of the marking, only small axons with thin myelin sheath were observed in the distal part, and the mean axonal diameter showed a significant difference between the two parts. Moreover, the mean internodal length was 172.4 +/- 13.4 microm in the distal part of the marking and 1019.0 +/- 56.2 microm in the proximal part. The internodal length also showed a significant difference between the two parts. Thus, the axonal diameter and internodal length were consistent with the characteristics of regenerating axons in the distal part. Furthermore, ultrastructural analysis also showed the histological characteristics of axonal regeneration. Thus, a transected proximal nerve stump may be lengthened by axonal regeneration during gradual stretching, and the stimulus of mechanical stretching may induce the growth of regenerating axons.