Myelin sheath remodelling in regenerated rat sciatic nerve.

Adult rat sciatic nerves were subjected to a crush lesion and allowed to survive during 2 weeks-11 months. Segments of regenerated nerve were removed from exsanguinated animals and subjected to physiological analysis and light microscopic examination of teased fibres. Application of the potassium channel blocking agent 4-aminopyridine (4-AP) to regenerated nerve segments had marked effects on action potential waveform and recovery properties. When applied to normal control nerves 4-AP had minimal effects. Examination of teased preparations from the same nerves revealed the presence of two classes of internodes along regenerated nerve fibres. A majority of conventional regenerated internodes exhibited lengths (L) of at least 150 microns. Maximal L reached about 350 microns after one month survival and 550 microns 10 months later. In the individual nerve the maximal L-values tended to increase with fibre size. Most convential regenerated internodes had L-values of 200-400 microns. In addition, scattered unusually short sheaths, with L-values of 10-150 microns, were found intercalated between conventional internodes. Some intercalated sheaths had a wrinkled irregular configuration and some lacked a light-microscopically distinct myelin layer. We suggest that the occurrence of unusually short and partly distorted sheaths along regenerated fibres reflects a nodal-internodal remodelling in response to longitudinal crowding. These sites might represent foci with aberrant functional properties, possibly accounting for the 4-AP sensitivity of regenerated nerve.