Quantitation of the Schmidt-Lanterman incisures in juvenile, adult, remyelinated and regenerated fibres of the chicken sciatic nerve

Morphometric studies of peripheral nerves in various species have shown that the number of Schmidt-Lanterman incisures in an internode is proportional to the fibre diameter. In the rat sural nerve it appears that following remyelination, the relationship between the numbers of incisures per internode and the fibre diameter remains unaltered despite the fact that the remyelinated internodes are uniformly short. We quantitatively examined the insertion of incisures in adult, juvenile, remyelinated and regenerated fibres of the chicken sciatic nerve. Remyelinated fibres were examined 100 and 200 days following the intraneural injection of diphtheria toxin, and regenerated fibres 200 days after a nerve crush. Our results show that the incisures are more abundant in the chicken than in all other species previously reported, and for both juvenile and adult hens the number of incisures in an internode is proportional to the fibre diameter. Following both remyelination and regeneration the internodal lengths were shorter than control fibres and the distances between the incisures were reduced by approximately 20% for all fibre diameters. Significantly, the numbers of incisures in an internode were related to the length of the remyelinated or regenerated internode and not to the fibre diameter. This finding is in marked contrast to previous reports for rat peripheral nerve. Our findings are discussed in relation to the hypothesis that the Schmidt-Lanterman incisures are vital to the maintenance of the myelin sheath and/or the associated axon.