A new approach toward analyzing peripheral nerve fiber populations. II. Foreshortening of regenerated internodes corresponds to reduced sheath thickness.

The new approach used in this study is based on the concept that axon caliber is not the only factor affecting the thickness of the myelin sheath. It is necessary to consider the entire geometric proportions of the internode, since sheath thickness corresponds to the relationship between axon caliber and the length of the internode. This type of analysis was applied to the regenerated internodes in rat sciatic nerves. Survival periods of 4, 9, 18 and 36 weeks were studied after lesions had been placed in young adult rats. The data show significantly thinner sheaths for regenerated fibers as compared with normal nerves, consistent with previous observations. This reduction in sheath thickness, however, corresponded quantitatively to the degree of foreshortening of internodes in the regenerated nerves. An average reduction of 10 in the quotient internode length/fiber caliber corresponded to a reduction of about 0.015 in the relative thickness of the sheath (quotient axon diameter/fiber diameter). This means that regenerated myelin sheaths are not truly hypoplastic; rather, they are adapted to the reduced internode length, and have the same relationship found for normal fibers. In partially damaged nerves there was a clear distinction in terms of sheath thickness between regenerated fibers and undamaged fibers. Demonstration of this phenomenon by scatter diagrams opens new possibilities for the quantitative assessment of neuropathies.