Remodelling of internodes in regenerated rat sciatic nerve: electron microscopic observations

Twelve adult rats received a crush lesion of the left sciatic nerve in the upper thigh. Following survival times of 2 weeks to 6 months, the animals were perfused with glutaraldehyde. Longitudinal thin sections from regenerated nerve segments were examined by electron microscopy. At survival periods of 2 weeks to 1 month, exceptionally short internodes composed of a Schwann cell lacking myelin, a more or less wrinkled Schwann cell-myelin sheath unit, or Schwann cell cytoplasm containing lamellated bodies and lipid droplets, were found intercalated between conventional regenerated myelin sheaths. Such intercalated sheaths were always bordered by sites with a nodal differentiation. Although much less frequent, short distorted myelin sheaths were also found at regeneration times of 2 and 3 months, but they have not been found in sections from 6-month specimens. In the long-term regenerated nerves many paranodes were distorted by prominent myelin folds and some nodes of Ranvier exhibited abnormal axon-Schwann cell networks and adaxonal Schwann cell protrusions. These observations show that internodes in regenerated rat sciatic nerves are subject to extensive remodelling. This includes internodal shortening and nodal migration, myelin sheath breakdown and demyelination, elimination of redundant Schwann cells and nodal fusion. These morphological changes may have important physiological implications.