Schwann cell responses during regeneration of myelinated fiber and experimental onion-bulb formation

Wallerian degeneration was induced by crushing the mouse phrenic nerve at the neck and subsequent regeneration in the distal segment was studied chronologically at the ultrastructural level. Bundles of regenerating axon sprouts appeared in the distal endoneurial tube (Bungner band) from 5 days later on and were subsequently sorted out into smaller bundles by proliferating Schwann cells in a similar way to that reported in developing nerve. It was found, however, that usually only one sprout became mature and myelinated after establishment of 1to1relationship with a Schwann cell, while other sprouts showed features of degeneration and appeared decreased in number with time. The Schwann cells which were associated with such degenerating sprouts gradually showed atrophy of their cytoplasm, transient collagen pocket formation and finally attenuated forming residual concentric cytoplasmic lamella surrounding the remyelinating axon (primitive onion-bulu). Based on this observation, repeated crush injuries, up to 4 times, were performed to examine whether advanced onion-bulb could by produced by repeating the regenerating process. The result showed that with the increase of the number of crush times, the number of supernumerary Schwann cells increased, occasionally forming multiple concentric layers surrounding the remyelinated axon. On the other hand, clusters of myelinating fibers, which were rarely seen following single crush injury, became also observed, although to a lesser extent. These observations suggest strongly that onion-bulb could be regarded as evidence of repeated episodes of regeneration of myelinated fibers