Patterns of Wallerian degeneration of myelinated fibres in short and long peripheral stumps and in isolated segments of rat phrenic nerve. Interpretation of the role of axoplasmic flow of the trophic factor.
The topographical level of nerve transection influences the time of appearance of Wallerian degeneration in the peripheral stump. After transection made in the distal portion of the nerve, degeneration appears earlier and is, for equal times, stronger than that observed under similar conditions after transection made near the beginning of the nerve. In nerves transected in the proximal part, the spatial pattern of degeneration along the peripheral stump depends on the time after neurotomy. A time span exists, different for each size class of fibres, within which the degree of degeneration decreases linearly with increasing distance from the site of transection. Within this time span, at increasing hours, the intercepts of calculated regression lines increase but the slopes change only slightly, so that an array of quasi-parallel lines is obtained. Departures from linearity occur at early times when some degree of degeneration has already appeared in the proximal part of the stump, whereas in its distal part all fibres still look normal. Another type of departure from linearity appears at late times (over 34 h). In isolated nerve segments the degeneration is weaker than in the peripheral stumps remaining in continuity with the nerve terminals and the longitudinal pattern of changes is strikingly altered. A unitary interpretation of these phenomena in terms of redistribution of the trophic factor by the bidirectional axoplasmic transport is proposed.