Nerve fibres in spinal cord impact injuries. Part 1. Changes in the myelin sheath during the initial 5 weeks.

The spinal cords of cats were subjected to an impact injury using a "weight dropping" technique and sequential changes in the sheaths of non-degenerate myelinated fibres studied over a 3-week period. By 1 1/2 h after impact fibres showed retraction of some lateral loops from one paranode. The extent and severity of this change increased over the first week so that partial and full thickness demyelination were seen frequently. Partial demyelination most commonly resulted from the internodal termination of the innermost lamellae at an internodal location often associated with a Schmidt-Lantermann incisure. Remyelination by both Schwann cells and oligodendroglia occurred at the end of the second week. Oligodendroglial myelin showed many features of immaturity, similar to those found during development. It is suggested that the very earliest myelin damage is mechanical but is aggravated by other factor(s) one of which is probably ischaemia. Within the most severely injured areas there is death of oligodendroglia and any surviving axons are remyelinated principally by Schwann cells. In intermediate and minimally damaged areas of white matter oligodendroglial remyelination predominates.