The non-directional pattern of axonal changes in Wallerian degeneration: a computer-aided morphometric analysis.

Wallerian degeneration was investigated to determine whether axonal changes occur progressively in a somatofugal or somatopetal direction or simultaneously along the length of the axon. Microtubule density was used as a measure of the extent of axonal degeneration and was assessed by a computer-aided analysis of electron micrographs. The left sural nerves of ten rats were crushed and 30 hours later axonal areas and axonal microtubule numbers were recorded from a large sample of axons at two sites 1 cm and 3 cm distal to the crush. The same recordings were made from the right unoperated nerve at two comparable sites. Statistical analysis of all the data provided no evidence for a somatofugal or reverse direction of degeneration. It is concluded therefore that in Wallerian degeneration axonal changes, as indicated by microtubule dissolution, occur simultaneously along the length of the axon. It is proposed that to interpret the conflicting published data on the direction of fibre degeneration, Schwann cell changes (e.g. myelin ovoid formation) and axonal changes (e.g. microtubule dissolution) should be considered independently since they have different aetiological mechanisms which may account for the differing experimental results reported.