All previous analyses of axonal responses to traumatic axonal injury (TAI) have described the ultrastructure of changes in the cytoskeleton and axolemma within 6 h of injury. In the present study we tested the hypothesis that there are, in addition, ultrastructural pathological changes up to 1 week after injury. TAI was induced in the adult guinea pig optic nerve of nine animals. Three animals were killed at either 4 h, 24 h, or 7 days (d) after injury. Quantitative analysis of the number or proportion of axons within 0.5-micro m-wide bins showed an increase in the number of axons with a diameter of less than 0.5 micro m at 4 h, 24 h, and 7 d, the presence of lucent axons at 24 h and 7 d and that the highest number of injured axons occurred about half way along the length of the nerve. A spectrum of pathological changes occurred in injured fibers-pathology of mitochondria; dissociation of myelin lamellae but little damage to the axon; loss of linear register of the axonal cytoskeleton; differential responses between microtubules (MT) and neurofilaments (NF) in different sizes of axon; two different sites of compaction of NF; loss of both NF (with an increase in their spacing) and MT (with a reduction in their spacing); replacement of the axoplasm by a flocculent precipitate; and an increased length of the nodal gap. These provide the first ultrastructural evidence for Wallerian degeneration of nerve fibers in an animal model of TAI.

Introduction:  Stretch‐injury to the optic nerve of the guinea‐pig results in cytoskeletal pathology. We tested the hypothesis that post‐traumatic changes may continue up to 1 week after injury. Material and methods:  Six animals were injured under controlled conditions. Under terminal anaesthesia, three controls and three animals were killed at 24 h or 1 week. Unbiased stereological analysis of the axonal cytoskeleton was undertaken. Results:  At 4 h, 20% of nerve fibres are injured (Jafari et al. J Neurotrauma 1998; 15: 955). In the present study, no evidence for a change in the proportion of injured nerve fibres at longer survivals was obtained. At 24 h, three types of injured fibre occur: (i) some are compacted as at 4 h; (ii) some, termed ‘degenerating’, show loss/increased spacing between neurofilaments and microtubules (P = 0.005); and (iii) in some, empty myelin figures occur. At 7 days, both degenerating fibres and empty myelin figures are increased (P = 0.032). Conclusions:  These results demonstrate that injured nerve fibres undergo secondary axotomy between 4 and 24 h and there are increased numbers of degenerating fibres at 7 days. However, not all injured fibres undergo axotomy between 4 and 24 h, because, at 7 days fibres undergoing axotomy are present.