Recovery from spinal transection in fish: regrowth of axons past the transection.

In fish axons reportedly cross a spinal transection, form synapses, and then cease growing or grow in abnormal directions. This obviously would not restore previous synaptic connections, and since fish recover function, it is often stated that recovery of function in fish seems to be independent of restoration of normal synaptic architecture. By contrast, the present study emphasizes that many axons in goldfish continue growing after they pass a spinal transection. Horseradish peroxidase was injected into caudal spinal cord in normal or spinal cord-transected fish. Neurons were labeled in the same places in operated and normal fish. Since the site of injection was 4 cm (18 segments) caudal to the transection, it seems clear that the axons of cells labeled in transected fish extended past the cut for almost the length of the spinal cord. Thus growth did not cease for these axons. Therefore, even though synapses undoubtedly form in the neuropil distal to the cut, many axons do not stop growing and restoration of previous synaptic connections may well be a requirement for return of function.