Axon Resealing: Filling in the Holes

When a nerve fiber is transected, the proximal stump must reseal before the processes of axon sprouting and axon regrowth can begin. Studies have shown that the open end of a cut axon rapidly constricts as the intracellular compartment becomes exposed to Ca ²⁺ and other ions in the extracellular fluid. Until recently, it was thought that a functional seal was subsequently produced by fusion of the axon membrane at the point of con striction. Such an event, however, has never been observed. In contrast to this simple hypothesis, a recent study indicates that the cut ends of squid and earthworm giant nerve fibers only partially close after transection, leaving significant holes that prevent functional resealing. In the giant myelinated axon of the earthworm, the hole is plugged by densely packed vesicles made within the axon shortly after transection. Because earthworm giant axons, like mammalian axons, are capable of regrowing after transection, this phenomena may provide a model of nerve fiber resealing in mammals. The Neuroscientist 1:253-254, 1995