[Stimulation and inhibition mechanisms in peripheral nerve regeneration]

To better analyse the early growth of peripheral nerve regeneration, we recently developed a film model. Following transection of a peripheral nerve, e.g. the common peroneal nerve in mice, both proximal and distal stumps of the transected nerve are sandwiched between two sheets of film, and kept in vivo for various timed intervals after axotomy. The regenerating neurites sprout not only from the nodes of Ranvier close to the transected nerve end but also from the terminal bulbs which are formed at the transected nerve end. All of the regenerating neurites consist of naked axons for at least 2 days after axotomy, and elongate on the film with a growth rate of 77 microns/day. On migrating from a parent nerve to the regenerating axons, Schwann cells promote the axons to grow with a 4 times higher speed. Thereafter, a distal nerve stump of the transected nerve release some stimulating factors toward the regenerating nerves, and the axonal growth rate is increased by approximately 1.5 fold. Some inhibitory factors, one of which is myelin-associated glycoprotein, are at the same time released from the distal nerve segment for a week from the 7th post-operative day, and keep new axons from sprouting and inhibit outgrowth of young naked axons in order to optimum regeneration and maturation of outgrowing pioneer axons. It means a so-called pruning phenomenon.