Severed nerves left with a significant gap between the ends do not regrow in adult mammals. Scarring rather than regeneration takes place, and loss of function is permanent. All is not bleak, however. Researchers here report on progress in guided nerve regrowth, using a implant that encourages regeneration of nerve tissue across a comparatively large distance. The prospects for recovery from damage to the peripheral nervous system are becoming brighter. Assuming it is accompanied by removal of scar tissue at the nerve ends, the regenerative approach illustrated here could, in principle, be applied well after an injury has taken place, and is thus particularly interesting.

Peripheral nerves can regrow up to a third of an inch on their own, but if the damaged section is longer than that, the nerve can't find its target. Often, the disoriented nerve gets knotted into a painful ball called a neuroma. The most common treatment for longer segments of nerve damage is to remove a skinny sensory nerve at the back of the leg - which causes numbness in the leg and other complications, but has the least chance of being missed - chop it into thirds, bundle the pieces together and then sew them to the end of the damaged motor nerve, usually in the arm. But only about 40 to 60% of the motor function typically returns.

Researchers have now created a biodegradable nerve guide - a polymer tube - filled with growth-promoting protein that can regenerate long sections of damaged nerves, without the need for transplanting stem cells or a donor nerve. The nerve guide returned about 80% of fine motor control in the thumbs of four monkeys, each with a 2-inch nerve gap in the forearm. The experiment had two controls: an empty polymer tube and a nerve graft. Since monkeys' legs are relatively short, the usual clinical procedure of removing and dicing a leg nerve wouldn't work. So, the scientists removed a 2-inch segment of nerve from the forearm, flipped it around and sewed it into place, setting a high bar for the nerve guide to match.

Functional recovery was just as good with the guide as it was with this best-case-scenario graft, and the guide outperformed the graft when it came to restoring nerve conduction and replenishing Schwann cells - the insulating layer around nerves that boosts electrical signals and supports regeneration. In both scenarios, it took a year for the nerve to regrow. The empty guide performed significantly worse all around.