A surprising discovery by Duke University biomedical engineers holds promise for regenerating damaged nerves. The new approach rivals the success of the existing standard of treatment, an autograft – surgical 
Very small tubes filled with two kinds growth factors show the regrowth of damaged nerve fibers (central area). Credit: Duke Universitytransplant using a nerve removed from elsewhere in the patient’s body.
Autografts rely on nerves that are not critical to body functions. However, the graft usually replaces one type of neuron with another, say, a sensory neuron standing in for a motor neuron.
An alternative approach, building a “nerve bridge” between the ends of a severed nerve, has inspired several researchers. The “bridge” is a tube filled with substances that could coax both ends of the nerve to grow back together. Finding the correct combination of tube material, growth factors, proteins, and other substances has proved elusive.
A group of salamanders provided the key to the discovery of macrophages that promoted the kind of healing the engineers were hoping to find. The amphibians had lost their tails. The team knew that macrophages, which usually serve a clean-up role in the body, are necessary for salamander tail regrowth.
Rather than recruiting adult macrophages to convert from cleaning to healing, they filled their nerve bridge with a biological signal shown to attract younger, undifferentiated cells destined to become pro-healing macrophages. This approach helped regrow nerves in rats.
"Instead of retraining the demolition and cleanup crew, we hired a new workforce with a future in construction," said Nassir Mokarram, assistant research professor of biomedical engineering. "The results were significantly better. This is the closest anyone has ever been to equaling the efficacy of an autograft, and we did it with nothing more than a tube and the recruitment of the body's own immune system."
The next step after this proof-of-concept experiment is to use a nanofiber rather than the tubes used for the nerve bridge.
