In a lab at the University of Pennsylvania, a plastic dish holds two rows of tiny black dots, pairs of them connected by dozens of thin, hairlike filaments. Each dot is a cluster of thousands of neurons, explains Douglas Smith, who is a professor of neurosurgery and the director of Penn's Center for Brain Injury and Repair. The fibers that stretch between them actually comprise thousands of axons, long, slender projections that conduct electrical impulses away from each neuron's central body. These bundles--each one a lab-engineered nerve--represent physical bridges that Smith hopes will help researchers like him mend previously irreparable injuries.

When sections of nerves in the body are severed or crushed, they die. Although the nerves can regenerate, they do it at the glacial pace of about one millimeter a day. And there's another catch: as new axons grow, they need the original nerve sheath--a protective membrane made up of several different kinds of cells--to guide them to the area that has lost function. That sheath begins to dis­integrate after about three months without a living nerve in it. "It's a race against time," says Smith. A nerve severed in, say, the wrist can span the short distance to the hand and heal in time to restore function. If the same nerve were cut near the shoulder, however, the person would almost certainly lose full use of that hand, since the new growth would not reach the hand before the sheath died.