Severed nerve regrows to bridge a gap.

Biodegradable chemicals have succeeded in encouraging severed nerves in rats to regenerate and span a 2/3-inch gap in tissue. Preliminary results of this work, reported last week in Chicago at the American Chemical Society's national meeting, suggest that these chemicals might become the basis for a device to treat trauma patients with significant damage to nerves in the extremities.

Moreover, the fact that the same polymeric material "can induce regeneration of two very distinct tissues [the sciatic (leg) nerve and skin]," says loannis Yannas of the Massachusetts Institute of Technology (MIT), "suggests very strongly that there is potential for regneration in other organs that has been significantly underrated." Yannas collaborated on the nerve regeneration device with colleagues at MIT, Case Western Reserve University in Cleveland and two Boston hospitals.

Yannas was a developer of the "artificial skin" used to grow new epidermis on human burn victims (SN: 1/30/82, p. 73). He says the nerve regeneration device employs the same chemistry: a plastic silicone outer layer (here a tube) filled with cowhide-derived collagen (connective tissue) that has been chemically bonded to a carbohydrate polymer -- glycosaminoglycan, or GAG -- derived from shark cartilage.

The collagen-GAG polymer acts as an initial "scaffold" to support the nerve endings' new growths. By the time the polymer had degraded -- six weeks after surgery -- "we saw continuous nerve fibers bridging the 15-millimeter gap," Yannas says. (In any final device, the plastic tubing would also have to be replaced with a biodegradable material.)

Yannas notes that work led by Swedish researcher Goran Lundborg showed that severed nerves guided by an empty silicone conduit could regenerate across a 6- to 12-mm gap -- but not across 15 mm. "So we worked at 15 mm," Yannas says, "to find out if our material was offering a significant advantage over what was thought to be the best previous work."

Last year Luis de Medinaceli and his colleagues at St. Elizabeth's Hospital in Washington, D.C., reported that they had restored nerve function in rats whose sciatic nerves had been severed, splinted back together and allowed to reattach (SN: 1/28/84, p. 52). Referring to Yannas's work, de Medinaceli says, "He is addressing an issue which I have not -- how to bridge a gap." Spanning gaps is "fundamentally important" to reconstructive surgeons, he says, but only if nerve function returns. And establishing that will be essential to proving the value of Yannas's device, he believes.

De Medinaceli likens nerve tissue to a trunk line of optical fibers transmitting telephone messages across the Atlantic. He notes that while merely reattaching two stump ends of the fiber bundle may be enough to restore most American callers to a party in Europe, if the original ends of each fiber are not matched precisely the American caller will reach a stranger, not the party dialed. In the body, he says, similar confusion may arise if nerve tissues don't know whom they're addressing. De Medinaceli says tests of how well the rats walk again show that his patched nerve ends match reasonably well the original connections.

Yannas concedes that similar tests need to be conducted on rats whose nerves grew back across a 15-mm gap. However, he says it's not unreasonable to suspect that enough valid reconnections can span a gap to restore function. Citing unpublished data on tests that his group conducted two years ago, he says rats regained the ability to walk after severed sciatic nerves regenerated across a 5-mm polymer-filled gap.