REGROWTH OF AXONS IN LESIONED ADULT-RAT SPINAL-CORD - PROMOTION BY IMPLANTS OF CULTURED SCHWANN-CELLS

Highly purified populations of Schwann cells were grafted into lesione d adult rat spinal cord to determine if they promote axonal regenerati on. Dorsal spinal cord lesions were created by a photochemical lesioni ng technique. Schwann cells derived from E16 rat dorsal root ganglia, either elongated and associated with their extracellular matrix or dis sociated and without matrix, were rolled in polymerized collagen to fo rm an implant 4-6 mm long which was grafted at 5 or 28 days after lesi oning. No immunosuppression was used. Acellular collagen rolls served as controls. At 14, 28 and 90 days and 4 and 6 months after grafting, animals were analysed histologically with silver and Toluidine Blue st ains and EM. The grafts often filled the lesion and the host borders t hey apposed exhibited only limited astrogliosis. By 14 days, bundles o f unmyelinated and occasional thinly myelinated axons populated the pe riphery of Schwann cell implants. By 28 days and thereafter, numerous unmyelinated and myelinated axons were present in most grafts. Silver staining revealed sprouted axons at the implant border at 28 days and long bundles of axons within the implant at 90 days. Photographs of en tire 1 mu m plastic cross-sections of nine grafted areas were assemble d into montages to count the number of myelinated axons at the graft m idpoint; the number of myelinated axons ranged from 517-3214. Electron microscopy of implants showed typical Schwann cell ensheathment and m yelination, increased myelin thickness by 90 days, and a preponderance of unmyelinated over myelinated axons. Random EM sampling of five Sch wann cell grafts showed that the ratio of unmyelinated to myelinated a xons was highest (20:1) at 28 days. These ratios implied that axons nu mbered in the thousands at the graft midpoint. Dissociated Schwann cel ls without matrix promoted axonal ingrowth and longitudinal orientatio n as effectively as did elongated Schwann cells accompanied by matrix. There was a suggestion that axonal ingrowth was at least as successfu l, if not more so, when the delay between lesioning and grafting was 2 8 rather than 5 days. Acellular collagen grafts did not contain axons at 28 days, the only interval assessed. In sum, grafts of Schwann cell s in a rolled collagen layer filled the lesion and were well tolerated by the host. The Schwann cells stimulated rapid and abundant growth o f axons into grafts and they ensheathed and myelinated these axons in the normal manner.