The meningeal sheath of the regenerating spinal cord of the eel, Anguilla

We describe here the meningeal sheath that encloses the spinal cord, and the sheath that develops when the cord regenerates after a total transection. This description is derived from electron and light microscopy. The sheath of the uninjured cord was found to be a single structure of two parts: an outer, thin melanocyte layer and an inner, thicker layer of 2 to 10 rows of fibroblasts, closely associated with collagen and elastic fibers. Soon after cord transection, the injured axons re-grow and, together with the reforming central canal, create a bridge that links the transected cord within 8 days of injury. This bridge is covered at first by a rudimentary meningeal sheath, formed of fibroblasts and macrophages, that later progressively thickens and becomes more compact. By about day 20, the fibroblasts are arranged as 16 to 20 loose rows that include bundles of collagen, oriented along the rostro-caudal axis of the cord. Even after 144 days, the meninx, although substantially thicker than normal because of the numerous fibroblast rows (20 to 30), still lacks the melanocyte layer. In cases in which the meninx at the transection site was mechanically and pharmacologically (6-hydroxydopamine) disrupted, bridge formation was essentially unchanged, and axonal regrowth continued; some regrowing axons, however, extruded from the denuded cord. Accordingly, our findings indicate that although the meningeal sheath is not essential for cord regeneration to take place, it may well facilitate recovery by providing mechanical guidance and support to the regrowing axons.