The fate of Schwann cell basement membranes in permanently transected nerves.

After peripheral nerve fiber degeneration, Schwann cell basement membranes (SCBM) persist, maintain the columnar orientation of multiplying Schwann cells, and provide pathways for regenerating axons to original target tissue. Despite the putative importance of these functions, there is little quantitative information on SCBM in chronic denervation. The number, integrity, shape, and size of SCBM were evaluated in transverse electron micrographs of peroneal nerves of groups of mice at various times after permanent sciatic nerve transection. With increasing time after transection, the SCBM fragment, the fragments become shorter, dispersed throughout the endoneurium and partially disappear. The discontinuity, dispersion, and partial disappearance of SCBM, which worsens with time after nerve transection, alters the scaffold by which neurites grow back to formerly innervated or appropriate target tissue. Without reaching appropriate target tissue, regenerating fibers may fail to develop or undergo retrograde atrophy and degeneration. The progressive changes of the SCBM may contribute to the demonstrated poor regeneration when nerve reconnection is delayed, or when proximal nerves are reconnected, so that much time may elapse before neurites grow back to distal nerve.