Abstract
Light and electron microscopic studies were made on the cut ends and the distal portion of the rat sciatic nerves at intervals from 1 day to 56 weeks after nerve section.
Degenerative changes were evident both in myelin sheaths and axons 24 hours postoperatively at the bilateral stumps, and these changes were able to observe mainly from the 4th day towards the end of the second week in the distal portion. Almost all the Schwann cells appear to survive during this stage of degeneration, undergo hypertrophy with fine filaments and increased numbers of cell organelles, and even multiply themselves. They contained various forms of myelin fragments in their cytoplasms. It is considered that the myelin sheaths are degraded in Schwann cells during Wallerian degeneration.
Three days after the nerve section, cells identified as macrophages were observed in the endoneurium of both cut ends and thereafter they became widely distributed along the distal portion of the nerve showing Wallerian degeneration. The cytoplasms of the cells appeared darkly staining and contained numerous vacuoles and various forms of lamellar debris. They had many finger-like processes at their periphery. In contrast with Schwann cells, the macrophages didn't possess cytoplasmic filaments and basal lamina. Occasional cells were found also inside the basal lamina of the Schwann cells and some were observed crossing the disrupted basal lamina. Injections of horseradish peroxidase around the devided nerves disclosed an uptake of the tracer by these debris-laden macrophages. It seems likely that macrophages play an active part in the breakdown and removal of the degraded myelin and axon in addition to Schwann cells.
The earlier signs of regeneration were found in sections of the proximal cut end and the junctional zone between the cut ends on the 7th day after nerve section. Smaller axons containing distinctive neurofilaments reappeared in relation to Schwann cells with beginning remyelinization. Groups of Schwann cells and their included axons became separated each other by cells with elongated processes. They displayed ultrastructural features of fibroblasts and gradually assumed an appearance of perineurial cells which were evidenced by the presence of basement membrane envelope and of multiple pinocytic vesicles in their elongated extensions. It seems that these cells play an important role in isolating the environment of the regenerating nerve fibers from the surrounding tissues.
Solitary cilia were observed in fibroblasts and Schwann cells degrading the myelin. Those observed in Schwann cells had a fibrillar configuration of 9 + 0, having nine peripheral doublets of microtubules with no axial complex.
