Repairing Process at the Proximal Stump of Transected Myelinated Axons, with Special Reference to Formation of the Demarcation Membranes

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Morphological changes at the proximal stump of transected myelinated axons were studied by electron microscopy in the mouse sciatic nerve. Immediately after transection, the distal end of the proximal axon stumps was filled with dense networks of disorganized neurofilaments. Degradation of the disorganized neurofilaments ocurred within a few minutes, leaving an empty space within a myelin sheath tube. At 5-10 minutes after transection, while the distal end of the remaining axon stump was still open to the empty space, small (50-100 nm in diameter) and large vesicles (200-400 nm in diameter) began. To accumulate, and within 20 minutes, an extensive terminal vesicle accumulation filling up the whole area of the axon stump began to form at the distal end of the living portion. During these early morphological changes, deep axolemmal invaginations occurred in the terminal vesicle accumulation. It is suggested that with this measure, the open end of the proximal axon stumps might be first completely sealed. In the axonal segment just proximal to the terminal vesicle accumulation, the small and large vesicles randomly accumulated not only in the peripheral but also in the central zone of the axon stumps among neurofilaments and microtubules. In these reacting axonal segments, characteristic demarcation membranes were observed in the vesicle-accumulated portions and more commonly at the border between the vesicle-accumulated and neurofilament-predominant portions. It is suggested that many of them form initially as the axolemmal invagination and extend through successive fusion of the large vesicles, while some, through fusion of the large vesicles with each other. It seemed that the vesicle-accumulated portions including the terminal vesicle accumulations can not survive for long in the myelin tube, and eventually degenerate. The findings of the present study suggest that the vesicle accumulation, formation of the demarcation membranes and degeneration of the vesicle-accumulated portions might be repeated during traumatic degeneration which progressively extends proximally as far as the axon stumps are enclosed by the myelin sheath tubes. Though these events have been considered to be degenerative, they can be regarded as a part of the repairing process, and with the demarcation membranes, the living axonal portions are always separated from the degenerating portions which are eventually cast off.
神戸大学の論文
1998-12-26

Repairing Process at the Proximal Stump of Transected Myelinated Axons, with Special Reference to Formation of the Demarcation Membranes

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