An electron microscopic study of the early changes distal to a constriction in sympathetic nerves

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Abstract
The ultrastructural changes distal to a constriction in unmyelinated postganglionic sympathetic axons have been studied in the cat splenic and hypogastric nerves at intervals up to 24h after operation. Since the immediate postoperative changes were identical with those found proximal to the lesion it was concluded that they were due to the mechanical effects of tying the ligature. Within a few hours of operation there was an accumulation of organelles immediately distal to the constriction. At all times this was greatest within 0.5 mm of the lesion and only rarely extended beyond the first 1 mm even after 24h. The organelles which accumulated included mitochondria, many of which were swollen, pleomorphic myelin figures, multi-vesicular bodies, agranular vesicles and vacuoles of various sizes. Vesicles with an electron dense core, i.e. granular vesicles did not accumulate distal to the constriction. Further from the constriction focal accumulations of some structures, in particular mitochondria and myelin figures, did occur but they were rare. In this part of the nerve there was generally a diminution in the number of organelles, and a marked alteration in the axonal morphology. Alternating swollen and narrow regions were common, and many axons were very irregular as if ‘collapsing’. While filaments and occasionally fine tubules persisted in some of these axons up to 24h after operation, there was a reduction in the endoplasmic reticulum. These findings are compared with those seen proximal to the constriction. Although there were some similarities, the absence of granular vesicles, the decrease in the axonal endoplasmic reticulum and the ‘collapse’ of the axons due to the decrease in their contents were notable differences. It is suggested that there is a differential movement of axoplasm and organelles distal to the constriction. Thus while granular vesicles and some axoplasm continue to move towards the periphery, mitochondria and a part of the axoplasm migrate proximally to the site of axonal injury and the region of maximal degenerative change. The movement of mitochondria is considered to indicate a reaction to injury rather than supporting the view that retrograde axoplasmic flow occurs in normal axons.