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

Abstract

The ultrastructural changes, proximal to a constriction, in unmyelinated postganglionic sympathetic axons have been studied in the cat splenic and hypogastric nerves. They were con stricted with a fine ligature, which was left in situ , and examined at intervals up to 24h after operation. Immediately after tying the ligature there were definite changes in the morphology and organelle content of the axons adjacent to the constriction. These were due to the mechanical trauma produced by tying the ligature. From 10 min onwards there was a rapid accumulation of organelles in axons, which became progressively more swollen. Eventually the plasma membranes of both axons and Schwann cells broke down and later the Sclrwann cell basement membrane ruptured , allowing axonal organelles to escape into the interstitial spaces. The accumulating organelles included vesicles with an electron dense core, mitochondria, large vacuoles and myelin figures. Filamentous structures and both fine and wide tubules were more prominent and more numerous after operation. These were accompanied by a marked increase in the number of agranular vesicles. Tubular structures, morphologically similar to both the agranular vesicles and the vacuoles, were also very numerous. This similarity in appearance and the fact that many of these agranular vesicles, vacuoles and tubules were found joined together by either filaments or fine tubules suggests that they are part of the same system . At all times the greatest accumulation of organelles and the most marked axonal swelling was found in the first 0.5 to 1 mm segment of nerve immediately adjacent to the constriction. Less marked changes, affecting principally the agranular vesicles and finer tubules to gether with focal accumulations of mitochondria were seen at more proximal levels from 4h onwards. It is suggested that the axonal swelling is due to the accumulation of axoplasm and organelles. Two possible interpretations are discussed: (1) that this represents the effects of obstructing the normal flow of axoplasm from the cell body to the periphery, and (2) that some features, in particular the increase in the size and number of the various tubules may be indicative of an active reaction to the effects of axonal injury. These two phenomena probably take place simultaneously and are accompanied by degenerative changes.