ULTRASTRUCTURE OF THE PRAWN NERVE SHEATHS

Abstract

The sheaths from freshly teased nerve fibers of the prawn exhibit a positive radial birefringence, consistent with their EM [electron microscopic] appearance as highly organized laminated structures composed of numerous thin cytoplasmic sheets of laminae bordered by unit membranes and arranged concentrically around the axon. The closely apposed membranes in these sheaths are fragile and often break down into rows of vesicles during fixation. Desmosomelike attachment zones occur in many regions of the sheath. The membranes within these zones resist vesiculation and thereby provide a "control" region for relating the type of vesicles formed in the fragile portions of the sheaths to the specific fixation conditions. It is proposed that during fixation the production of artifactual vesicles is governed by an interplay of three factors direct chemical action of the fixative on the polar strata of adjacent unit membranes, osmotic forces applied to membranes during fixation, and the preexisting natural relations between adjacent membranes. It is found that permanganate best preserves the continuity of the membranes but will still produce vesicles if the fixative exerts severe osmotic forces. The results support other reports of the importance of comparing tissues fixed by complementary procedures so that systematic artifacts will not be described as characteristic of the natural state.