Ultrastructural studies of the superior cervical trunk of the mouse: Distribution, cytochemistry and stability of fibrous elements in preganglionic fibers

Abstract

The ultrastructure of axons in the preganglionic cervical sympathetic trunk of the mouse is described with emphasis on the number, distribution and stability of fibrous elements in the axoplasm. Neurofilaments outnumbered microtubules in myelinated and non‐myelinated axons of all sizes, and the ratio of neurofilaments to microtubules in non‐myelinated axons at each point studied was fairly consistent and independent of axonal diameter. The density of neurofilaments and microtubules, however, was greater in axons of progressively smaller dismeter. In non‐myelinated axons and small myelinated axons neurofilaments were uniformly distributed throughout the axoplams resulting in minimum and maximum interfilament distances of 300 Å and 500 Å respectively; the spacing of fibrous elements within any one axon was dependent upon its diameter and position with respect to the superior cervical ganglion in the preganglionic trunk. The maximum interfilament distance was also found in large myelinated axons where neurofilaments, occurring in fascicles, were separated by distances of approximately 500 Å. Cytochemical staining of axons with lanthanum hydroxide, ruthenium red or alkaline bismuth delineated the delicate filamentous matrix interconnecting microtubules, neurofilaments and other organelles in the axoplasm. Alkaline bismuth stain was most intense in myelinated axons where heaviest deposition of reaction product was associated with neurofilaments. Treatment in vitro of the cervical sympathetic trunk with 5 × 10⁻⁵ M vinblastine sulfate dissociated microtubules and induced formation of crystalline arrays of “tubular” elements. A uniform center to center spacing of 250–300 Å was found for crystalloids in nonmyelinated axons; however, in myelinated axons the center to center spacing was not uniform and varied in the range 300–600 Å. Neurofilaments and their surface projections were unaffected by vinblastine. Fixation in the presence of lanthanum enhanced delineation of crystalloid elements. Exposure of 0–4°C for up to three hours had no consistent effect on microtubules or neurofilaments. In contrast, cold treatment disrupted the delicate axonal matrix and resulted in the formation of aggregates of coarse flocculent material in the axoplasm.