No abstract

The localization of 3H-labeled cholesterol in nerves undergoing degeneration and regeneration was studied by radioautography at the electron microscope level . Two types of experiments were carried out : (a) Cholesterol-1,2 3H was injected intraperitoneally into suckling mice . 5 wk later, Wallerian degeneration was induced in the middle branch of the sciatic nerve, carefully preserving the collateral branches . The animals were then sacrificed at various times after the operation . During degeneration, radioactivity was found over myelin debris and fat droplets . In early stages of regeneration, radioactivity was found in myelin debris and regenerating myelin sheaths . Afterwards, radioactivity was found predominantly over the regenerated myelin sheaths . Radioactivity was also associated with the myelin sheaths of the unaltered fibers . (b) Wallerian degeneration was induced in the middle branch of the sciatic nerves of an adult mouse, preserving the collateral branches . Cholesterol-1,2-'H was injected 24 and 48 hr after the operation and the animal was sacrificed 6 wk later. Radioactivity was found in the myelin sheaths of the regenerated and unaltered fibers . The results from these experiments indicate that : (a) exogenous cholesterol incorporated into peripheral nerve during myelination remains within the nerve when it undergoes degeneration . Such cholesterol is kept in the myelin debris as an exchangeable pool from which it is reutilized for the formation of the newly regenerating fibers, especially myelin . (b) exogenous cholesterol incorporated into the nerves at the time that degeneration is beginning is also used in the formation of new myelin sheaths during regeneration . (c) mature myelin maintains its ability to incorporate cholesterol .

The regeneration of transected mouse sciatic nerves using semipermeable acrylic copolymer tubes to enclose both stumps has been qualitatively assessed from 1 to 30 weeks post-operative. Quantitative morphometric analysis of electron micrograph montages of complete transverse sections of the segment regenerated between stumps has permitted determinations of the percents of total area occupied by the various tissue constituents--blood vessels, epineurium, perineurium, endoneurium, myelinated axon/Schwann cell units, and unmyelinated axon/Schwann cell units. Significant differences were found in the total cross-sectional area of segments regenerated through tubes of 1.0 mm versus 0.5 mm internal diameters. Segments regenerated with the distal stump inserted in the tube contained significantly greater percentages of neural units and were significantly larger at 8 weeks post-operative compared to segments regenerated for 9-10 weeks with the distal stump avulsed. The morphometric method permits rapid quantitation of sizeable electron micrograph montages which at 1300 X permit all types of tissue components, including the unmyelinated axons, to be visualized.

The node of Ranvier has been the object of particular attention from students of the nerve fiber primarily for two reasons. One of these, of special concern to anatomists, is due to the prolonged controversy ensuing from Ranvier's initial concept of the Schwann cell as a structural unit giving rise to, as well as providing natural limits to, the internodal lengths of the myelin sheath. Another has been the urge to provide anatomical loci for the measurable electrical properties of the nerve fiber, both at rest and during impulse propagation. Such anatomical-physiological correlations have hinged on the continuities and discontinuities thought to exist both structurally and functionally throughout the extent of the fiber.Our own studies (1, 2) on the formation of myelin in peripheral nerves of infant mice and embryonic chicks have led to the conclusion that the myelin sheath is formed by a process of infolding and spiral wrapping of the surface of the Schwann cell. This conclusion was based on an electron microscope study of transverse sections of nerve fibers in the early stages of myelination. The spiral wrapping of the myelin sheath provided a plausible mode of increasing the thickness of myelin, but the manner in which the myelin sheath is elongated to cover the entire internodal region of the axon was not apparent, nor were there any hints in the transverse sections described in these earlier studies concerning the formation or appearance of discontinuities such as SchmidtLantermann clefts or nodes of Ranvier. The present report deals with a study of longitudinal sections of infant mouse sciatic nerves, with particular emphasis on the length and forms of the early segments of myelin, as well as the relationship between the Schwann cell, the myelin sheath, and the axon at regions of discontinuity of the myelin.