A Comparison of Nodes of Ranvier in Sciatic Nerves with Node-Like Structures in Optic Nerves of the Mouse

In a study of myelin formation in thin sections of mouse optic nerve, several node-like discontinuities in the myelin sheath have been observed. Since previously published electron micrographs of such discontinuities in nerve fibers of the rat central nervous system (1, 2) lacked sufficient detail to resolve the glial cytoplasm--myelin lamellar relationships, it appears worthwhile to present this comparison of nodal structures in the peripheral and central nervous systems of the mouse. 1 Since the term node of Ranvier is so widely used in the description of peripheral nerve fibers, and since there are differences in structure between the peripheral and central fibers, the analogous region of fibers of the central nervous system is here referred to simply as a "node." Aside from the well known histological features of the peripheral nodes of Ranvier, at tention has been called to several aspects of its structure in electron micrographs: (a) the high concentration of mitochondria in the Schwann cell cytoplasm near the node (4); (b) the overhanging of outer myelin lamellae at the node (5), and the abrupt change in direction of myelin lamellae at the node (4, 6, 7), both bringing the termination of each myelin lamella into close apposition with the axon surface at the node; (c) the non-syncytial nature of the Schwann cell junctions at the node in immature myelinating fibers (5) ; (d) the interdigitita-