The submicroscopic organization of vertebrate nerve fibres: An electron microscope study of myelinated and unmyelinated nerve fibres

Abstract

1. I. Thin serial frozen sections of fresh peripheral and central myelinated nerve fibres from the frog, rat, cat and rabbit, in addition to spinal cord segments of frog and rat, were examined with the electron microscope after freezing-drying or fixation with osmium. Supplementary investigations included parallel observations by phase contrast microscopy and examination of thin sections of embedded nerve, replica-adhesion and dissociation preparations. 1. 1. The following structures are described in the internode portion of the peripheral myelinated nerves: 1.1. a) The myelin sheath of fresh nerve separates into layered membranes which closely resemble the myelin lamellae of fixed whole nerve. Lamellar membranes of this type with an average thickness of 80 Å and a granular fine structure have been found in all nerve types. By means of ultracentrifugation of fresh whole nerve these laminated membranes can be warped in a reproducible fashion, indicating that they are probably preformed structures in the sheath. Dissociation of the fresh, unfixed membranes into rod shaped granules 60–80 Å long is produced by trypsin digestion. The concentric laminated structure of the sheath, consisting of individual layers approximately 80 Å thick, could be confirmed in all nerve types examined. 1.2. b) Oblique clefts corresponding to the Schmidt-Lantermann incisures were observed in the myelin sheath, and further structural details of the neurilemma membrane, its associated fibre bundles and the Schwann cells noted. Numerous opaque, oval bodies (0.5–1 μ) with a granular internal structure were found close to the Schwann cell. 1.3. c) An arrangement is described for obtaining successive electron diffraction patterns and electron micrographs from restricted areas of the same object in the specimen holder of an RCA EMU electron microscope, without breaking the vacuum. With this arrangement, characteristic cross-grating and Kikuchi line patterns have been recorded from crystalline structures resembling myelin lamellae in osmium fixed preparations. The evidence available suggests that these crystalline lamellae are osmium products formed in the nerve during the process of fixation. 1.4. d) The axon of fresh nerve fibre sections which have been frozendried or fixed, contains filaments of indefinite length, with a diameter of 100–200 Å and regularly spaced axial discontinuities. These filaments show a predominantly longitudinal arrangement within the fibre and can aggregate to form compact bundles resembling neurofibrils. Dense oval corpuscles and clear vesicular bodies are also seen in the axon. 2. 2. In thin sections and dissociation preparations from the lateral funiculus of the frog and rat spinal cords, new types of submicroscopic fibres were found which have diameters ranging between 0.1 and 1 μ, and can be identified as nerve fibres by their connection with larger myelinated fibres and their fine structure. These fibres have regularly spaced fusiform enlargements occupied by dense bodies resembling Schwann nuclei, and consist of a thin sheath formed by a single tubular membrane, which contains long filaments 100 Å in diameter and a variable number of other structures. The finest fibres consist of a single membrane and only one central filament. The sheath membrane of these fibres is 60–80 Å thick and exhibits the same granular fine structure as the layered membranes of myelinated nerve fibres. 3. 3. Unmyelinated nerve fibres from the spinal cord and the sympathetic chains of frog and rat are constituted by compact bundles of nodose filaments, having a diameter of 100–200 Å and indefinite length, and are invested with a single granular membrane approximately 60–100 Å thick. Dense slender bodies resembling nuclei are attached to these membranes. 1. II. The findings presented here indicate that the structural elements of all types of nerve fibres are fundamentally similar, and point to a common pattern of organization in myelinated fibres of all sizes and in the so-called unmyelinated nerve fibres.