Electron Microscope and Low-Angle X-Ray Diffraction Studies of the Nerve Myelin Sheath

Abstract

The myelin sheath of nerve fibers represents one of the most highly organized components of biological systems, which, despite its marked lability, has proved to be uniquely suitable for a systematic analysis by combined application of physical and chemical techniques. Extensive polarized light studies (4,34,(48)(49)(50)(51) of the strong birefringence of the sheath suggested it to be built up of concentrically disposed protein lamellae alternating with submicroscopic layers of lipide molecules oriented with their long axes in the radial direction. Low-angle x-ray diffraction studies (1, 5-7, 21-32, 51-55) supported this concept and furnished quantitative data on the highly ordered arrangement of the lipide, protein, and water components in the normal myelin structure. The fundamental x-ray diffraction spacings of 170 A found in amphibian and of 180 to 185 A in mammalian nerves were assumed to correspond to the thickness of the concentric lipide-protein layers deduced from polarized light investigations.The postulated layers and their exceptionally regular concentric arrangement in the myelin sheath were subsequently observed directly in the electron microscope (8-20, 35). However, this remarkable demonstration of structural regularity at the macromolecular level achieved by direct and indirect methods is still inadequate if it cannot be referred to the specific localization of chemical components in the sheath. Thus, the concentric laminated structure observed in electron micrographs of thin nerve sections is simply a pattern of the selective deposition of osmium at certain sites, and cannot as yet be interpreted in terms of specific regions containing lipides, lipoproteins, or protein constitutents. Likewise, the low angle x-ray diffraction patterns recorded from normal nerve can only furnish general information on the dimensions and approximate distribution of scattering groups in the radial direction of the sheath. Further attempts to correlate these structural parameters with the chemical composition of the sheath are thwarted by our lack of knowledge of the chemical nature and * Permanent address:

Current Models for the Structure of Biological Membranes

“…In any case, the fixation and staining of authentic bilayers of membrane lipids and lipoproteins in cases where the preservation of the lamellar structure has been demonstrated by X-ray diffraction (259,263) clearly shows that the triple-layered structure seen in micrographs is compatible with the Danielli model . For natural membranes the myelin sheath has so far offered similar evidence (80) and recent studies on isolated membrane fractions both with X-ray diffraction and with electron microscopy (86) also confirm this conclusion.…”

Ultrathin Frozen Sections

“…Gelatin embedding of osmium tetroxide-fixed tissue followed by partial dehydration has been used to obtain ultrathin sections at room temperature and these were reported to be sticky (10). Even when gelatin-impregnated tissues were thoroughly dried in a vacuum before sectioning at room temperature, posttreating with as little as 2% glycerol facilitated sectioning (7).…”

Peripheral myelin of Xenopus laevis: Role of electrostatic and hydrophobic interactions in membrane compaction

“…~180 Å for mammals, ~177 Å for rodents, ~171 Å for frog, and ~161 Å for fish (Schmitt et al, 1935;Fernandez-Moran and Finean, 1957;Hoglund and Ringertz, 1961;Finean and Burge, 1963;Moody, 1963;Blaurock, 1967;Akers and Parsons, 1970;Caspar and Kirschner, 1971;Chandross et al, 1978;Blaurock et al, 1986;Kirschner et al, 1989;Mateu et al, 1990). These differences derive mostly from differences in the widths of the extracellular and cytoplasmic appositions between membrane bilayers, and correlate generally with the sizes of the respective domains of P0 or P0-like proteins Inouye and Kirschner, 1990;Inouye and Kirschner, 1991;Avila et al, 2007).…”