Myelin Membrane Structure and Composition Correlated: A Phylogenetic Study

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Abstract
We have correlated myelin membrane structure with biochemical composition in the CNS and PNS of a phylogenetic series of animals, including elasmobranchs, teleosts, amphibians, and mammals. X‐ray diffraction patterns were recorded from freshly dissected, unfixed tissue and used to determine the thicknesses of the lipid bilayer and the widths of the spaces between membranes at their cytoplasmic and extracellular appositions. The lipid and protein compositions of myelinated tissue from selected animals were determined by TLC and sodium dodecyl sulfate‐polyacrylamide gel elec‐trophoresis/immunoblotting, respectively. We found that(l) there were considerable differences in lipid (particularly gly‐colipid) composition, but no apparent phylogenetic trends; (2) the lipid composition did not seem to affect either the bilayer thickness, which was relatively constant, or the membrane separation; (3) the CNS of elasmobranch and teleost and the PNS of all four classes contained polypeptides that were recognized by antibodies against myelin Po glycoprotein; (4) antibodies against proteolipid protein (PLP) were recognized only by amphibian and mammalian CNS; (5) wide extracellular spaces (ranging from 36 to 48 Å) always correlated with the presence of Po‐immunoreactive protein; (6) the narrowest extracellular spaces (∼31 Å) were observed only in PLP‐containing myelin; (7) the cytoplasmic space in PLP‐containing myelin (∼31 Å) averaged ∼5Å less than that in Po‐containing myelin; (8) even narrower cytoplasmic spaces (∼24Å) were measured when both Po and 11–13‐kilodalton basic protein were detected; (9) proteins immu‐noreactive to antibodies against myelin P2 basic protein were present in elasmobranch and teleost CNS and/or PNS, and in mammalian PNS, but not in amphibian tissues; and (10) among mammalian PNS myelins, the major difference in structure was a variation in membrane separation at the cytoplasmic apposition. These findings demonstrate which features of myelin structure have remained constant and which have become specifically altered as myelin composition changed during evolutionary development.