Spinal cord myelin is vulnerable to decompression

Abstract

Spinal cord white matter is the major site of tissue damage resulting from decompression sickness (DCS or "the bends"). Damage is thought to result from bubble nucleation within the tissue. Why DCS occurs predominantly in the spinal cord and not in the brain is not known; neither is the exact pathological mechanism by which the spinal cord is damaged, nor how multiple sclerosis (MS)-like symptoms may ensue. To investigate the molecular basis of white matter damage, we subjected myelinated mouse tissues to varying durations of decompression, and then after recompression to one atmosphere, examined them for changes in myelin structure and composition. X-ray diffraction showed that the myelin period in spinal cord decreased by 4%, whereas those of optic and sciatic nerves were stable. The change in period was accompanied by a change in membrane bilayer profile--i.e., relative to control, the width of the bilayer decreased by approximately 6 A, whereas the interbilayer spaces each increased by approximately 3 A. The changes in electron density levels suggested a redistribution of matter from the interbilayer spaces into the lipid headgroup layers. By contrast with these structural changes, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and high-performance thin layer chromatography (HPTLC) revealed no noticeable change in myelin composition--i.e., there was no release of myelin-specific proteins or lipids. Our findings indicate that spinal cord myelin has an inherent structural vulnerability that may facilitate the targeting of this tissue during pressure changes.