Ultrastructure appearances of distal segment of rat optic nerve after 72 hrs of NMDA injection. Transverse sections show separation and vacuolation of myelin sheath (A & B). Longitudinal sections display nodal blebs (C, arrows), abnormal accumulation of altered organelles (D, arrow), myelin whorls (E, arrow) arising from the inner myelin and forming mesaxon (F, arrow). The cytoskeleton surrounding the myelin whorls appears normal at this stage (E & F). Bars = 2 μm.