Axonal membrane structure during acute experimental allergic encephalomyelitis (EAE) was examined with freeze-fracture electron microscopy. Axons without myelin sheaths were prevalent within EAE spinal cords. Often these axons were associated with astrocytic processes, though membrane specializations were not observed at these sites. The demyelinated axons exhibited a highly asymmetrical partitioning of intramembranous particles (IMP), with approximately 2,000 particles/mu;m2on P-faces and approximately 150/mu;m2on E-faces. This distribution and density of IMP is similar to myelinated internodal membrane. The IMP were generally randomly distributed along the axons. However, in some regions, E-faces of demyelinated axons without paranodal-like membrane specialization in the vicinity displayed a greater than normal (approximately 500/mu;m2) particle density. Many of the IMP in these regions of increased density were of a large (10 nm) diameter. Axonal membrane bounded by a single set of paranodal oligodendroglial loops (lsquo;heminodalrsquo;) was also observed, and the axolemma adjacent to the terminal glial loop exhibited a gradient of morphologies. The E-faces of presumed heminodal membrane most often displayed a moderately low density of IMP. However, in several instances, heminodal membrane exhibited a moderately high IMP density (approximately 1,100/mu;m2), similar to that observed within normal nodal membrane. In all cases, a high percentage of the E-face IMP within heminodal membrane were large. The results demonstrate that acute demyelination is associated with a maintenance of the integrity of certain components of the axolemma and an apparent dedifferentiation in other constituents.
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