Macrophages remove damaged myelin. A tangle of astrocytic processes fills the gaps of lost tissue. Remaining oligodendrocytes attempt to make new myelin, but this process is ineffective because gliosis creates a barrier between the myelin producing cells and their axonal targets. With time, plaques reach a burned-out stage consisting of demyelinated axons traversing glial scar tissue. Although myelin is preferentially affected, axon loss may be significant.
Active lesions are cellular because of inflammatory cells, macrophages, and reactive astrocytes.
Activity is often confined to the borders of plaques. Myelin stains reveal the lesions unequivocally. Grossly, MS plaques appear as irregular, sharply demarcated, gray areas in the white matter. They are usually multiple. Plaques are randomly distributed.