Some cases are progressive from the start or go into a progressive phase later. The pathology of MS develops around blood vessels. In most cases, the inflammatory reaction subsides only to appear at another location or at another time. Some lesions expand at their periphery while activity in their center subsides. The pathological process may be arrested at any time, sometimes after partial demyelination.
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.