The CNS is composed of different cell types including neurons and oligodendrocytes. Neurons are composed of a cell body and an extension, the axon, that is surrounded by myelin, a protective sheath. This sheath plays a role in the survival of the axon and allows rapid transmission of information between the brain and the rest of the body. Oligodendrocytes are the cells used to make the myelin sheath.
In MS, myelin is the target of the disease process, which is triggered by an inflammatory reaction. At this signal the macrophages destroy the myelin sheath, respecting the axon at first. The demyelinated lesions are usually well delineated and are called plaques. As the outbreaks progress, the oligodendrocytes will be less and less able to repair damaged myelin sheaths, the axons will become scarce, the old plaque is the place of a scar tissue rich in astrocytes that corresponds to astrocytic gliosis or sclerosis. The study of the cellular and tissue mechanisms of MS seeks to identify factors controlling the inflammatory phase, facilitating remyelination, protecting the axons.