Oligodendrocytes were first described by Robertson in 1899 as small cells with a few processes of variable length. In 1921, del Rio Hortega published a detailed histological description of these cells and coined the descriptive word “oligodendrocyte.” The localization of these cells along axonal tracts, around neuronal cell bodies, and in proximity to capillaries served as the basis for del Rio Hortega’s classification of oligodendrocytes into three subgroups: interfascicular, perineural satellite, and perivascular. The heterogeneity of oligodendrocytes has also been observed at the ultrastructural level and by the use of immunocytochemical markers (for review and ref. see Espinosa de los Monteros and de Vellis, 1988; Goldman and Vaysse, 1991). However, there is no evidence for more than one oligodendrocyte lineage (Skoff and Knapp, 1991).