Certain problems in defining the oligodendrocyte cell type should be pointed out at the onset. The interfascicular oligodendrocyte is commonly defined as the cell responsible for the formation and maintenance of central myelin. Direct demonstration of the connections between oligodendrocyte somas and myelin sheaths is inherently very difficult, however, and it is possible that there are substantial numbers of cells in white matter, resident among the myelinrelated oligodendrocytes, that do not directly husband myelin segments. This possibility must be seriously considered because it is now known from tissueculture studies (detailed in Section 6) that oligodendrocytes may express myelin-specific components when not directly connected to myelin sheaths. Also, recent detailed studies of remyelination in adult white matter suggest that glial reserve or stem cells (resident, but as yet unrecognized, in white matter) are responsible for the production of new oligodendrocytes prior to remyelination.