In vitro analysis of the oligodendrocyte lineage in mice during demyelination and remyelination

Myelination in the developing CNS requires the coordinated interaction of several types of glial cells (reviewed in Raine, 1984; Wood and Bunge, 1984; Raft, 1989) . In vitro studies of optic nerve from newborn rat have characterized a bipotential progenitbr cell that can give rise to oligodendrocytes or to type 2 astrocytes, 'and is therefore called an oligodendrocyte-type 2 astrocyte (O-2A) progenitor cell. Type 2 astrocytes express antigens that are also present on the surface of O-2A progenitor cells. The localization and function of type 2 astrocytes in vivo is not clear at the present time. Another type of astrocyte, called the type 1 astrocyte; appears before type 2 astrocytes and arises from a glial precursor distinct from the O-2A progenitor (Raft et al., 1984) . Type 1 astrocytes secrete growth factors that affect the proliferation, migration, and/or differentiation of O-2A progenitors (reviewed in Raft, 1989; Dubois-Dalcq and Armstrong, 1990) . Upon maturation, oligodendrocytes elaborate extensive processes which wrap around CNS axons to form myelin (reviewed in Paine, 1984) . Perinodal astrocytes, possibly type 2 astrocytes, extend processes to nodes of Ranvier, which are electrically excitable regions of axons between adjacent myelin sheaths (ffrench-Constant and Raft, 1986b; ffrench-Constant et al., 1986) . Both oligodendrocytes and perinodal astrocytes seem to play a role in efficient saltatory conduction of nerve impulses; myelin acts as an electrical insulator while perinodal astrocytic processes may serve various functions associated with impulse propagation at nodes (discussed in Ritchie, 1984; Black and Waxman, 1988) .