MYELINATION BY MATURE OVINE OLIGODENDROCYTES IN-VIVO AND IN-VITRO - EVIDENCE THAT DIFFERENT STEPS IN THE MYELINATION PROCESS ARE INDEPENDENTLY CONTROLLED

The ability of isolated mature post-myelination ovine oligodendrocytes to myelinate was investigated in tissue culture and in vivo. In cultu re, although the cells adhered preferentially to rat dorsal root gangl ia (DRG) axons, sent out processes that encircled and wrapped them, pr oliferated, and synthesised myelin proteins (MBP), no myelination was found. This failure to find myelination occurred despite the fact that the oligodendrocytes both in the present experiments and in previous studies elaborated membranous structures that have been shown chemical ly and structurally to be similar to normal central nervous system mye lin. These findings contrasted with those seen when neonatal rodent gl ial cells were added to similar DRG neuron cultures, in which myelinat ion readily occurred. When the same adult ovine oligodendrocytes were transplanted into the brains of Shiverer mice, normal compact myelin w as formed, proving that the cells were capable of myelination and sugg esting that cross-species incompatibility was probably not a major fac tor in the lack of myelination in vitro. It is possible that the failu re of ovine oligodendrocytes to myelinate DRG axons is due either to t he relatively low number of supporting glial cells, such as astrocytes or microglia which may be necessary for satisfactory myelination, or that some other factor in the microenvironment is lacking; in any even t, these results point to the complexity of oligodendrocyte-axon inter actions. It is clear that each of the events, from adherence to prolif eration to wrapping and the myelin compaction may be under the control of a different signal and may operate through a distinct mechanism, e ven though each process is dependent on the other. The results also po int to the potential usefulness of this model system for deciphering s uch signals and mechanisms. (C) 1993 Wiley-Liss, Inc.