Evidence that axons are mitogenic for oligodendrocytes isolated from adult animals.

Recovery from demyelinating events in the adult central nervous system (CNS) may depend in some situations on the generation, by cell division, of new oligodendrocytes (ODCs). Adult ODCs, identified by morphological criteria, have been shown to divide in vivo in response to induced demyelination or to physical trauma, although the factors that elicit this response are unknown. Adult ODCs do not proliferate in vitro under normal conditions (ref. 4 and P.M.W., unpublished observations). Encouraged by observations that axons provide a mitogenic stimulus for Schwann cells and support the proliferation of embryonic ODC progenitors, we have studied the response of adult ODCs, identified by immunocytological criteria, to axons of dorsal root ganglion neurones in culture. We have now found that adult ODCs proliferate in vitro when neurones are present but not when neurones are absent, suggesting that neurons can elicit ODC division under the conditions prevailing in our culture system.