OLIGODENDROCYTES ARE NOT INHERENTLY PROGRAMMED TO MYELINATE A SPECIFIC SIZE OF AXON

Current studies support the morphological classification of oligodendr ocytes proposed by Del Rio Hortega ([1922] Bol. R. Soc. Esp. Hist. Nat . 10:25-29; [1924] C.R. Sec. Biol. 91:818-820), in which cells either myelinate multiple internodes that are associated with small axons, or they myelinate restricted/single internodes of large-diameter axons. The reasons why an oligodendrocyte myelinates a particular calibre of axon are unknown. Because progenitors are generated in restricted, sub ventricular zones, an intrinsic program would imply that germinal cent res contain a mixture of cells, each committed to myelinate axons of a particular size. Conversely, each cell could have the potential abili ty to myelinate any size axon. We tested this latter hypothesis that o ligodendrocyte progenitors are uncommitted in their ability to myelina te a particular axon size. We introduced oligodendrocyte lineage cells from the optic nerve, which normally encounter only small-diameter ax ons, to a myelin-deficient environment containing a large range of axo n sizes. Dissociated, mixed glial cells from the optic nerve were char acterised immunocytochemically and were grafted into the spinal cord v entral column of neonatal, myelin-deficient rat mutants. Examination o f the patches of myelin produced by these cells at different times aft er transplantation revealed that optic nerve oligodendrocytes were cap able of producing a widespread, nonselective myelination of axons that were destined to have both small or large calibres. Thus, an axonal o r local signal, and not an intrinsic program, is probably responsible for the previously described oligodendrocyte diversity. J. Comp. Neuro l. 399:94-100, 1998. (C) 1998 Wiley-Liss, Inc.