AXON-MYELIN SHEATH RELATIONS OF OLIGODENDROCYTE UNIT PHENOTYPES IN THE ADULT-RAT ANTERIOR MEDULLARY VELUM

Axon-oligodendrocyte relations of Rip-immunolabelled and dye-injected oligodendrocyte units are characterised in the adult rat anterior medu llary velum (AMV). Each oligodendrocyte unit comprised the oligodendro cyte cell body, processes and the internodal myelin segments they supp ort. Oligodendrocyte units corresponded to classically described type I/II or type III/IV unit phenotypes which respectively myelinated disc rete populations of small and large diameter axons, delineated by a my elinated fire diameter of 2-4 mu m (diameter of the axon plus its myel in sheath). Within units, mean fibre diameter was directly related to mean internodal length and inversely related to the number of myelin s heaths in the unit. The relationship between fibre diameter and intern odal length was retained in units which myelinated axons of different diameters, indicating that axon diameter was an important determinant of the longitudinal dimensions of myelin sheaths. We also show that ty pe III/IV units maintained a far greater volume of myelin than type I/ II units. It was concluded that type I/II and III/IV oligodendrocytes represent two functionally and morphologically distinct phenotypes who se distribution densities were determined by the diameter and spatial dispersion of axons.