Can oligodendrocytes attached to myelin proliferate?

The uptake of thymidine by oligodendrocytes in the brains of adult mice was examined after the induction of cortical traumatic lesions in an attempt to determine whether mature oligodendrocytes actively attached to myelin sheaths were capable of proliferating. In view of the great difficulty in visualizing the connection between a given oligodendrocyte and a myelin sheath even in the normal adult animal, the neuropil was made edematous in the traumatized animals in order to separate out the components and to study the oligodendrocyte processes. Uptake of tritiated thymidine was found in oligodendrocytes, as well as in endothelial cells, astrocytes, and microglia. The percentage of labeled oligodendrocytes appeared low in relation to the total number of the oligodendrocytes. In addition, in a few labeled cells, cytoplasmic processes could be seen extending to and apparently forming the myelin sheath. The possibility that a differentiated cell still attached to myelin may at the same time be able to proliferate is of great significance in understanding the potential for remyelination and recovery, in the adult, because it would imply that any oligodendrocyte is a candidate for proliferation, rather than only uncommitted or immature glial cells, which may be limited in the mature brain.