Early myelination involves the dynamic and repetitive ensheathment of axons which resolves through a low and consistent stabilization rate

Oligodendrocytes in the central nervous system exhibit significant variability in the number of myelin sheaths that are supported by each cell, ranging from just a few to about 50 (1-4). This begs the question, why does sheath number vary so widely across individual oligodendrocytes? Myelin production during development is dynamic and involves both sheath formation and loss (5-9). However, how these parameters are balanced to determine overall sheath number has not been thoroughly investigated. To explore this question, we combined extensive time-lapse and longitudinal imaging of oligodendrocytes in the developing zebrafish spinal cord to quantify sheath initiation and loss. Oligodendrocytes repetitively ensheathed the same axons multiple times before stable sheaths were formed. Interestingly, while there was a variable number of ensheathments produced by each oligodendrocyte, ~80-90% of these structures were always lost, which was an unexpectedly high, but consistent rate of sheath destabilization. The dynamics of this process indicated rapid membrane turn-over as ensheathments were formed and lost repetitively on each axon. To test whether membrane recycling impacts this sheath stabilization rate the endocytic pathway was disrupted by expressing a dominant-negative mutant form of Rab5. Oligodendrocytes over-expressing this mutant lost an even higher percentage of sheaths. Overall, oligodendrocytes initiate a highly variable number of axonal ensheathments, but these cells all exhibit a similar sheath stabilization rate, which is dependent on the endocytic recycling pathway.