Ca ²⁺ activity signatures of myelin sheath formation and growth in vivo

During myelination, individual oligodendrocytes initially over-produce short myelin sheaths, which are either retracted or stabilized. By live-imaging oligodendrocyte Ca²⁺ activity in vivo, we find that high-amplitude, long-duration Ca²⁺ transients in sheaths prefigure retractions, mediated by calpain. Following stabilization, myelin sheaths grow along axons, and we find that higher-frequency Ca²⁺ transient activity in sheaths precedes faster elongation. Our data implicate local Ca²⁺ signaling in regulating distinct stages of myelination.