CNS live imaging reveals a new mechanism of myelination: the liquid croissant model.

The overall morphology and with it associated the formation of myelin is generally thought to be resolved. Based on electron microscopic findings more than half a century ago, the current model of myelination describes all myelin membranes to run in parallel with the longitudinal axis of the axon and to form a smooth surface, reminiscent of a rolled up carpet. However, different studies in the past demonstrated a distinct myelin morphology with an uneven myelin surface contour that challenges the established concept. Even though the current model of myelination has since been recognized as insufficient, CNS myelin formation has not yet been investigated in real-time with the requisite technique and resolution. We therefore traced myelin growth in murine organotypic cerebellar slice cultures using high-resolution confocal live imaging, light and electron microscopy and assessed myelin morphology in young and adult mice by confocal microscopy. Our data verify that the myelin surface is indeed not smooth but runs in a bidirectional, regularly spaced coil along the axon in both young and adult mice. Time-lapse imaging revealed that the growth of coiled myelin turns emerges during myelin formation. We therefore propose the "liquid croissant" model as a new concept of myelination that overcomes not only some of the incongruences of previous myelination theories, but potentially also explains the development of certain myelin pathologies observed in remyelination and axonopathies.