Blood-brain barrier permeability towards small and large tracers in a mouse model of osmotic demyelination syndrome

•BBB opening towards small tracer is delayed compared to oligodendrocyte loss in ODS.•Increased BBB permeability is concomitant to decreased expression of junction’s proteins.•Ultrastructure of endothelium or junctional complexes does not show major disruption.•Endothelium shows numerous profiles of enlarged plasmalemmal (tubulo-)vesicles.•BBB breach includes subendothelial cavities close to damaged astrocyte end-feet.