In a Neuroscience perspective in Science Redmond and Chan summarize work of Ruckh et al. showing that factors present in the blood of younger mice (introduced by joining the circulatory systems of a young and old mouse) enhance repair of the myelin sheath around neuronal axons in the older mice. Here is a graphic from the summary:
Legend - The circulatory system of an “old” mouse (gray) with a demyelinated lesion was surgically joined with that of a healthy “young” mouse (white. The old mouse exhibited enhanced remyelination relative to the control, an old-old mouse pair. Remyelination depended on the recruitment of circulatory factors from the young mouse, including macrophages. Resident “old” oligodendroglia retain remyelination potential even as they age, but macrophage-mediated clearance of inhibitory myelin debris from the lesion may become impaired.