In old mice, a several-week exposure to the blood of young mice causes their bone marrow stem cells to act “young” again, a team of Howard Hughes Medical Institute (HHMI) researchers has found, consistent with other recent studies that show stem-cell aging may be reversible.

Exposure to a younger animal’s blood somehow pushed the older animal’s hematopoietic stem cells (which give rise to all the cells of the blood system) back to a more youthful state, in which they were fewer in number but recovered nearly all of their blood-cell-generating capacity.