Young blood revives aging muscles, Stanford researchers find  so
comes from previous work. Rando had found that satellite cells
in younger muscles begin producing a protein dubbed Delta in
response to muscle damage. Older muscles maintained the same
pre-injury levels of Delta even after muscle damage. However, in
the current study he found that satellite cells in elderly mice
joined to younger partners ramped up Delta production to
youthful levels after an injury.  The group confirmed their
results by putting satellite cells from old and young mice in a
lab dish with either old or young blood serum. Old satellite
cells in old serum and young satellite cells in young serum both
behaved as expected. But when old satellite cells were bathed in
young serum they cranked up their production of Delta and began
dividing. Likewise, young satellite cells decreased the amount
of Delta they produced when in a dish with older serum and
divided less frequently.  Rando said that it may be a general
phenomenon that a person'