Bone marrow stem cells continually adjust the production of red blood cells, white blood cells, and platelets, generating millions of cells every second. Red blood cells transport oxygen, white blood cells support immune defenses, and platelets allow blood to clot. Young stem cells rejuvenate old miceResearchers transplanted blood stem cells from young mice into older animals and saw renewed function in both the blood-producing system and the immune system. “We managed to temporarily draw older blood stem cells out into the bloodstream in large numbers at the same time as we injected new healthy blood stem cells. Before transplantation, the researchers expanded the supply of young, healthy blood stem cells in the laboratory because obtaining enough cells naturally is difficult.