A study involving samples flown on four SpaceX resupply missions to the International Space Station has revealed that space travel accelerates the aging of blood-forming stem cells crucial for blood and immune system health. Stem cells are cells within the body that can develop into various types of cells. The studied cells, called human hematopoietic stem and progenitor cells, make all of the blood cells in the bone marrow - the soft and fatty tissue residing inside bones - including red blood cells that carry oxygen, immune system white blood cells that fight infections and platelets that clot the blood. The cells during spaceflight became overactive, depleting their reserves and exhausting their ability to rest and recover, a characteristic that lets stem cells regenerate over time, according to the researchers. The researchers detected differences in the response of the stem cells to space travel depending upon the donor.