Hematopoietic stem cells (HSCs), cells that turn into blood cells, have been heavily researched for decades, and have been successful in treating blood-related diseases in humans, such as leukemia and lymphoma, for much of this time. Recently, they were even successfully used in treating a patient with both leukemia and HIV.
Like many significant findings in science, their discovery was not made in search for such cures, but it was stumbled upon while dealing with another serious medical issue of the time: radiation exposure. During and immediately following World War II, scientists tried desperately to treat people who had been exposed to lethal doses of radiation. Transplants of spleen and bone marrow tissue were found to rescue these victims. It was later found that HSCs present in these tissues were behind these healing properties; researchers discovered this by lethally irradiating rats and mice and similarly rescuing them with transplants of different types of cells. The formation of the National Marrow Donor Program in the 1980s greatly increased the availability of these cells for research. HSCs have been successfully used clinically in humans since the 1950s, and to this day they are one of the few adult stem cells widely used in clinical therapies.
Hematopoietic stem cells are able to become, or “differentiate” into, all of the cells in the hematopoietic system. The hematopoietic system includes all the different kinds of blood cells, from myeloid elements (including red blood cells, white blood cells, and platelets) to lymphatic elements (such as T cells). These tissues are some of the most rapidly dividing cells in the body, and as such are generally the first cells hurt by radiation. For this reason, therapies using HSCs allows replenishment of the cells most damaged after exposure.