A 7-year-project to develop a barcoding and tracking system for tissue stem cells has revealed previously unrecognized features of normal blood production. New data from Harvard Stem Cell Institute scientists at Boston Children's Hospital suggests, surprisingly, that the billions of blood cells that we produce each day are made not by blood stem cells, but rather by their less pluripotent descendants, called progenitor cells. The researchers hypothesize that blood comes from stable populations of different long-lived progenitor cells that are responsible for giving rise to specific blood cell types, while blood stem cells likely act as essential reserves. The work, supported by a National Institutes of Health Director's New Innovator Award and published online on October 5, 2014 in Nature, suggests that progenitor cells could potentially be just as valuable as blood stem cells for blood regeneration therapies. This new research challenges what textbooks have long maintained: i.e., that blood stem cells maintain the day-to-day renewal of blood, a conclusion drawn from their importance in re-establishing blood cell populations after bone marrow transplants—a fact that still remains true. But, because of a lack of tools to study how blood forms in a normal context, nobody had been able to track the origin of blood cells without doing a transplant. Boston Children's Hospital scientist Fernando Camargo, Ph.D., and his postdoctoral fellow Jianlong Sun, Ph.D., addressed this problem with a tool that generates a unique barcode in the DNA of all blood stem cells and their progenitor cells in a mouse. When a tagged cell divides, all of its descendant cells possess the same barcode.
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