Blood Stem Cells Conceal Unequal Predispositions: Self-Renewal Comes in Two Different Types, Study Finds
Sanders, Laura, Science News
All blood stem cells are not created equal, a new study finds. Two distinct kinds of self-renewing blood cells have been spotted in mice, muddying a simplistic view of stem cell categories. Knowing how these different types of stem cells behave may help scientists better understand and treat blood diseases.
"The definition of a stem cell, as you look closer, gets more complicated," comments stem cell researcher Timm Schroeder of the German Research Center for Environmental Health's Institute of Stem Cell Research in Neuherberg. The new study, appearing March 5 in Cell Stem Cell, adds to a growing body of evidence that "black-and-white characterizations might not be right," says Schroeder, who was not involved with the study.
In the blood, millions of diverse cells die every second. To keep up with this loss, stem cells continually divide to create the correct balance of cell types, which include oxygen-carrying red blood cells and a menagerie of immune cells.
"For the longest time, people always thought there was one single type of blood stem cell in the bone marrow that continually replenished the blood sys tern throughout the life of a person," says study coauthor Grant Challen of Baylor College of Medicine in Houston.
Recent studies have hinted that blood stem cells have distinct behaviors, but all scientists had to go on was information about how the cells acted once they were implanted in mice. Challen and colleagues identified subpopulations of stem cells before they were implanted and then watched as the cells behaved differently as they divided.
"We're the first group to actually identify them using different markers," says Challen.
His team used a special dye to stain stem ceils removed from mouse bone marrow. Some stem cells expelled the dye at different rates, which, along with other well-known stem cell markers, allowed the researchers to sort these cells into two classes based on fluorescence. This dye difference told researchers that the stem cells looked different, but not whether the cells acted differently, too.
Researchers next transplanted several hundred cells, by type, into mice lacking blood stem cells. Over the next few months, the researchers monitored what kinds of blood cells were produced by the stem cells. While each type of stem cell was able to produce every kind of blood cell, the team found a clear difference in productivity: One type of stem cell made many more red blood cell precursors than the other type. And the second type made more of certain kinds of white blood cells, immune cells called T and B cells, than its counterpart. These strong biases for generating different kinds of cells were evident even when the researchers injected a single stem cell into a mouse and watched as the stem cell repopulated the entire blood system. …