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Models for cell division. Stem cells may be divided in the following three possible ways: A, Symmetric division: a stem cell divides into two progenitor cells; B, Symmetric renewal: a stem cell divides into two stem cells. C, Asymmetric division: a stem cell divides into one progenitor cell and one stem cell. (To see this illustration in color the reader is referred to the web version of this article at www.liebertonline.com/ars).
