How to renew a stem cell

The unique cell cycle characteristics of stem cells are defined, at least in part, by a polycomb transcription factor called Bmi-1, according to Anna Molofsky, Ricardo Pardal, Sean Morrison, and colleagues (University of Michigan, Ann Arbor, MI). Bmi-1 is unique because, unlike other important cell cycle proteins such as Myc, it is needed in stem cells but not in their offspring. Thus, it is important as much for what it is not as for what it is.

The loss of Bmi-1, the researchers find, leaves mice deficient in stem cell capacity. The mice survive using only restricted progenitor cells that are limited in both the types of cells that they can generate and the numbers of divisions in which they can do so. The resulting depletion in cells leads to growth and neurological defects, and early death.

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