Creating the right environment

Like baby chicks, stem cells need a nest—a niche within the body where they reside and self-renew. Oguro et al. (page 2247) find that, for blood stem cells, this nest is built by the same gene silencer that also helps them self-renew.

This silencer is Bmi1, which turns off two cell cycle inhibitor genes, Ink4a and Arf, to promote the self-renewal of hematopoietic stem cells (HSCs). Oguro et al. showed that defective HSC self-renewal in Bmi1-deficient mice can be restored by deletion of these cell cycle regulators, as previously observed in neural stem cells. But the new results show that HSC levels gradually decline as these mice age.

The authors wondered whether the progressive HSC problem might be due to defects in the HSC niche—the bone marrow...