The tortoise and the hare?: Two distinct intestinal stem cell populations

Adult stem cells have the capacity to self-renew and to regenerate tissues long-term, in both homeostasis and wound repair. Genetic lineage tracing studies have identified two principal stem cell populations in the intestine. One population consists of actively cycling Lgr5+ cells residing at the crypt base. The other population consists of quiescent Bmi1+ cells that largely reside at approximately the +4 cell position directly above the Paneth cells in the crypt. Recent studies demonstrate a functional relationship between these two intestinal stem cell (ISC) populations. This review provides an overview of ISCs and the ISC niche.