Stem cells in the small intestine. Representative architecture of a crypt in the small intestine. Quiescent stem cells, or label-retaining cells (infrequently cycling stem cells can be labeled with ³H thymidine or BrdU for a long time), reside immediately above the uppermost Paneth cell at the fourth position and express Bmi1. Cycling stem cells, or columnar basal cells, lie between Paneth cells at the bottom of a crypt and express Lgr5.