Label retention and cell cycle entry upon initiation of emergency hematopoisis. (A) After pulse-labeling with green fluorescent protein (GFP) or 5-bromo-2′-deoxyuridine (BrdU), a small fraction of HSCs retain high levels of label (LRCs), indicating a state of continued quiescence (dormancy). Almost all repopulation potential resides within the dormant population. (B) Continuous transition states exist between dormant and actively cycling HSCs, allowing the hematopoietic emergency response to be adjusted to the signal strength. The dormant HSC compartment is resilient to acute stress but can be exhausted under chronic stress.