[Stress response mechanism in hematopoietic stem cells.]

At steady state, hematopoietic stem cells(HSCs), the most undifferentiated cells in the hematological system, are kept quiescent in the cell cycle. Upon hematological stresses, including radiation, anti-cancer medication, infection, and transplantation, bone marrow HSCs enter the cell cycle and robustly repopulate the entire hematopoietic system via multi-lineage differentiation and self-renewal, partly due to the alteration of their surrounding microenvironment or niche. Such hematological repopulation activity is termed "stress hematopoiesis," an activity essential for homeostatic maintenance of blood production. Understanding stress hematopoiesis is also critical to establish efficient methods enabling expansion in vitro of HSCs, since forced activation of the cell cycle by cytokine supplementation induces a stress-like cellular state.