Slow-cycling/quiescence balance of hematopoietic stem cells is related to physiological gradient of oxygen

Objective Regulation of hematopoiesis depends on cytokines, cellular interactions, transcription, and metabolic factors. Among the latter, O 2 has been neglected for a long time. Recently, an increasing number of publications evidenced the regulatory role of physiological low O 2 concentrations (0.1−5%; similar to those in bone marrow) on the in vitro behavior of hematopoietic stem cells. This brief review utilizes the article of Eliasson and colleagues in this Journal to summarize the major results and questions about the relationships between O 2 and hematopoiesis. Materials and Methods In order to be concise and interesting for readers unfamiliar with this field, we selected only the most significant data that either reinforce or contradict the conclusions of Eliasson et al., but we also provide references of reviews with a more detailed bibliography. Results A critical analysis of some key publications provides partial answers to three important questions: is the term hypoxia appropriate to describe physiological low O 2 concentrations? Is a very low O 2 level sufficient to control the quiescence/slow cycling balance of hematopoietic stem cells? Is the O 2 concentration able to modify the effect of cytokines on hematopoietic stem cells? Conclusions We propose to use in situ normoxia instead of the confusing term hypoxia when working with normal cells at physiological low O 2 concentrations. We suggest that a very low O 2 concentration is necessary but not sufficient to induce hematopoietic stem cell quiescence. We review some articles showing that O 2 variations modify the effect of cytokines.