Under homeostasis, low numbers of haematopoietic stem cells (HSPC) are detectable in the bloodstream of mammals. Here, we show the mechanisms behind the robust circadian oscillations of circulating HSPC in mice. In normal 12h light-12h darkness cycles (LD) there were 2–4-fold fluctuations in Lin-Sca-1+c-kit+ cells and competitive repopulating stem cell units (CRU; 16 weeks) between the peak (5h after the initiation of light) and trough (12h later). Circadian fluctuations of HSPC were entrained by photic cues since the pattern was dramatically altered when mice were subjected to continuous light or a “jet lag” of 12h. Because CXCL12 mediates HSPC migration, we evaluated whether it is subjected to circadian control. Indeed, CXCL12 protein and mRNA levels in the bone marrow (BM) fluctuate under all light conditions in antiphase with circulating HSPC. Since the sympathetic nervous system (SNS) affects G-CSF-induced HSPC mobilization (
Disclosure: No relevant conflicts of interest to declare.