Hematopoietic stem cells (HSCs) regenerate blood cells throughout the lifespan of an individual.  The HSC pool has to accommodate to the cellular stresses associated with its life-long activity. HSCs are protected in specific bone marrow areas called “niches”. New technologies allow the analysis of different bone marrow compartments and the functional implications on HSCs.  HSCs persist for a lifetime; there is a balance between their proliferation and their quiescent (sleeping) state. Therefore, the HSCs cell cycle is controlled by factors within and outside the cell.