ABSTRACT
In humans, hematopoietic tissues generate ~ 1011-1012 mature blood cells per day to maintain normal hematologic functions (Suda et al., 2011). These mature blood cells originate from hematopoietic stem cells (HSCs), which exhibit the essential features of stem cells: self-renewal and differentiation. Extensive previous work reveals that HSCs are not a homogenous cell population, but rather can be classifi ed into at least two HSC populations according to the length of the cell cycle (Wilson et al., 2008). While the majority of HSCs cycle relatively actively, with a dividing interval of 9-36 days, a minority of HSCs are more quiescent, with a longer cell-cycle duration of 56-145 days (i.e., dormant HSCs) (Wilson et al., 2008). Over the two year lifespan of a laboratory mouse, the rapid-cycling HSCs divide ~ 20 times, whereas the slow-cycling HSCs divide fewer than fi ve times. Long-term repopulation of HSCs after bone marrow (BM) transplantation is accomplished by dormant HSCs.
