ABSTRACT
The hematopoietic system faces a lifelong need for replenishment as its cellular elements are lost to physiologic turnover or in response to insult from infection or trauma. The engine that drives this cycle of loss and renewal is the hematopoietic stem cell (HSC)—a rare, self-renewing, multipotent cell type that resides in the adult bone marrow in mammals. The HSC sits at the base of a well-de‹ned hematopoietic hierarchy, giving rise to progressively more differentiated progenitor and, ultimately, mature cell populations.
