Mechanisms of stem cell ageing

There are many definitions of the term ageing and perhaps even more theories that seek to explain its causes (Balcombe and Sinclair 2001). From a physiological standpoint, ageing beyond reproductive maturity is often viewed as a progression of multisystem deficits in tissue function. In adult mammals, tissue homeostasis is maintained by stem cell populations that reside in, or migrate between, a variety of adult tissues. These stem cells ensure proper tissue function by generating new cells to replace those lost or damaged over time. Despite the presence of adult stem cells in muscle, nervous, gastrointestinal, hematopoietic, and other tissues, each of these systems exhibit functional decline with age (Edwards et al. 2002, Campisi 2003, Kondo et al. 2003, Penninx et al. 2003, Pinto et al. 2003, Linton and Dorshkind 2004, Balducci and Ershler 2005, Fulle et al. 2005, Kamminga 2005, Bauer et al. 2006, Keller 2006, Theise 2006). It is compelling to consider that functional changes in the stem cell compartment of adult tissues precedes and perhaps contributes to the manifestation of ageing phenotypes. In this chapter we will review literature describing the effects of ageing on the well-characterized stem cells of the hematopoietic system. In this system, ageing is accompanied by immune compromise, anemia, and a dramatic increase in the incidence of malignancy (Edwards et al. 2002, Campisi 2003, Penninx et al. 2003, Pinto et al. 2003, Linton and Dorshkind 2004, Balducci and Ershler 2005). Several studies support the hypothesis that these ageing phenotypes stem from functional changes in hematopoietic stem cells (HSCs).