mice exhibit tissue stem-cell defects or diminished oxidative defence and ROS-induced stem-cell depletion (Box 2). Although these genetic models are suggestive, a major question remains about whether numerical or functional decline in tissue stem cells contributes to declining health in the elderly and underlies at least some aspects of ageing. In this regard, an agerelated analysis of well-defined stem-cell types in three organ systems — haematopoietic, neural and muscular — with varying regenerative profiles has proven particularly instructive.
