Many tissues turn over during adult life, and declines in this process are associated with the progression of aging. Whether renewal is continual (as in the intestinal villi) or episodic (as in hair follicles), it is mainly attributed to somatic stem cells. One fundamental question is whether a decline of tissue renewal reflects the lifelong accumulation of external insults or the internal progression of a clock within stem cells? On pages 613 and 575 of this issue, Wang et al. (1) and Matsumura et al. (2), respectively, gain insight into this phenomenon by examining hair follicle growth and aging.
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