Adult tissue-specific stem cells both self-renew and generate functional progeny. Mammalian hair follicles, which are characterised by cyclical phases of growth (anagen), regression (catagen) and rest (telogen), are an ideal system in which to investigate the homeostasis of an adult stem cell population. On , Jonathan Epstein and co-workers show that Hopx, which encodes an atypical homeodomain protein, is specifically expressed in long-lived stem cells in the basal bulge of the mouse telogen hair follicle. Hopx+ cells, they report, contribute to all the lineages of the mature hair follicle. In addition, the researchers identify a previously unknown progenitor population in the lower hair bulb of anagen-phase follicles and show that these Hopx-expressing cells contribute to the cytokeratin 6-positive inner bulge niche cells in telogen that regulate the quiescence of adjacent hair follicle stem cells. Because Hopx expression also marks other adult stem cell populations, the researchers suggest that tissue-specific stem cell populations might share homeostatic mechanisms.
