Dormancy in the stem cell niche

Tissues characterized by constant turnover contain post-mitotic, terminally differentiated cells originating from highly proliferative progenitors, which in turn derive from a relatively small population of stem cells. At the population level, self-renewal and differentiation are the possible outcomes of stem cell proliferation; overall, however, stem cells are quiescent if compared with their direct progeny. The recent discovery of a particularly quiescent, or dormant, subpopulation of hematopoietic stem cells (HSCs) raises a number of fundamental questions. As stem cell fate is influenced by the signals integrated by the stem cell niche, will dormant HSCs reside in specific dormant niches? Is the mechanism of dormancy common to multiple regenerating tissues or specific to the hematopoietic system? If cancer is maintained by a few cancer stem cells, do they also contain a subpopulation of dormant cells, and could this be exploited for therapeutic purposes?