Transcription factors collaborate with epigenetic regulators to maintain undifferentiated stem cells. The polycomb family chromatin regulator, Bmi-1, is required for the maintenance of postnatal stem cells in multiple tissues, including the hematopoietic and nervous systems, but not for the proliferation of most restricted progenitors in the same tissues (Lessard and Sauvageau, 2003, Molofsky et al., 2003 and Park et al., 2003). The trithorax protein Mll is required for the maintenance

of HSCs, but not for the proliferation of restricted myeloid and lymphoid progenitors (Jude et al., 2007 and McMahon et al., 2007). Mll is also required for Selleckchem NSC 683864 neurogenesis by CNS stem cells, but not for gliogenesis (Lim et al., 2009). Differences between stem cell self-renewal and restricted progenitor proliferation are not absolute, as some restricted progenitors, such as lymphoid

progenitors and cerebellar granule precursor cells, also depend on Bmi-1 for their proliferation (Leung et al., 2004 and van der Lugt et al., 1994). Nonetheless, these transcriptional and epigenetic mechanisms do not generically regulate the proliferation of all cells, even when the mechanisms www.selleckchem.com/products/SRT1720.html are widely conserved among stem cells in multiple tissues. Cell-cycle regulation also distinguishes stem cells from restricted progenitors in the same tissues. In some adult tissues, the stem cells are quiescent most of the time, whereas most restricted progenitors divide more frequently. A good example is the hematopoietic

system, wherein only a few percent of HSCs are in cycle at any one time (Kiel et al., 2007) and a subset of HSCs divide only once every few months (Foudi et al., 2009 and Wilson et al., 2008). Although most restricted hematopoietic progenitors divide much more frequently, there are some restricted hematopoietic progenitors, including lymphoid progenitors (Pelayo et al., 2006), that can reversibly enter aminophylline and exit the cell cycle over long periods of time, much like HSCs. As a consequence, bromo-deoxyuridine label retention is not a sensitive or specific marker of HSCs (Kiel et al., 2007) but can be used in concert with other HSC markers to identify a slowly dividing subset of HSCs (Foudi et al., 2009 and Wilson et al., 2008). There is also evidence that some adult neural stem cells (Doetsch et al., 1999, Morshead et al., 1994 and Pastrana et al., 2009) and hair follicle stem cells (Blanpain et al., 2004, Cotsarelis et al., 1990 and Tumbar et al., 2004) are quiescent much of the time. However, quiescence is not a defining feature of stem cells, because stem cells in each of these tissues divide rapidly during fetal development (Lechler and Fuchs, 2005, Morrison et al., 1995 and Takahashi et al.