Expression of the proneural bHLH transcription factor Ascl2

is associated with stemness and is absolutely required for intestinal stem cell maintenance. Active Notch is required for Ascl2 expression and its loss results in precocious crypt cell differentiation [8 and 10]. The proneural protein Atoh1 acts as a master regulator of fate specification Birinapant of the secretory lineage [2 and 11]. Ascl2 expression is maintained by active Notch signalling that also acts to suppress Atoh1. Expression of Atoh1 is cell-autonomously inhibited by Hes proteins and in the absence of Notch signalling, crypt stem cells precociously differentiate into secretory goblet cells [7 and 12]. The spatial organisation of cells expressing Notch ligand and receptor in the crypt evokes a classic lateral inhibition scenario for control of stem versus secretory fate (Figure 2). Stem cells towards the crypt base found preferentially adjacent to Delta-expressing Paneth cells, express Notch receptor [13• and 14], signaling pathway and are maintained in an undifferentiated state by constant Notch signalling

and suppression of Atoh1 [7, 9, 15 and 16], As migrating cells lose contact with Paneth cells and the high Notch signalling they confer, they become poised between secretory and non-secretory fate. Lineage selection may then arise by stochastic variation in Delta expression leading some cells to express higher levels than others. This initial stochastic imbalance in Delta expression becomes reinforced allowing only a subset of cells (Delta high, Atoh high) rising up the crypt to become committed to a secretory fate while the rest become absorptive enterocytes. This regulation and functional organisation readily explains a binary fate in a supra-Paneth cell poised population but fits less well with a subsequent downstream cascade of secretory lineage choices specified after a series of cell divisions each progressing unidirectionally towards a more restricted fate. Moreover, recent evidence derived from regenerating systems casts doubt both on the existence of stable populations of

progenitors and the irreversibility of lineage specification. For many years it has also been known Metalloexopeptidase that intestinal regeneration following damage is not solely a function of surviving stem cells expanding to restore homeostasis (Figure 3) [17]. Following radiation induced injury the clonogenic fraction of crypt cells is elevated suggesting that these might correspond to the abundant and immature absorptive cells present within the early transit-amplifying compartment of the lower crypt. In support, specific ablation of the key Lgr5+ population using targeted diptheria toxin is not catastrophic as non-Lgr5+ cells (Bmi1+) cells are able to act as a replacement stem cell pool at least for a limited time [18].