Fig. 6. Wnt and Notch signalling in binary cell-fate decisions. (A) Pluri- or multipotent stem cell populations self-renew and differentiate. In the case of multipotent cells, stem cells are rare, self-renew infrequently and when they differentiate, they do so into an amplifying cell population, often called a transit-amplifying (TA) compartment. Ample evidence links Wnt signalling (red) to self-renewal and Notch signalling (blue) to TA cell population maintenance (see text and Table 1 for details). (B) A different mode of differentiation is characteristic of progenitor cells that can differentiate into one of two fates. Wnt and Notch signalling often act as permissive signals for either of the alternative fates. Under appropriate differentiation conditions, embryonic stem cells behave like progenitors and also have a binary option between neuroectoderm and endomesoderm (see Table 1). (C) In the vertebrate intestine, Wnt signalling promotes a stem cell fate, whereas Notch signalling is essential for the TA fate. TA cells can then differentiate into either of two fates, absorptive and secretory, that depend on Wnt or Notch signalling, respectively. It is possible that the activity of one pathway involves the repression of the other (see also Table 1).