Fig. 10. Models of fate determination of the endodermal epithelium and invagination. (A) Notch1 activation instructs endodermal epithelial cells to immature glandular progenitor fate. Remaining epithelial cell takes luminal fate under the influence of luminal inducers. Until Notch1 signaling is inactivated, progenitor glandular cells remain immature. As Notch signaling stops, immature glandular cells undergo gland cell differentiation. (B) Until stage 28, epithelial cells are homogenous. Slightly before stage 29 (stage 29–), when Delta1 expression starts, luminal differentiation and specification of glandular progenitors commence. From stage 29 onwards, as Notch1 signaling is gradually inactivated, glandular progenitors undergo differentiation and invaginate into mesenchyme. Luminal inducer(s) compete with Notch pathway and decrease number of gland progenitor.