In traditional linear hierarchy models, self-renewing PSCs residing in the basal cell (BC) layer give rise to transit-amplifying cells (TACs) of intermediate phenotypes that may express both basal and luminal cell markers during their maturation. These cells theoretically possess transient self-renewal activity and produce large numbers of terminally differentiated secretory luminal cells (LCs). In bifurcated models, basal cells and luminal cells represent separate epithelial cell lineages. These lineages may be sustained by intermediate transit-amplifying cells and/or lineage-restricted basal and luminal cell progenitors (BP and LP, respectively).