Figure 1.
A model of the cellular hierarchies that may exist in the epithelium of the human breast lobule. Cells in an intermediate position include Hoechst dye‐effluxing SP cells and some of these express steroid receptors, p21CIP1, Msi1 and CK19. Msi1 and Delta/Notch signalling may regulate self‐renewal of the stem cell and production of the transit‐amplifying population through a process of asymmetric cell division. After a small number of cell divisions, transit‐amplifying cells exit from the cell cycle and differentiate into myo‐ or luminal epithelial cell lineages characterized by the markers CALLA and CK14 (myoepithelial) or MUC1 and CK18 (luminal).