Fig. 4. Symmetric and asymmetric divisions of stem or progenitor cells. (A) Stem or progenitor cells can divide symmetrically and generate either two stem or progenitor cells (expansion), or two differentiated cells (depletion). Alternatively, a stem or progenitor cell can divide asymmetrically and generate one stem or progenitor cell (self-renewal) and one differentiated cell (differentiation). Asymmetric cell division is the predominant type of stem or progenitor cell division during adult mammalian tissue homeostasis. (B) Mechanisms of normal asymmetric cell division and its potential perturbation in cancer. The stem or progenitor cells occupy specific positions within tissues that are called niches and are defined by the presence of the particular neighboring cells and cell-cell and/or cell-substratum adhesion structures. The mother cell asymmetrically localizes the Par3-Par6-aPKC complex (red) and cell fate determinants (green), and orients the mitotic spindle in such a way that basal cell fate determinants are inherited by only one daughter. Segregation of cell-fate determinants to one daughter assures that this cell acquires a fate different to that of the mother cell. Asymmetric cell division may fail because of an inability to asymmetrically localize cell fate determinants (Drosophila lgl, dlg, scribl, pros, brat and mira mutants), an inability to correctly orient the mitotic spindle (Drosophila pins, mud mutants) or both of these defects (Drosophila polo, aurA mutants).