Figure 1.

Symmetric vs. asymmetric cell division in epithelial cells. Schematic depiction of a polarized dividing cell show two modes of cell division. During symmetrical division, spindle orientation and determinant protein (e.g., Numb) localization are not coordinated. Determinants segregate equally, giving rise to two equal (stem) cells. During asymmetric division, spindle orientation and determinant protein (e.g., Numb) localization are coordinated, giving rise to a differentiating cell and a stem cell. Thus, the difference in Numb (green) expression levels between two daughter cells mediates asymmetric cell division, whereas lack of Numb inheritance by both daughters will allow them to execute the stem cell self-renewal program by maintaining Notch1 activity and thus allowing symmetric cell division, as we reported in distal lung epithelial stem cells (El-Hashash and Warburton, 2012).