We have discussed the importance of asymmetric division in generating cell diversity during development and in maintaining the number of stem cells in a population. What mechanisms underlie the ability of cells to become asymmetric before cell division to give rise to cells with different fates? Cell asymmetry is a concept we have met before, under the name of cell polarity, so let us first review what it means for a cell to be polarized.

Cell polarity—the ability of cells to organize their internal structure, resulting in changes in cell shape and the generation of regions of the plasma membrane with different protein and lipid compositions—has been introduced in several chapters. For example, we have seen that polarized intestinal epithelial cells have an apical domain with abundant microvilli separated from the basolateral domain by tight junctions (see Figures 17-1 and 20-11). Epithelial transport requires these cells to have different transport proteins in the apical and basolateral membranes (see Figure 11-30). As we will see later in this section, these epithelial cells are responding to extracellular signals that instruct them how to polarize. These cells represent just one example of cell polarity—essentially all cells in animals are polarized, and we discuss several examples in which the underlying mechanisms have been defined. What emerges are three core principles of cell polarity. First, cells have an intrinsic polarity program, as revealed by their remarkable ability to polarize in the absence of external cues. As we will see in our examples, a master and common regulator of this program is the small GTPase Cdc42. Second, this intrinsic polarity program can be directed by external or internal cues. Third, the polarity of individual cells is often maintained by intracellular mutually antagonistic complexes. We first discuss the intrinsic polarity program in budding yeast because, given that all the components of the mechanism are shared with animals, the principles uncovered in yeast are likely to be conserved. We then turn our discussion to examples in which cells respond to external cues to establish cell polarity depending on antagonistic interactions. Finally, we discuss an example of asymmetric cell division that gives rise to a daughter stem cell and a differentiated cell.