Cells can polarize in response to external signals, such as chemical gradients, cell-cell contacts, and electromagnetic fields. However, cells can also polarize in the absence of an external cue. For example, a motile cell that initially has a more or less round shape can lose its symmetry spontaneously even in a homogeneous environment and start moving in random directions. One of the principal determinants of cell polarity is the cortical actin network that underlies the plasma membrane.

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... on in this network generated by myosin motors can be relaxed by rupture of the shell, leading to polarization. In this chapter, we discuss how simplified model systems can help us to understand the physics that underlies the mechanics of symmetry breaking.