Tuning cell shape change with contractile ratchets.

Throughout the lifespan of an organism, shape changes are necessary for cells to carry out their essential functions. Nowhere is this more dramatic than embryonic development and gastrulation, when cell shape changes drive large-scale rearrangements in tissue architecture to establish the body plan of the organism. A longstanding question for both cell and developmental biologists has been how are forces generated to change cell shape? Recent studies in both cell culture and developing embryos have combined live imaging, computational analysis, genetics, and biophysics to identify ratchet-like behaviors in actomyosin networks that operate to incrementally change cell shape, drive cell movement, and deform tissues. Our analysis of several cell shape changes leads us to propose four regulatory modules associated with ratchet-like deformations that are tuned to generate diverse cell behaviors, coordinating cell shape change across a tissue.