Separation anxiety: stress, tension and cytokinesis

Abstract

Cytokinesis, the physical separation of a mother cell into two daughter cells, progresses through a series of well-defined changes in morphology. These changes involve distinct biochemical and mechanical processes. Here, we review the mechanical features of cells during cytokinesis, discussing both the material properties as well as sources of stresses, both active and passive, which lead to the observed changes in morphology. We also describe a mechanosensory feedback control system that regulates protein localization and shape progression during cytokinesis.

Figure 1. Mechanical properties during different phases of cytokinesis

A. Cell progression through cytokinesis can be characterized by a number of distinct morphological changes that correspond to different mechanical phases. After rounding, Phase 1 describes the change from a spherical to a cylindrical cell. Phase 2 is characterized by furrow ingression. Phase 3 is characterized by the thin bridge connecting the daughter cells. B. During cytokinesis, proteins such as myosin II and cortexillin localize to the cleavage furrow, while others are found globally or more concentrated at the poles. These localizations give rise to spatially dependent mechanical properties. Also shown are the different stresses (radial, σ_rr, and compressive, σ_zz) acting during cytokinesis as well as the spatially dependent cortical tensions (T_f and T_d at the furrow and daughter cells, respectively) which, when coupled to the local curvatures (arising from radii R_f and R_d) give rise to local Laplace-like pressures that help drive cytokinesis. C. A viscoelastic model of the cell encompasses a mostly viscous cytoplasm and a cortex that, though primarily elastic, also includes a viscous component [20]. D. The diagram depicts the Laplace pressure generated at the curved interface between fluid surfaces.

Figure 2. A mechanosensitive system regulates cytokinesis

A. During cytokinesis, signals believed to come from the spindle direct cortical proteins such as a myosin II or cortexillin to the future site of the cleavage furrow. B. External stresses, such as those imposed by a micropipette aspirator can cause changes in morphology. C. Stresses in the actin can give rise to cooperative recruitment of myosin II/cortexillin to the site of stress [13, 48]. D. Recruited myosin II works against the external stress enabling the cell to exit the micropipette (E) at which point normal division can proceed (F).