Animal cells undergo dramatic changes in shape, mechanics and polarity as they progress through the different stages of cell division. These changes begin at mitotic entry, with cell-substrate adhesion remodelling, assembly of a cortical actomyosin network and osmotic swelling, which together enable cells to adopt a near spherical form even when growing in a crowded tissue environment. These shape changes, which probably aid spindle assembly and positioning, are then reversed at mitotic exit to restore the interphase cell morphology. Here, we discuss the dynamics, regulation and function of these processes, and how cell shape changes and sister chromatid segregation are coupled to ensure that the daughter cells generated through division receive their fair inheritance.

During gastrulation of the mouse embryo, individual cells ingress in an apparently stochastic pattern during the epithelial-to-mesenchymal transition (EMT). Here we define a critical role of the apical protein Crumbs2 (CRB2) in the gastrulation EMT. Static and live imaging show that ingressing cells in Crumbs2 mutant embryos become trapped at the primitive streak, where they continue to express the epiblast transcription factor SOX2 and retain thin E-cadherin-containing connections to the epiblast surface that trap them at the streak. CRB2 is distributed in a complex anisotropic pattern on apical cell edges, and the level of CRB2 on a cell edge is inversely correlated with the level of Myosin IIB. The data suggest that the distributions of CRB2 and Myosin IIB define which cells will ingress and we propose that cells with high apical CRB2 are basally extruded from the epiblast by neighboring cells with high levels of apical myosin.