An organism's shape depends on morphogenetic cell and tissue movement, but how do individual cells contribute to tissue behaviour? To find out,Gorfinkiel, Martinez Arias and colleagues have quantitatively analysed cell-shape changes during Drosophila dorsal closure (DC) (see p. 1889). Halfway through embryogenesis, the embryo's epidermis has a gap that is covered by an extraembryonic tissue, the amnioserosa (AS). During DC, concerted tissue movements of the epidermis and the AS close this gap. Using a novel image-analysis method, the authors track the cell-shape changes of each individual AS cell (ASC) during DC and quantify them in normal embryos and in embryos in which apoptosis, microtubule dynamics or adhesion is disturbed in the AS and/or epidermis. In all cases, they observe spatial and temporal differences in ASC deformation and a correlation between ASC deformation rates and the zippering speed of the two epidermal sheets that close the gap. They suggest, therefore, that mechanical constraints from tissues outside the AS can modulate AS cell-shape changes and thus collective tissue behaviour.