End-on imaging: a new perspective on dorsoventral development in Drosophila embryos

Drosophila ventral furrow formation has frequently been used as a model to study developmentally-regulated cell-shape changes. However, a technique to follow all cellular changes during this process within a single living embryo has been lacking. We describe a novel technique, called "end-on imaging", to collect time-lapse images of transversely mounted living embryos. End-on imaging revealed several new features of dorsoventral development. First, we observed a wave of syncytial nuclear divisions predicting the location of the ventral furrow. Second, we determined that there is a 5-min gap between the end of cellularization and the start of ventral furrow formation, suggesting that the two processes may share the same pool of cytoskeletal components. Lastly, we show that apical-membrane flattening, the first step in ventral furrow formation, is due to the ventral cells pushing against the vitelline membrane, rather than flattening the dome-shaped, apical surfaces of these cells by a pulling or constriction motion.