The plasma membrane flattens out to fuel cell-surface growth during drosophila cellularization

Cell-shape change demands cell-surface growth, but how growth is fueled and choreographed is stilldebated. Here we use cellularization, the first complete cytokinetic event in Drosophila embryos, to show that cleavage furrow ingression is kinetically coupled to the loss of surface microvilli. Wemodulate furrow kinetics with RNAi against theRho1-GTPase regulator slam and show that furrow ingression controls the rate of microvillar depletion. Finally, we directly track the microvillar membrane and see it move along the cell surface and into ingressing furrows, independent of endocytosis. Together, our results demonstrate that the kinetics of the ingressing furrow regulate the utilization of amicrovillar membrane reservoir. Because membranes of the furrow and microvilli are contiguous, we suggest that ingression drives unfolding of themicrovilli and incorporation of microvillar membrane into the furrow. We conclude that plasma membrane folding/unfolding can contribute to thecell-shape changes that promote embryonic morphogenesis.