Actomyosin cortex integration with complex plasma membrane topography in the early <i>Drosophila</i> embryo

Most animal cells display widespread plasma membrane (PM) folding. It is unclear how cortical tension is generated and controlled over cell surfaces with such PM topography. Our results highlight the early syncytial Drosophila embryo as a model of cortical actomyosin network integration with complex PM topography. Over the embryo surface, before arrival of peripheral nuclei, actomyosin networks entwine across a dense field of PM infoldings. Actomyosin network and PM topography changes are closely coupled during synchronous mitotic cycles and following experimental perturbations. Actomyosin activity is required for periods of condensed spacing between PM infoldings, when the integration of actomyosin networks and PM topography seems to form a tensile, composite material. These cyclic condensations are preceded by periods of expanded spacing between PM infoldings driven by Arp2/3 activity. Without Arp2/3 activity, the actomyosin cortex and PM topography gain an aberrant configuration, excessive tension is evident, and embryo surface distortions occur. Overall, PM topography seems integral to actomyosin cortex function and regulation.