Cell packings and myosin network architectures underlie local tissue mechanical behaviors in the fly embryo

Epithelial tissue sheets play key roles in shaping the embryonic body form in part by possessing the ability to locally flow and remodel like a fluid or stretch and bend like a solid. Dramatic examples of both behaviors occur in the fly embryo. The ventral region of the embryo undergoes in-plane stretching and out-of-plane bending during ventral furrow formation just before the neighboring lateral region rapidly flows to double its length during body axis elongation. However, it is not understood how local tissue properties are regulated between these neighboring regions of the embryo to give rise to distinct solid-like and fluid-like tissue behaviors. Here, we develop a framework for analyzing regional cell packings and supracellular myosin network architectures in the fly embryonic epithelium during ventral furrow formation and body axis elongation. We find cell packing signatures for each morphogenetic event that coincide with spatiotemporal transitions in tissue mechanical behavior across the embryo. These cell packing signatures are associated with distinct features of local myosin network architecture. Our findings suggest that solid-fluid tissue mechanical behaviors are spatially and temporally regulated to sculpt the embryonic body form and will serve as a foundation for studying spatiotemporal patterns of tissue mechanics in developing embryos.