Control of Tissue Flows and Embryo Geometry in Avian Gastrulation

Embryonic tissues undergo coordinated flows during avian gastrulation to establish the body plan. Here, we elucidate how the interplaybetween embryonic and extraembryonic tissues affects the chick embryo’s size and shape. These two distinct geometric changes areeach associated with dynamic curves across which trajectories separate (kinematic repellers). Through physical modeling andexperimental manipulations of both embryonic and extraembryonic tissues, we selectively eliminate either or both repellers in modeland experiments, revealing their mechanistic origins. We find that embryo size is affected by the competition between extraembryonicepiboly and embryonic myosin-driven contraction—which persists when mesoderm induction is blocked. Instead, the characteristicshape change from circular to pear-shaped arises from myosin-driven cell intercalations in the mesendoderm, irrespective of epiboly.These findings elucidate modular mechanisms controlling avian gastrulation flows and provide a mechanistic basis for the independentcontrol of embryo size and shape during development.