Embryonic area homeostasis in avian gastrulation

In the early chick embryo, tens of thousands of cells in a single layer undergo large-scale vortical movements, breaking an initial circular symmetry to set the bilateral symmetry of the body plan. As they recirculate, cells in the embryo divide at uniform rates, but primarily ingress along a midline called the primitive streak. Strikingly, the embryo's area remains nearly constant throughout. Its cells become more tightly packed even as surrounding extraembryonic cells are stretched by outwardly crawling edge cells. We propose a self-organizing mechanochemical model wherein mechanosensitive contractile actomyosin activity in the embryo resists expansion to keep the embryo's size constant. Our model reproduces the observed dynamics of cell number densities, embryo shape change, and area homeostasis in wild type and perturbations obtained by mechanically confining the embryo and by inhibiting contractile activity. Together with our mechanochemical model for self-organized tissue flows and active stress patterns, we can now also explain embryo shape change and area homeostasis during chick gastrulation. Such self-regulating processes are ubiquitous in living systems and can be critical for robust development and the control of synthetic morphogenesis.