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Abstract Collective migration of epithelial cells drives diverse tissue remodeling processes. In many cases, a free tissue edge polarizes the cells to promote directed motion, but how edge-free or closed epithelia initiate migration remains unclear. Here, we show that the rotational migration of follicular epithelial cells in theDrosophilaegg chamber is a self-organizing process. Combining experiments and theoretical modeling, we identify a positive feedback loop in which the mechanosensitive behavior of the atypical cadherin Fat2 synergizes with the rigid-body dynamics of the egg chamber to both initiate and sustain rotation. Mechanical constraints arising from cell–cell interactions and tissue geometry further align this motion around the egg chamber’s anterior–posterior axis. Our findings reveal a biophysical mechanism — combining Fat2-mediated velocity–polarity alignment, rigid-body dynamics, and tissue geometry — by which a closed epithelial tissue self-organizes into persistent, large-scale rotational migrationin vivo, expanding current flocking theories. 

Abstract Coherent structures—flow features that organize material transport and deformation—are central to analyzing complex flows in fluids, plasmas, and active matter. Yet, identifying such structures on dynamic surfaces remains an open challenge, limiting their application to many living and synthetic systems. Here, we introduce a geometric framework to extract Lagrangian and Eulerian coherent structures from velocity data on arbitrarily shaped, time-evolving surfaces. Our method operates directly on triangulated meshes, avoiding global parametrizations while preserving objectivity and robustness to noise. Applying this framework to active nematic vesicles, collectively migrating epithelial spheroids, and beating zebrafish hearts, we uncover hidden transport barriers and Lagrangian deformation patterns—such as dynamic attractors, repellers, isotropic and anisotropic strain—missed by conventional Eulerian analyses. This approach offers a new perspective on soft and living matter, revealing how geometry and activity can be harnessed to program synthetic materials, and how Lagrangian strain and principal deformation directions can help uncover mechanosensitive processes and directional cues in morphogenesis. 

Abstract Embryonic tissues undergo coordinated flows during avian gastrulation to establish the body plan. Here, we elucidate how the interplay between embryonic and extraembryonic tissues affects the chick embryo’s size and shape. These two distinct geometric changes are each associated with dynamic curves across which trajectories separate (kinematic repellers). Through physical modeling and experimental manipulations of both embryonic and extraembryonic tissues, we selectively eliminate either or both repellers in model and experiments, revealing their mechanistic origins. We find that embryo size is affected by the competition between extraembryonic epiboly and embryonic myosin-driven contraction—which persists when mesoderm induction is blocked. Instead, the characteristic shape 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 independent control of embryo size and shape during development. 

Gastrulation is an essential process in the early embryonic development of all higher animals. During gastrulation, the three embryonic germ layers, the ectoderm, mesoderm and endoderm, form and move to their correct positions in the developing embryo. This process requires the integration of cell division, differentiation and movement of thousands of cells. These cell behaviours are coordinated through shortand long-range signalling and must involve feedback to execute gastrulation in a reproducible and robust manner. Mechanosensitive signalling pathways and processes are being uncovered, revealing that shortand long-range mechanical stresses integrate cell behaviours at the tissue and organism scale. Because the interactions between cell behaviours, signalling and feedback are complex, combining experimental and modelling approaches is necessary to elucidate the regulatory mechanisms that drive development. We highlight how recent experimental and theoretical studies provided key insights into mechanical feedback that coordinates relevant cell behaviours at the organism scale during gastrulation. We outline advances in modelling the mechanochemical processes controlling primitive streak formation in the early avian embryo and discuss future developments.