Geometry-driven self-organization of migrating cells and chiral filaments

Biological systems, such as cytoskeletal filaments and multicellular collectives, exhibit rich self-organization driven by their intrinsic activity and physical constraints. Across scales, geometry plays a central role: the shape of active units and the structure of their environment strongly influence how order emerges in these far-from-equilibrium systems. In this thesis, we combine minimal active-matter models with bottom-up in vitro experiments to identify the principles by which geometry and activity shape collective behavior in biological matter. At the subcellular scale, we investigate how the physical properties of chiral FtsZ filaments govern their large-scale organization. Using a mesoscopic model validated by reconstituted filament assays, we show that filament shape, intrinsic curvature, and flexibility, together with activity-driven straightening, determine whether filaments form chiral rings, polar bands, or nematic-like states. Our model shows that small changes in filament mechanics produce large shifts in collective structures, revealing how the geometry of individual filaments amplifies through activity to control patterns relevant for bacterial cytokinesis. At the cellular scale, we examine how geometry of the external environment influences collective cancer cell migration. Experiments in controlled microfabricated obstacle environments demonstrate that geometric disorder alone can induce a transition from collective to single-cell invasion. A minimal model of active particles in disordered geometries reproduces these effects, demonstrating that cell dissemination results both from local geometrical constraints and a global integration of spatial disorder over time. Surprisingly, this transition is accompanied by a change in the universality class of interface dynamics, revealing that cell-environment interactions can shift the collective behavior of an invasive front purely through mechanics. In the last system, we study how cell-intrinsic geometry and chirality affect collective motion in confined and unconfined settings. Using a minimal model inspired by in vitro experiments, we analyze motile cells that exhibit persistent rotation due to intrinsic left-right asymmetry. In circular confinement, this simplified chiral model reproduces several experimentally observed features within specific parameter regimes. At high densities in unconfined environments, however, intrinsic chirality becomes masked even before the onset of the jamming transition. These results show that single-cell geometric asymmetry does not necessarily translate into large-scale chiral flows, offering new insight into how forces and symmetries shape collective motion in dense cellular systems. Overall, this work uncovers how geometric constraints – embedded in the intrinsic architecture of an active filament or encoded in the complexity of a microenvironment couple to active forces to generate different classes of complex behaviors.