Morphogenesis, the process by which tissues acquire their shape, hinges on a finely orchestrated collective motion of cells autonomously choreographing themselves to a well-defined final position. Accumulating evidence shows that many biological tissues behave as active nematics, both in vitro and in vivo. The collective motion of cells is controlled by the nematic order, and topological defects have been proposed as morphogenic organizers via active stresses. However, the generation and control of tissue-scale forces involved in morphogenesis remain poorly understood, in particular within 3D surfaces.

We aim to understand how geometry and topology controls the spontaneous organization of cells that drives morphogenesis, i.e. the growth from a 3D surface to tissues with complex shapes. To investigate this phenomenon, I grow cells on the surface of deformable capsules and monitor the nematic field, cellular flows, and tissue growth. Capsule shape can be altered, to control the local gaussian curvature and its anisotropy. Shell rigidity can be tuned, and forces inferred from the elastic deformations of the shell.

The nematic field is shown to depend on confinement and curvature. Confinement on a surface of finite area constrains the number of defects, while the topology of a surface dictates the total nematic charge, +2 in the case of our spherical capsules. Indeed, four equidistant +1/2 defects are observed in the actin network of a monolayer of C2C12 on a spherical capsule. Furthermore, the contribution of the anisotropy of curvature pushes defects from regions of high anisotropy towards the poles, as seen on the surface of ellipsoidal capsules. Subsequent growth of the monolayer shows the formation of multilayers with orthogonal orientation. The high long-range contractile stresses due to the nematic ordering leads to anisotropic folding of the capsule and complex morphogenetic-like events.