Tissue fracture and healing dynamics govern extreme plastic shape changes in the Trichoplax adhaerens, a simple, early divergent animal

While epithelial tissues provide robust mechanical support, they also exhibit local ‘flows’ during morphogenesis and development. In adults, epithelial tissues undergo repeated stretching (e.g. lungs), and an inability to withstand stretch can cause fractures leading to diseases. Tissue fractures so far have been associated with negative consequences, and we do not yet know if fractures can be beneficial. Here, we have discovered a novel fracture-based mechanism by which epithelial tissues can exhibit fast and extreme plastic shape changes in a simple, early divergent animal - the Trichoplax adhaerens (phylum Placozoa). Fracture dynamics play a critical role in the lifecycle of Placozoans by dictating asexual reproduction by binary fission. Using innovative experimental in-toto imaging and tagging techniques, we quantitatively demonstrate that innate mechanical forces (shear and tension) arising from organismal motility govern tissue fracture dynamics. These physiological fractures can either enlarge or ‘heal’ rapidly – leading to dramatic tissue shape change. These tissues exemplify a novel paradigm in soft-active-living-matter since they ‘fluidize’ by creating transient, local, stochastic ‘soft zones’ that exhibit glassy dynamics and a yield stress behavior in living animals.