Tissue fracture dynamics govern extreme plastic shape changes in a simple, early divergent animal

Tissue mechanics dictate shape and form in all animals, and is commonly regulated by genetics. Here, we have discovered a novel fracture-based mechanism by which epithelial tissues can exhibit extreme plastic shape changes in a simple, ancestral animal - the Trichoplax adhaerens. We found that adult animals are capable of continuous, real-time (~sec) shape changes exhibiting both solid-like (elastic) and liquid-like (plastic) tissue properties, with fluid-like patterns such as vortices and shear zones. We employ live imaging, novel bead-based tagging and engineering mechanics data analysis to quantitatively demonstrate how forces mainly govern tissue fractures. These animals employ ciliary-driven motility to generate global mechanical forces that facilitate organismal shape changes and induce local tissue stresses. When local stresses exceed the yield point, the tissue is surprisingly able to sustain physiological fractures without any detrimental effects to the animal’s health. These fractures either propagate to form larger holes or 'heal' rapidly (~hr) resulting in permanent shape change. We show that fracture dynamics plays a critical role in the entire life cycle of these animals, especially during asexual reproduction, where an animal ‘splits into two’ by binary fission.