Abstract
In animals, epithelial tissues mainly provide a protective barrier function, but these tissues are also subjected to mechanical forces during physiological activities such as locomotion. Studying the response of tissues subjected to dynamic forces is an important area of research. Here, we investigate the mechanics of epithelial tissues in a marine animal, the Trichoplax adhaerens, that lacks both muscles and neurons. It has a simple body plan consisting of two epithelial layers with ciliated cells enclosing a layer of fiber cells and no organ system. These asexually reproducing organisms do not possess a fixed shape and the cells are in contact with each other by means of adherens junctions. Recently, it has been demonstrated that mechanical forces due to their own motility give rise to localized ventral and dorsal fractures at fast loading time scales. These tissue fractures lead to formation of holes, some of which heal over time while others break into long strings that eventually rupture giving rise to daughter organisms. In silico models of these tissues have revealed a transition from ductile to brittle properties. The mechanical driving forces underlying these phenomena are thought to originate from the interaction of the animal’s lower epithelium with the substrate. The lower epithelium consists of cells with cilia that exert a transient adhesion, which allows the organism to walk over a substrate. However, the traction forces generated by the ciliary motion have not yet been measured. Here, we fill in this gap by utilizing the traction force microscopy (TFM) technique. TFM has been extensively used to study traction forces of single cells and during collective cell migration. The TFM technique involves microscopic timelapse imaging to quantify the displacement field due to deformation of a soft elastic substrate by the ciliary adhesions to compute traction forces. Here, we show that TFM can successfully measure traction forces in this animal, and we also demonstrate the application of monolayer stress microscopy to calculate the principal stresses.