Such cell sheets are typically produced by allowing cells to come to confluence within a stencil or patterned substrate to form a monolayer with a desired geometry 34, 35. Specifically, we sought to harness mechanical tissue interactions in the context of both tissue biophysics and cell-sheet engineering, where the latter aims to produce and harvest intact cell monolayers to create scaffold-free, high-density tissues 24. Here, we sought to broadly investigate questions such as: how do tissues of different shapes interact with each other, and what happens when many tissues simultaneously interact? Our goal was to then develop these fundamental concepts into broad “design principles” for assembling composite tissues in a controlled way. Furthermore, colliding monolayers with differences in Ras gene expression were able to displace one another 31, 32, while epithelial tissue boundaries were found to induce waves of cell deformation and traction long after the tissues had collided 33. For instance, the interplay between repulsive Eph/ephrin and adhesive cadherin cell–cell interactions regulate tissue boundary roughness, stability, and cell fate 26, 27, 28, 29, 30. Thus, recent research has focused on the formation and dynamics of tissue–tissue boundaries. In particular, tissue–tissue interfaces underlie both biological processes such as organ separation and compartmentalization 18, 19, as well as biomedical applications such as tissue-mimetic materials 20, 21, 22 and engineered tissue constructs 23, 24, 25. In places where tissues meet, the resulting tissue is a living composite material whose properties depend on its constituent tissues. Indeed, cell–cell interactions give rise to behaviors such as contact inhibition 2, 3, 4, collective cell migration 5, 6, and cell-cycle regulation 7, 8, 9, 10, which underlie physiological functions such as tissue development and healing 11, 12, organ size control 13, 14, morphogenetic patterning 15, and even pathological processes such as tumor invasion 16, 17. This concept is increasingly apropos as interdisciplinary research pushes deep into the coordinated cell behaviors underlying even “simple” tissues. Similar content being viewed by othersĪ biological tissue is a cellular community or, as Virchow wrote in the 19th century 1, “a cell state in which every cell is a citizen”. Overall, our work provides insight into the mechanics of tissue collisions, harnessing them to engineer tissue composites as designable living materials. Finally, we introduce TissEllate, a design tool for self-assembling complex tessellations from arrays of many tissues, and we use cell sheet engineering techniques to transfer these composite tissues like cellular films. We present a physical model of tissue interactions that allows us to estimate the bulk modulus of the tissues from collision dynamics. Next, we propose that genetically identical tissues displace each other based on pressure gradients, which are directly linked to gradients in cell density. First, we determine rules for tissue shape changes during binary collisions and describe complex cell migration at tri-tissue boundaries. Here, we studied collisions between monolayer tissues with different geometries, cell densities, and cell types. While cell-cell interactions have been intensely investigated, less is known about tissue-tissue interactions. Tissues do not exist in isolation-they interact with other tissues within and across organs.
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