Cells as liquid crystals, and what happens when they can't align

The organization of cells in tissues is crucial in determining their properties and functionalities. From the embryonic organization to mature tissues, cells are often arranged in well-determined patterns, which control the mechanical properties of the tissue and the ability to sense the environment. Many cell types have a characteristic anisotropic shape, and show an intrinsic ability to align with their neighbors. This behavior is analog to nematic liquid crystals (LCs). One special feature of LCs is the presence of topological defects, regions where the nematic order is lost and where stresses are concentrated in small regions. Monolayers of cells show alignment defects analog to LC topological defects. There is increasing evidence that such defects have a biological role, and their importance has been discussed in processes such as cell apoptosis, formation of 3D protrusions, growth of tumors stroma and cell migrations from tumors.
To better understand the role of these disordered regions in tissues, we induce topological defects in cell monolayers and study the cells behavior. Our goal is to connect biological properties of cells to the LC properties of the monolayers that the cells can form. We use topography to investigate the behavior of cells near induced topological defects, each characterized by their topological charge. Using shallow patterned grooves, we can impose defects with various charges without totally constraining the cells, and we study the cells’ behavior. Our data show a very different behavior of fibroblasts 3T6 and epithelial cells EpH-4 near topological defects with azimuthal or hyperbolic orientation of cells (+1 and -1 topological charge respectively), both in the ability to align and in the distribution of cell density within the monolayers. Finally, the behavior of cells near defects can provide a unique way to determine the size of the defect core.