Investigating the density variation of fibroblasts near topological defects

Fibroblasts in dense populations have long-range nematic order. They also display topological defects, disordered regions within the ordered fluid. We grow 3T6 fibroblasts in monolayers on poly-dimethyl-siloxane patterned with ridges with heights ranging from 1.5 to 14 microns. These ridges are designed to guide the cells into forming topological defects with azimuthal circular alignment and hyperbolic alignment at controlled locations, allowing us to study how they impact the behavior of the fibroblasts. Near circular defects the cells reach a higher density than the rest of the monolayer while near the hyperbolic defects the density remains low. We investigate the role of collective migration in this behavior by varying the height of the topographic features and studying their resulting distributions and dynamics. By increasing the height of ridges, we decrease the probability of cells crossing ridges, limiting their ability to migrate collectively. However, we find that even on substrates with 14um ridges, taller than the cells, the locally increased density near circular defects is not suppressed, but rather enhanced. This finding supports another proposed mechanism for this density variation, which is due to a difference in the cell proliferation near defects.