Epithelial cell monolayers expand on substrates by forming finger-like protrusions, created by leader cells, in the monolayer boundary. Information transmission and communication between individual entities in the cohesive collective lead to long-range order, vortical structures, and disorder to ordered phase transitions. We ask the following questions: what makes a leader? What is the role of followers in leader cell formation? We used a particle-based model to simulate epithelial cell migrations on substrates of 9.4 kPa, 21 kPa and 33 kPa stiffness. The dynamics of cellular motion in the ensemble are governed by orientational Vicsek and inter-cellular interactions between neighboring particles. The model also includes bending, curvature-based motility, and acto-myosin contractile cable forces on the contour, in addition to density dependent noise and cell proliferations. We show that border forces are essential in the leader cell formation and the overall areal expansions of epithelial monolayers on substrates. Radial velocities and areal expansions of the monolayer agree with experiments reported for epithelial cells on substrates of varied stiffness. Ordering in follower cells within a specific region of the monolayer was apparent on substrates of higher stiffness and occurred prior to the emergence of leader cells. We demonstrate that regions of increased cell density occur behind the leader cell edge on all three substrates. Finally, we assessed the role of cell divisions on the ordering of velocities in the monolayer. These results demonstrate that monolayer heterogeneities, caused by density instabilities in the interior regions, correlate with leader cell formation during epithelial migrations.
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