Alignment of vascular endothelial cells as a collective response to shear flow

When cultured endothelial cells (ECs) are exposed to shear flow, initially cobblestone-like ECs spontaneously elongate and align along the flow direction, acquiring a similar architecture to that of native endothelium in blood vessels. Though previous works have revealed how individual cells sense and respond to shear flow, little is known about the contribution of cell–cell interaction to this phenomenon. Here, we reveal that the response of ECs to shear flow is cell density-dependent, and therefore possibly mediated by collective cell behavior (i.e. cell–cell interaction). A cell density threshold was identified, below which ECs never formed an aligned structure by shear flow exposure. This threshold cell density corresponded to the two-dimensional percolation threshold, suggesting that the aligned structure formation requires a connecting network of ECs. We also observed an optimum cell density at which both elongation and alignment were completed within a minimum time period. A possible mechanism of the EC response to applied shear flow is discussed.