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Although chemosensing by individual cells is stochastic, multi-cellular organisms exhibit highly regulated response to stimulations. The key elements to understand orders in chemosensing are cellular communications and the existence of pacemakers. To study the collective behaviors in chemosensing induced by these two factors, we studied spatial-temporal calcium dynamics of fibroblast cells in response to ATP in microfluidic devices. We found gap junction communications led to faster, more synchronized, and correlated responses compared to isolated cells. We demonstrated the existence of pacemakers and how they dictated the initial responses in the presence of gap junctions. By further studying the calcium dynamics of cells embedded in a thin hydrogel film, where cellular communications were only through diffusing molecules, we conclude that gap junctions are essential to generate prompt …