When an individual cell needs to move somewhere, it manages just fine on its own. It extends protrusions from its leading edge and retracts the trailing edge to scoot itself along, without having to worry about what the other cells around it are doing. But when cells are joined together in a sheet of tissue, or epithelium, they have to coordinate their movements with their neighbors. It’s like walking by yourself versus navigating a crowded room. To push through the crowd, you have to communicate with others by talking (“Pardon me”) or tapping them on the shoulder. Cells do the same thing, but instead of verbal cues and hand gestures, they use proteins to signal to each other.