Concept of autonomous cell movement. (a) Each cell has two states, in stay and in move, for cell movement. A cell in stay is activated by signals from different cell type, and begins to move, changing its state into in move-state. The cell in move continues to move for short periods and then ceases from moving and returns to in stay. (b) Idea of activation mechanism: The signals from different cell types increase an inner state of a cell in stay. If the inner state of the cell exceeds a threshold, the cell in stay is activated and becomes in move-state.