Figure 4

Dynamic skeletons. A. Two consecutive images of a chemotaxing Dictyostelium cell; A₂ shows the latter frame. The images are 30 seconds apart. B. Cell boundaries and their respective pruned skeletons: S _n for the earlier frame and S_n+1for the latter one. C. By comparing the regions in the two images, a difference map is obtained describing the deformation of the cell from one image to the other. White regions () are growing, while black regions () are withdrawing. D. Branches of the skeleton in the latter frame (red lines) signal protrusion activities if they are pointing at regions in , and branches of the skeleton in the earlier frame (blue lines) signal retractions if they are pointing at regions in . The starting positions of these activities are decided by the points where S_n+1or S _n intersects the boundary curve of the earlier frame (red dots for protrusions and blue dots for retractions). The size of an activity is calculated as the length of the part of associated branch that resides in the growing (L₁ in D₁) or withdrawing (L₂ in D₂) area. E. The relative angle of a protrusion (θ₃) or retraction (θ₄) is defined when the cell is moving in response to a gradient of chemoattractant, based on the center of the cell in the earlier frame (the black dot) and the starting point of the activity (red dot for protrusion and blue dot for retraction).