Fig. 4. High-resolution dynamics of protrusive activity. Movie 4 (see http://dev.biologists.org/supplemental) shows the extension and retraction of cellular protrusions at high magnification over a 104 minute time interval beginning 2 hours and 10 minutes into the recording. (A) Three time points from Movie 4. The left column shows QH1 experimentally labeled PECs, the middle column depicts the corresponding DIC images and the right column is a composite of the two. Red and yellow asterisks denote the relatively fixed position of two neighboring endothelial cell clusters – the two asterisks do not move substantially during the 26 minutes. By contrast, the corresponding red and yellow arrowheads indicate the presumed tips of two cell protrusions – note that these protrusions converge over the 26 minute interval. Their contact at 182 minutes is marked by a red outline on the image denoted by a black asterisk. The corresponding composite image, on the right, demonstrates that the DIC feature outlined in red is labeled with the QH1 antibody. (B) The post-experimental labeling of the same field at 12 hours and 7 minutes (727'). The left image is the experimental QH1 labeled cells, while the middle frame displays the post-experimental QH1 vasculature. After 12 hours, the original features in A (asterisks) have moved slightly, but are traceable to the positions of the red circles (see Movie 4). The green arrowhead in the middle frame points out the continuity of contact between the endothelial cell clusters. The frame on the right shows the combined fluorescence from QH1 injected 12 hours earlier in red (or yellow), and the post-fixation QH1 fluorescence labeling in green (or yellow), yellow indicates positions of overlap. Scale bar: 225 µm. The time of image acquisition is shown in the lower corner of each epifluorescence frame.