Fig. 3

Structure of the linear invasion strands in vivo. a Live B16-F0 (non-metastatic) and B16-F10 (metastatic) tumors in mouse skin imaged with epifluorescence microscopy (cell nuclei marked with Histone-2B/mCherry, green). Images are representative of least six tumors from at least three independent mice per group. Insets show finger-like invasion strands moving away from the main tumor mass. Scale bar is 100 μm. b Enlarged images corresponding to the tips of the invasion strands. Few (~2–4) B16-F10 cells, so-called tip cells, detached from the invasion strands and exhibited trajectories that over limited observation time appeared ballistic (see also Fig. 4b, Zone 5). These tip cells were observed only in the very outer zone 5 which was not included in Lévy walk analysis because of small number of cells in this zone. Scale bar is 100 μm. Quantification of (c) tumor growth (quantification based on tumor volume and expressed as an increase over volume measured on day 1; time, days) and (d) invasion strand length on day 6. Invasion strands were statistically longer in metastatic tumors (average ~400 μm in B16-F10 vs. ~100 μm in B16-F0, red horizontal lines). Error bars in c are standard deviations based on ≥ 30 invasion strands from at least six tumors of three individual mice. e The widths of invasion strands near the base (i.e., beginning of strand at tumor’s edge), center between base and tip, and the tip region (recorded on day 6; error bars give standard deviations of ≥7 invasion strands from at least four tumors of three individual mice). f Individualized (with separation above two nuclear diameters) versus compact cell positioning in leading tips of invasion strands