Dispersal is due to a chemoattractant present in serum.

<p>All panels show data from melanoma cells migrating in chemotaxis chambers as described <a href="http://www.plosbiology.org/article/info:doi/10.1371/journal.pbio.1001966#pbio.1001966-MuinonenMartin1" target="_blank">[15]</a>. (<b>A–B</b>) Cells migrate from conditioned medium towards fresh medium. WM1158 cells were randomly attached to a coverslip and assembled in a chamber in 48 hour WM1158 cell conditioned medium. The medium in one chamber was replaced with fresh medium, while the other was left alone. Tracks of individual cells are shown as coloured lines (A). Cells move towards the fresh medium, as shown by the spider plot (B) showing all cell tracks. (<b>C–D</b>) Example images showing WM239A metastatic melanoma cells after 21 hours in serum-free medium (C) and a 0%–10% FBS gradient (D). Coloured paths show centroid tracks from time 0. (<b>E</b>) Quantitative analysis of chemotactic responses. “Spider” plots (large panels), rose plots, mean chemotactic index, and Rayleigh test for directionality are shown for cells in serum-free medium and a 0%–10% FBS gradient (<i>n</i>>100 cells in three independent experiments for both conditions). Spider plots show strong chemotaxis in FBS gradients; in serum-free medium only random movement is seen. Rose plots show overall movement from 6–12 hours; the proportion of total cells in each sector is shown on a log scale, with red lines representing the 95% confidence interval. The majority of cells in the FBS gradient move in the direction of the chemoattractant. Rayleigh tests statistically confirmed this highly significant unimodal directionality. Graphs of chemotactic index were generated from the same data.