Chemotaxis followed by Retrotaxis in "U"-Shaped Channels. Neutrophil-like HL-60 cells with Hoechst-stained nuclei migrate along the arms of the U-shaped channels in a chemoattractant gradient (fMLP, 100nM). After passing the tip of the U , which has the highest concentration of fMLP, neutrophil continue to migrate along the channel, in the opposite direction of the gradients. The gradient is visualized using a 3kD dextran-conjugated dye (shown in red): chemoattractant levels are highest at the tip of the U and diffuse outward towards the cell loading channel (not shown). Some bleaching of the fluorescent molecule occurs during the movie.