Approaching branches slow and avoid contact even in intact cultured kidneys. (a) shows an example frame from Additional file 2: Movie S1, a Hoxb7-cre x ROSA-eYFP kidney developing for a total of 6 days in culture (this construct causes the ureteric bud to fluoresce). (b) shows the speed of advance of branch tips that were not approaching other branches (the majority were of this type), at different times of culture. The mean speed is nearly constant over the culture, falling by only 12%. This is important for interpretation of panel (c), which plots the closing speed of tips that are approaching one another against their separation. There is an inverse relationship between approach speed and log of distance, suggesting that the closing speed of branches decreases, even to a stop, as separation decreases (82 observations; R = 0.67; significance = 4.8x10-12). The change is is much larger than the 12% reduction of scalar speed in (b), so cannot be accounted for simply by the culture ageing at the same time that tips approach other branches.