Evidence for self-avoidance in the developing tubule trees of real cultured kidneys. Single ureteric buds, isolated, surrounded by mesenchyme and cultured, generate reniform trees in both the model (a) and reality (d). Pairs of ureteric buds cultured on collision courses with one another are predicted by the model to produce trees that are distorted but that avoid collision (b): this does indeed happen in reality (e). Where three ureteric buds are aimed towards one another, the model (c) and real cultures (f) generate straight `no-man-s land', tip-free zones between them: these can be seen between the arrows in (f). Additional file 11: Figure S4f shows the same image false-coloured to indicate more clearly which branches belong to which tree. Ureteric buds trees are stained with anti-calbindinD28k. Scale bars are 100 μm.