Branching position is determined by tubule geometry and is consistent with the concentration profile of secreted diffusible inhibitor(s). Frequency maps 24 hours after induction of branching for (A) curved tubules, (B) bifurcated tubules, and immunofluorescence staining of actin (red) and nuclei (green) of (C) fractal trees. Branch sites in (C) are denoted by arrows; image stitched from multiple fields. Calculated concentration profiles of diffusible inhibitors for (D) curved tubules, (E) bifurcated tubules, and (F) fractal trees predict lowest local concentration of inhibitors where branching was found to be induced experimentally. Scale bars, 50 μm.