Involvement of the BMP signalling in self-avoidance. (a) In the presence of the inhibitor of TGFβ-superfamily signalling, AlkiII, collision avoidance between co-cultured ureteric buds fails and branches make contact (arrowheads). In addition, branches become long and spindly. (b) Gremlin, a more specific inhibitor of signalling by BMPs, also causes collision avoidance between ureteric bud trees to fail, but the branch pattern is generally more normal than in AlkiII. (c) In some ureteric buds cultured in Gremlin, branches run almost parallel rather than diverging, and show inter-tree collisions. Anti-BMP7 also causes collision avoidance to fail between trees (d) and within trees (e). (f) Shows the frequency of collisions quantitatively: none of the inhibitors causes collisions in every case, but each is significantly different from the controls, in which collisions were never seen (p values are in the main text). (g) Shows the incidence of parallel branches in Gremlin-treated buds. This was the only treatment to produce this effect reliably. (h, i) show the Six2-positive caps (green) over ureteric bud tips (red) in control and AlkiII-treated kidneys respectively; (j) shows quantitatively what is apparent visually from (h, i); the drug makes no detectable difference to the thickness of this cap, so the ability of AlkiII-treated tips to collide does not result from disappearance of a Six2+ `fender'.