Fig. 3. Blocking the Erk MAP kinase pathway using PD98059 inhibits ureteric bud branching. (A) The ureteric buds of E11.5 kidney rudiments cultured for 72 hours in normal medium branch extensively in an organotypic manner. (B,C) Inclusion of PD98059 (15 µM), an inhibitor of Mek-activation of Erk1/2 MAP kinases, greatly decreases the number of branches produced by the ureteric bud although some elongation of existing branches continues. (D) When kidney rudiments are grown for 40 hours in PD98059 (10 µM) and then transferred to normal medium for a further 48 hours, the central, oldest parts of the ureteric bud tree show sparse branching but the outer (newest) parts show a more normal branching density, suggesting that the cells recover from PD98059 treatment. (E) A dose-response curve for PD98059, showing how the number of tips formed by the ureteric bud tree declines with increasing (PD98059); error bars represent the standard error of the mean.