Fig. 5. Evidence from tissue recombinations (A-F) and isolated ureteric buds (G,H) that MAP kinase is required in the ureteric bud itself. (A-C) Lung epithelium undergoes branching morphogenesis in normal medium (A) and also in the presence of 25 µM PD98059 (B), which is sufficient to block ureteric bud morphogenesis in kidneys; even at 75 µM PD98059 (C), some branching continues. Lung mesenchyme is therefore capable of supporting epithelial branching in the presence of PD98059. Ureteric buds recombined with lung mesenchyme and cultured in the presence of GDNF branch organotypically (E), though not as extensively as they do when recombined with kidney mesenchyme (D). In the presence of 25 µM PD98059, however, ureteric buds show markedly reduced branching in lung mesenchyme (F; two ureteric buds are shown in this figure). (G) Isolated ureteric buds cultured in the presence of 50 ng/ml GDNF retain their epithelial organisation, though they round up a little to form almost spherical cysts (this effect has been described previously by Sainio et al. (Sainio et al., 1997)). In the presence of PD98059, however, the ability of GDNF to maintain epithelial organisation of the ureteric bud is lost, and over 24 hours its cells spill out to form a disorganised mass as if no GDNF were present (H).