Fig. 2. Directional migration and its dependence on PDGFR signaling. (A-C) Comparison of mesoderm migration on untreated tissue culture plastic (A), FN-coated plastic (B) and conditioned substratum (C). (D-I) Migration of PDGFR-inhibited mesoderm on normal conditioned substratum (see explanatory scheme above). Inhibition of PDGFR signaling by expression of dominant-negative PDGFR37 mRNA randomizes the direction of migration (D). Directed migration is restored by co-expression of wild-type PDGFR (E). Expression of missense PDGFR does not affect mesoderm migration (F). Mesoderm explants moving on conditioned substratum were treated during the 1 hour migration period with tyrphostin AG 1296 (G), control tyrphostin AG 43 (H) or Wortmannin (I). Both AG 1296 and wortmannin randomize the direction of migration. n, number of explants tested; results from at least three independent experiments. Explants in C,E,F,H prefer the animal pole side significantly (one-sided sign test, significance level =0.005 for (E) and 0.0005 for all others). In the same test, explants in A,B,D,G,I show no preference for the animal pole at any significance level.