Fig. 4. Inhibition of FGF signaling in vitro prevents feather placode induction but not the maintenance of established feather buds. Skin explants isolated at HH28, prior to placode formation (ppf; A-H), were cultured in the presence of DMSO or Su5402 for 16 hours in vitro and analyzed by whole-mount in situ hybridization for Shh, Bmp2, ß-catenin or Fgfr1 expression. Explants isolated at HH29, when the first buds have been established (eb; I-N), were separated along the midline as indicated by the doted line in A, and halves were either fixed directly after isolation (I,K,M) or cultured for 16 hours in the presence of Su5402 (J,N) or DMSO (L), and expression of Shh (I-L) or Bmp2 (M,N) was compared in the two halves.