Fig. 4. Fgf10 and Fgfr2b expression patterns and rescue of the Fgf10^-/- mammary defect by recombinant FGF10. (A,B) In situ hybridization for Fgf10 on an E10.0 (30 somite) wild-type embryo; whole-mount (A) and a 40 µm vibratome section (B) along the plane indicated by the line in A. Fgf10 expression is present in the lateral plate mesoderm of the flank. (C-H) Radiolabeled in situ hybridization on Hematoxylin counterstained sections shows Fgf10 expression (red) in the somites (white contours in C,D), and Fgf2b expression (red) in the surface ectoderm (E-H). The arrowed brackets in E-H indicate the Wolffian ridge (WR) or flank. The mammary line (ml) overlying the hypaxial bud of the somite (black contour in H) expresses elevated levels of Fgfr2b. (I-L) Whole-mount in situ hybridization for Wnt10b, showing mammary placode formation (arrows) in a flank of a control (I,K) or Fgf10^-/- embryo (J,L), cultured with an implanted bead (circle) soaked in BSA (I,J) or rFGF10 (K,L). (M,N) Nuclear Fast Red stained section at the level of the lines through the flank in L. Boxed area in the insets are magnified; broken black lines outline the placodes. Abbreviations: LPM, lateral plate mesoderm; nt, neural tube; s, somite stage; som, somite; WR, Wolffian ridge; ml, mammary line. Panels A-H are not to scale. Scale bars: 1 mm in I-L; 100 µm in M,N.