Fig. 7. A model combining ventral elongation of the somites and somitic Fgf10 expression with maturation of the surface ectoderm and the acquisition of a mammary cell fate at the position of mammary placode 3. (A) A model for the molecular interactions leading to a mammary cell fate in the surface ectoderm. GLI3 (purple dots) and an unidentified factor or factors X act upstream of Fgf10 (pink dots) in the somites. Somitic FGF10 activates ectodermal FGFR2B (green), via diffusion and/or delamination of somitic cells. Sufficient levels of activation are required for Wnt10b/TOPGAL expression (blue) and multilayering of the surface ectodermal cells as hallmarks of a mammary cell identity. (B) Schematic drawings of body halves of progressively older embryos, illustrating the correlation between somitic elongation, somitic Fgf10 expression and ectodermal maturation. For convenience, the body size is kept the same for all ages. Broken vertical lines indicate the mammary line (ml) and Wolffian ridge (WR). Increasingly darker shades of pink indicate increasingly higher levels of Fgf10 expression. The surface ectodermal cells are cuboidal (thicker black line) at the WR, whereas dorsal and ventral to the WR they are squamous (thinner black line). Ventrally on the flank, ectodermal cells become cylindrical (green) and enlarge (orange) coinciding with low levels of Wnt10b and TOPGAL expression. These cells give rise to a stratum intermedium (blue), with elevated Wnt10b and TOPGAL expression, indicating a mammary cell fate. (C) Model of cumulative FGF10/FGFR2B signaling in the flank leading to progressive maturation of the ectoderm, and eventually conversion into mammary epithelium. See main text for explanation. Color coding corresponds to color coding of B. Abbreviations: G, dorsal root ganglion; som, somite.