To date, it has remained unclear what signals subdivide the surface ectoderm of embryos into different developmental fields, such as those that form mammary epithelium. But now, on p. , Veltmaat et al. reveal that the induction and positioning of one of the five mammary placodes in the mouse depends on Gli3-mediated somitic Fgf10expression gradients, which activate ectodermally expressed FGF receptors and lead to Wnt10b expression. Using Wnt signalling and ectodermal multilayering as markers of mammogenesis, the researchers investigated mammary placode development in mouse mutants with altered somitic Fgf10gradients. They report that mammary line formation is impaired and placode 3 is absent in embryos in which the somitic Fgf10 gradient is shortened(as in Gli3 null embryos) or less Fgf10 is expressed overall. The researchers suggest, therefore, that a combination of somitic elongation and somitic Fgf10 gradients induces the differentiation of the surface ectoderm into mammary epithelium at the position in mice that corresponds to the position of the human breast.
