Fig. 6.

Genetic pathway integrating FGF, Wnt and Tbx3 in epithelial–mesenchymal interactions regulating mammary gland initiation. Fgf signalling through FgfR1, blocked by SU5402, maintains Tbx3 expression and leads to expression of Pyst1 and Lef1. Fgf10 signalling is also required for mammary gland development as shown in knock-out mice. Wnt signalling, blocked by CK1-7, also maintains Tbx3 expression. Tbx3 null mutants do not form mammary buds and expression of Wnt10b and Lef1 is absent (Davenport et al. 2003), suggesting that Tbx3 controls their expression. Fgf8, Fgf9, Fgf4, Fgf17 are expressed in ectoderm or mammary knob epithelium. Pyst1 expression depends on signalling through FgfR1.