Figure 2.

A model of mammary line specification and placode formation. Fibroblast growth factor (Fgf)10 from the ventrolateral portion of the somites acts in concert with Wnts expressed within the epidermis to specify the mammary line. This induces TBX3 expression, which in turn induces the expression of specific Wnts and Fgfs within the mammary line. These molecules act in an autocrine fashion and cooperate with other signaling pathways to form 10 pairs of placodes at specific locations along the original mammary line.