Fig. 7. A model for early stages of limb bud growth. (A) Schematic representation of genetic interactions in early limb bud initiation. The thicker the arrow, the more crucial the interaction. See text for details. (B) Major steps in early limb bud formation. TBX5 in the lateral plate mesoderm (LPM) activates Fgf10, which in turn signals to the surface ectoderm (SE) to activate WNTs and FGFs, which then signal back to maintain Fgf10 levels, and subsequently Tbx5 expression. Lef1 and Tcf1 cooperate with TBX5 to sustain Fgf10 levels. SO, somites; IM, intermediate mesoderm.