Figure 1

Growth factor interactions during lung bud outgrowth and lung bud arrest. In the left hand panel, a bud is beginning to extend. Fibroblast Growth Factor 10 (FGF10) expression is shown as a clump of green mesenchymal cells that chemoattracts the epithelium, shown in brown, towards the pleura shown in white. Sonic hedgehog (SHH) is expressed at low levels, which facilitates the chemotactic activity of FGF10. Bone Morphogenetic Protein 4 (BMP4) also plays key roles in bud extension. In the right hand panel, the bud has extended and is undergoing bud arrest. FGF10 has induced Sprouty2 (SPRY2) expression in the epithelium to a high level, which inhibits further chemotaxis in response to FGF10 signaling. BMP4 is also induced at a higher level and inhibits cell proliferation and hence bud extension. SHH acts through Patched (PTC), to negatively regulate Fgf10 expression in the mesenchyme near the bud tip. The net result is inhibition of cell proliferation and chemoattraction, culminating in bud arrest.