Ways and means by which prominent signaling molecules and transcription factors control lung development. The middle (small) diagram shows the different parts (germ layers) of a lung bud (proximal and distal). Figure 7A: Location of signalling molecules and the pathways by which they control growth, budding, and differentiation of the airway epithelium and surrounding mesenchyme. The shading shows the proximal-distal extent of the lung bud epithelium. Figure 7B: Signaling programs in the distal lung bud bring about mesenchymal- and epithelial cell proliferation as well as airway branching. Details to these processes can be obtained from the text and particularly from the following comprehensive reviews: Metzger and Krasnow , Perl and Whitsett , Roth-Kleiner and Post , Cardoso and Lü , Lu and Werb , De Langhe and Reynolds , Affolter et al. , Warburton et al. , [15, 16, 18, 29, 98]. Modified after Ornitz and Yin .