| Development of the lung requires the precise coordination of three cell layers: an inner epithelium, the surrounding mesenchymal stroma, and an outer mesothelium. As established by classical recombination experiments, epithelial-mesenchymal interactions between these tissue layers control tissue survival, morphology, and differentiation. This thesis work addresses the molecular basis of this reciprocal signaling.; Early in development, mesenchymal signals control the stereotypic branching morphogenesis of the lung epithelium. Using a tissue culture system in which isolated endoderm is incubated with Fgf-loaded beads, we demonstrate that mesenchymal Fibroblast growth factor 10 (Fgf10) promotes outgrowth of lung epithelium. In culture, Fgf10 induces both proliferation and chemotaxis of lung endoderm. Additionally, an Fgf10 bead induces ectopic domains of another secreted signaling molecule, Bone morphogenesic protein 4 (Bmp4), in isolated endoderm. Thus, Fgf10 has dual roles in directing the outgrowth of a new bud, and subsequently inducing Bmp4.; To investigate the in vivo role of Bmp4 in lung endoderm, we overexpressed the Bmp antagonist Noggin in distal lung epithelium using the Surfactant Protein C promoter/enhancer. Abrogation of Bmp signaling in the lung epithelium results in a reduction in distal cell types and a concurrent increase in proximal cell types, as demonstrated by histology and in situ analysis of marker genes. These results suggest that Bmp signaling is important for establishing the proximal-distal axis of the respiratory epithelium.; Tissue interactions are also critical for lung mesenchyme development. Using an explant system, we show that exogenous N-Sonic Hedgehog (N-Shh) protein sustains the survival and proliferation of isolated mesenchyme in a dose dependent manner, and affects the expression of mesenchymal cell markers. In N-Shh exposed explants, these products are upregulated throughout the mesenchyme, but not in the periphery. This peripheral exclusion zone correlates with the presence of the mesothelial layer, which expresses Fibroblast Growth Factor 9 (Fgf9). Recombinant Fgf9 protein inhibits the differentiation response of the mesenchyme to N-Shh. These results suggest a model in which Shh and Fgf9 coordinately regulate the proliferation and differentiation of the lung mesoderm. |
