Embryonic mouse lung epithelium was separated from its mesenchyme and cultured under mesenchyme-free conditions. When covered with Matrigel, the cultured epithelium underwent branching morphogenesis in medium containing acidic fibroblast growth factor (aFGF), in which the epithelial cells constructed a simple columnar cell layer forming a lumen, as seen in normal development. The epithelial growth and branching morphogenesis induced by aFGF was completely inhibited by an antibody against aFGF. Heparin caused extra epithelial growth in cooperation with aFGF, but its use resulted in luminal expansion instead of enhanced branching. Basic FGF induced abnormal morphogenesis of the epithelium, though the lumen formed was lined by a simple columnar cell layer. Epidermal growth factor could not maintain epithelial cell growth, and the epithelium became a smaller and smoother ball than that at the start of cultivation. When covered with a collagen gel instead of Matrigel, the epithelium remained in its initial form, neither newly branching nor becoming a smooth ball, in the presence of aFGF. These results show that the epithelium of lung rudiments was able to branch under mesenchyme-free culture conditions in which a basement membrane matrix and aFGF were substitutes for the mesenchyme.

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