BUD FORMATION PRECEDES THE APPEARANCE OF DIFFERENTIAL CELL-PROLIFERATION DURING BRANCHING MORPHOGENESIS OF MOUSE LUNG EPITHELIUM IN-VITRO

Cell proliferation is an essential requirement for epithelial expansio n and tubular branching; however, little is known of how these events are coupled during morphogenesis. We have previously shown that, in th e absence of mesenchyme, fibroblast growth factor 1 (FGF-1) elicits bu dding of the mouse lung epithelium cultured in a basement membrane mat rix. Although bud formation seems to be the manifestation of a localiz ed response of lung epithelial cells to FGF-1, it is unclear whether b udding results from induction of differential rates of cell proliferat ion within the epithelium. We performed continuous labeling and pulse- chase experiments in FGF-1-treated mesenchyme-free lung epithelial cul tures at distinct stages of bud induction using bromodeoxyuridine (Brd U), to determine when and to what extent cell proliferation contribute s to bud formation. When explants were incubated with BrdU either befo re bud induction (0-18 hr in culture) or at the onset of budding (24-3 0 hr), labeled nuclei were found distributed throughout the entire exp lant. In contrast, BrdU incubation after the onset of budding (30-48 h r) resulted in labeling concentrated in the budding areas, and a decre ase of labeling toward the proximal region of the explant, between bud s. These results demonstrate that differential rates of cell prolifera tion between bud and nonbud areas do not appear until when buds are al most completely formed. Thus, in the developing lung epithelium in vit ro, bud outgrowth is not triggered by induction of localized cell prol iferation, Dev. Dyn. 1998;213:228-235. (C) 1998 Wiley-Liss, Inc.