Abstract
During the development of the lung, branching morphogenesis of the epithelium requires the presence of bronchial mesenchyme. Although it has been demonstrated that continuing morphogenesis is dependent upon growth through proliferation, there is no direct evidence to suggest that the specific pattern of branching is a consequence of differential mitotic activity which is influenced by the appropriate mesenchymal tissue. The induction of supernumerary buds in the tracheal epithelium of the mouse by transplantations of bronchial mesenchyme provides a means for analysing the role of cell proliferation in the formation of bronchial buds. We have addressed the problem by examining the ability of the tracheal epithelium, at different stages of development, to respond to bud induction by bronchial mesenchyme, and attempted to correlate this with the labeling index and 3H‐thymidine uptake in the trachea at the respective stages. It has been found that both the 11‐day‐old and 12‐day‐old tracheal epithelia initiate supernumerary buds in response to bronchial mesenchyme although the initiation in 12‐day‐old embryonic trachea occurred somewhat later than in the 11‐day‐old explants. The 13‐day‐old trachea failed to elicit buds after the tracheal mesenchyme had been stripped following treatment with 3% trypsin‐pancreatin. A decrease in cell proliferation in the tracheal epithelium (as revealed by autoradiography and scintillation counting following 5‐hour labeling with tritiated thymidine) between 11 and 13 days of development could contribute to these varying responses to bud induction. Further autoradiographic studies whereby the labeling index in the induced bud and the tracheal epithelium of the same explant have been compared, have revealed a continued high level of labeling in the branching bud and a concomitant decrease in labeling of the trachea. These data suggest that the bronchial mesenchyme either acts as a mitotic stimulator or provides conditions permissive to continued mitotic activity.