Mammalian lung development: Interactions in formation and morphogenesis of tracheal buds

Mesoderm present in early developing lung is of two sorts: bronchial, which can evoke formation of bronchial buds from tracheal epithelium; and tracheal, which inhibits bronchial morphogenesis. The latter process correlates with the presence of highly ordered sheaths of collagen and mesoderm cells near the epithelial surface. In contrast, bronchial mesoderm lacks such regularly arrayed components. Initial formation of supernumerary buds on the trachea can be evoked by nonspecific mesoderms, but such buds never branch. Bronchial mesoderm is required if the latter morphogenetic process is to occur. Thymidine labeling experiments fail to provide evidence that differential mitotic activity leads to initial bud formation.