Significance of apoptosis in morphogenesis of human lung development: light microscopic observation using in situ DNA end-labeling and ultrastructural study.

To clarify the significance of apoptosis in lung morphogenesis, the occurrence and localization of apoptosis were investigated in developing human lungs, using TdT-mediated dUTP-biotin nick end labeling (TUNEL) and ultrastructural observation. Under light microscopic observation, apoptotic cells were demonstrated in the epithelial lining of the developing respiratory tubes and their growth ends (bronchial tips). Small numbers of apoptotic cells were also observed in the mesenchymal cells surrounding the immature respiratory trees. The apoptotic cells tended to aggregate at the bronchial tips and the bifurcation. The ultrastructural study disclosed the presence of apoptotic cells characterized by condensation of nuclear chromatin, fragmented nuclei (apoptotic bodies), and rather well-preserved cytoplasmic organelles, in the developing bronchial epithelium. In conclusion, the observations confirm that the occurrence of apoptosis plays an important role in lung morphogenesis.