Study on the DNA synthesis of pulmonary cells in aging mice by light microscopic radioautography

The DNA synthesis and morphological changes of the pulmonary cells of 11 groups of mice from fetal day 16 to 22 months after birth were studied by light microscopic radioautography after 3H-thymidine incorporation. A detailed investigation was first carried out to determine the localization and the frequency of 3H-thymidine incorporation with the age-related change of pulmonary cells in the mouse lung. The results showed that the DNA synthesis of pulmonary cells changed with aging. The examination of radioautograms demonstrated that the activity of DNA synthesis of type 1 epithelial cells was very low, which reached the peak on the third day after birth, decreased gradually with aging, and lost the ability from postnatal 6 months onwards. On the other hand, the labeling indices of type 2 epithelial, interstitial and endothelial cells were the highest on the fetal day 16, then fell down with the developing of the lung, but increased again on the third postnatal day, and decreased gradually with age. These data provided the evidence that the DNA synthesis of type 2 epithelial, interstitial and endothelial cells were the largest on fetal day 16, then decreased with the developing of the lung due to aging.