The radioautograms of human lungs were studied after in vitro incubation with tritiated thymidine. Our materials consisted of the lungs of 12 normal foetuses (crown-rump length varied from 7·5 to 16·5 cm.) and of 11 adults (32 to 68 years of age). In the foetal lungs the uptake of tritiated thymidine was highest in the epithelium surrounding small tubular spaces, forerunners of future bronchial epithelium, and in the undifferentiated mesenchyme. The percentage of labelled cells was scarce in the alveolar wall cells of adults (0·05% to 0·65%), was unrelated to age, and appeared to be most frequent in phagocytic elements (so-called alveolar cells). There was an uptake of tritiated thymidine in scattered free alveolar macrophages and in a few interstitial cells. The media and adventitia of blood vessels did not show labelling in most cases. The labelling of epithelium of bronchioles and respiratory bronchioles varied from 0·20% to 0·70%. The findings of other investigators in the study of cell turnover in the lungs of laboratory animals are discussed and compared with the data obtained in the present study.
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