Circadian rhythm and proliferation of lung alveolar wall cells during postnatal growth in mice

The circadian rhythm of deoxyribonucleic acid (DNA) synthesis and mitosis in lung alveolar wall cells was studied in the mouse at 4 h intervals during postnatal days 4 and 5. Rhythmic variations in DNA synthesis and mitosis were observed both days, but their peak activities differed. On day 4, the greatest DNA synthesis occurred at 10 p.m. and the highest mitotic rate occurred at 2 p.m. On day 5, the greatest DNA synthesis and the highest mitotic rate both occurred at 10 p.m. On postnatal day 4, both DNA synthetic and mitotic rates showed peaks that occurred at the same time both at 2 p.m. and at 10 p.m. On postnatal day 4, both DNA synthetic and mitotic rates showed peaks that occurred at the same time both at 2 p.m. and at the 10 p.m. On postnatal day 5, 2 DNA synthesis peaks were observed, one at 10 a.m. and one at 10 p.m., with only 1 mitotic peak at 10 p.m. The length of intermitotic phases S and G2 of lung alveolar wall cells appears to be 5.5 h and 3 h, respectively.