Circadian variation of DNA replication in hamster cheek pouch epithelium analysed by tritiated thymidine labelling, flow sorting and autoradiography: no resting S phase cells in the normal epithelium.

In the normal hamster cheek pouch epithelium, cell proliferation takes place with a pronounced circadian rhythm. We tested our previous hypothesis that all cells having S phase DNA content are actively synthesizing DNA and thus participating in the daily cohort of proliferating cells. We found no evidence of resting S phase cells in the normal epithelium. Using labelling with tritiated thymidine followed by fluorescence activated cell sorting according to DNA content and by autoradiography of the sorted nuclei, it was demonstrated that during the 24 h period almost all cells with mid S phase DNA content were active in DNA synthesis.