DNA-synthesizing cells in oral epithelium have a range of cell cycle durations: evidence from double-labelling studies using tritiated thymidine and bromodeoxyuridine

Abstract Mouse tongue epithelium is characterized by a circadian variation in the number of DNA-synthesizing cells (labelling index, LI). Cells undergoing DNA synthesis were labelled with tritiated thymidine (³H]TdR) at 0300 (peak LI) or 1200 h (low LI). The fate of these cells was assessed by injecting animals with bromodeoxyuridine (BrdU) at intervals from 12–48 h after ³H]TdR, to follow them from one cell cycle to the next. Labelling was revealed by combining ³H]TdR autoradiography with immunoperoxidase detection of BrdU in the same sections.
A single peak in the appearance of double-labelled cells was seen at 44 h, if ³H]TdR was given at 1200 h; following 3H]TdR at 0300 h, a peak of double labelling was seen at 48 h with the possibility of smaller peaks at 24 h and 36 h.
These results show that the 24 h periodicity in LI in this tissue is associated with a predominant cell cycle duration of 44–48 h, but that a few cells cycle more quickly. Double labelling with ³H]TdR and BrdU provides a useful method for establishing cell cycle duration by labelling S-phase cells in successive cell cycles.