Circadian influence on the frequency of labeled mitoses method in the stratified squamous epithelium of the mouse esophagus and tongue

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Abstract
The frequency of labeled mitoses method (FLM) was used to study the cell kinetics in the stratified squamous epithelium of the mouse esophagus and tongue. FLMs were generated by injecting tritiated thymidine (TdR) at two different phases of the mouse circadian system: TdR was injected into one group of mice at 0900 and into a different, second group of mice at 2100. Three variables were monitored for each group; (1) the FLM, (2) the mitotic index and (3) the grain count over the labeled mitotic figures. In both the esophagus and the tongue there was a circadian rhythm in the mitotic index with the peak occurring during the first half of the diurnal phase and the trough occurring during the first half of the nocturnal phase. The FLM curves from each group revealed the following data: Transit time in G2+1/2M (TG2+1/2M) Transit time in S phase (TS) hours hours Esophagus — TdR at 0900 2.5 5.3 Esophagus — TdR at 2100 4.5 3.5 Tongue — TdR at 0900 4.2 5.1 Tongue — TdR at 2100 4.2 5.5 Second waves of labeled mitoses were seen in the esophagus and tongue when TdR was injected at 0900. No second wave of labeled mitoses occurred in either the esophagus or the tongue when TdR was injected at 2100. For the esophagus the grain counts of the labeled mitotic figures in each group indicated that the rate of DNA synthesis was greater during late S phase, whether the TdR was injected at 0900 or 2100. During most of the S phase the rate of DNA synthesis was significantly greater in the TdR at 2100 group, which is the time of the circadian system when the esophagus demonstrated a relatively short Ts. In the epithelium of the tongue there were a few statistical differences in the rate of DNA synthesis between the groups and during the duration of the S phase within either group, but these data were difficult to interpret. The data demonstrate the influence of the circadian system in cell kinetic work and in the interpretation of data obtained with the FLM in renewing populations of cells which are not totally asynchronous.