Over a 24-h period, groups of mice were given a single injection of colchicine (to collect blocked metaphases) and tritiated thymidine (to label nuclei synthesizing deoxyribonucleic acid). Epithelial nuclei in the oesophagus, trachea and ureter were examined and counted in paraffin sections: the duration of deoxyribonucleic acid synthesis (Ts) was calculated from the numbers of blocked metaphases and labelled nuclei, the duration of the post-synthetic gap (TG2) was estimated from the proportion of blocked mataphases labelled, and the cell cycle time (Tc) was calculated from Ts and the proportion of nuclei labelled. In each epithelium the different layers seen by light microscopy were analysed separately.

Ts was probably the same for the basal and superficial cells in the trachea (about 8 h), and was probably the same for the basal, intermediate and superficial cells in the ureter (about 5 h). In the oesophagus Ts was 8.5 h.

TG2 was probably the same for the basal and superficial cells in the trachea (3.6 h), and probably the same for the basal, intermediate and superficial cells in the ureter (about 4.6 h). In the oesophagus TG2 was 2.8 h.

Tc was about 380 h (basal cells) and 1400 h (superficial cells) in the trachea, and about 8000 h (basal and intermediate cells) and 2700 h (superficial cells) in the ureter. In the oesophagus Tc was 41 h.

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