The influence of injected tritiated thymidine on the mitotic circadian rhythm in the epithelium of the hamster cheek pouch

ABSTRACT The initial effect of an injection of TdR‐5‐ 3 H (1 μCi/g body weight; 6 Ci/mmol) in the cheek pouch epithelium of the Syrian hamster is an increase in the mitotic index. The increase is observed 1–5 hr after injection, depending upon the time of day when the injection is given, and is followed by compensatory variations in mitotic index. This deviation from the normal circadian rhythm in the mitotic index appears to depend on the fraction of G 2 ‐cells at the time of injection. The main effect is a shortening of t g2 . No effect is observed after injection of non‐radioactive TdR or isotonic saline. The results of the present experiment emphasize that unexpected results may be obtained when using mitotic indices from animals labelled with 3 H‐TdR, as well as the risks of using the PLM‐method in a partially synchronized system.