Mitotic rate of thyroid follicular cells in untreated and goitrogen-treated rats: variation with time of day.

The paper presents the results of investigations into the mitotic rates of thyroid follicular cells of adult male rats maintained under controlled conditions of temperature, photoperiod and handling. The mitotic rate in eight successive 3 hour periods of the day showed a clear mitotic rhythm, with a maximum at 12.00--15.00 hours in untreated rats. The mitotic rate over the 24 hour period was 7.6+/-0.83(S.E.)metaphases/10000 cells/hour. Rats treated with the antithyroid drug, carbimazole, for three weeks were manifestly hypothyroid, as shown by decreases in oxygen consumption and rate of growth. With carbimazole the mitotic rate of the thyroid follicular cells was increased three-fold, and the mitotic rhythm was no longer detectable. In an experiment to test the efficiency of vincristine sulphate, the accumulation of metaphases after 1 mg/kg or 2 mg/kg vincristine was linear for 0.5--8 hours, although, at the higher dose, the number of metaphases was reduced after 8 hours. The mean daily mitotic rate in untreated rats was approximately commensurate with the rate of increase of body weight, which supports the classification of follicular cells as an 'expanding cell population'.