Cell population kinetics of thyroid follicular cells in infant rats

T cell population kinetics of thyroid follicular cells in rats were studied by means of autoradiography and a statmokinetic technique. During the first fortnight after birth no significant changes in the mitotic index (MI) and labelling index (LI) were found. In the next 2 weeks a constant decrease in the number of proliferating cells occurs. In 10-day old animals 40% of the follicular cells were in the cell cycle (GF); 3.25 +/- 0.77 (SEM) % in the S phase and 0.18 +/- 0.04% in mitoses (MI). Day-night changes in the LI and mitotic rate (MR) indicated a peak value at 13.30 hours with a lowest value at 22.30 hours. The mean LI and MR averaged over the whole 24 hr were 3.1 +/- 0.1% and 122.2 +/- 18.1%, respectively. In 10-day old animals, using the fraction of labelled mitoses (FLM) method the median cell cycle time (TC) was 79 hr and the phase durations were TG1--64.6 hr, TS--8.2 hr and TG2--5.1 hr. The decrease in the number of proliferating cells with the age of the animals is considered to be a result of both cell cycle prolongation and in growth fraction reduction.