[The mitotic activity of the follicular and parafollicular (C) cells in the thyroid of rats with hypercalcemia].

Slices of the thyroid gland were impregnated with silver nitrate and stained with hematoxylin. Separate calculation of the index of calchicin mitosis of the follicular and parafollicular (C-) cells was made. The material of 37 mature albino rats was analyzed. Calcium-rich diet was given to 22 animals during 10 days. Different levels of hypercalcemia were achieved by different doses of vitamin D2 (from 1 to 10 thousand MU daily, 11 experimental groups). The mitotic index of C-cells was found to grow but under mild hypercalcemia (10% of the control level). Further elevation of the calcium concentration in blood was followed by a decrease of the amount of the dividing parafollicular cells down to control values. In rats fed by the diet optimal for the stimulation of C-cells division (11 rats, 2000 MU as a daily dosage of vitamin D2) within the interval from 1 to 10 days increased mitotic activity of the follicular cells was found from the 3rd day, while that of C-cells only as late as the 9th-10th day. In all the experimental rats the mitotic index of the follicular thyrocytes was 3.5-4 times as high as the mitotic index of C-cells. Relatively not high proliferative activity of parafollicular cells might be due to their pronounced polyfunctional character and their participation in complex paracrine interactions with other cell elements of the thyroid gland.