[Morphological characteristics of the thyroid glands of Macaca rhesus monkeys normally and in different types of hypokinesia].

Thyroid glands and C-cells of Macaca rhesus were examined histologically, electron microscopically and morphometrically. The examinations were performed on 6 intact and 7 hypokinetic rhesus monkeys which were kept in a suspension device for 2 months, small cages for 2 months or in a head-down position (7 days in a horizontal position and then 11 days in a head-down position at -6 degrees). The experiment with a diminished motor activity and a normal support function of the lower limbs (maintenance in small cages), in spite of its long duration, produced no structural changes in the parenchyma or C-cells. The 18-day bed rest test was followed by a hypofunction of the thyroid gland, inhibition of hormonal synthesis and secretion without distinct changes in C-cells. Unlike previous experiments, the suspension study led to hyperplasia and hypertrophy of C-cells and increase of their nuclear volume. Activation of C-cells can be regarded as an adaptive reaction aimed at stabilizing bone calcium.