Estimation of number of follicles, volume of colloid and inner follicular surface area in the thyroid gland of rats

Volume is an important variable in assessing the growth and involution of the thyroid gland. The functional unit in the thyroid is the follicle, which consists of thyrocytes surrounding colloid. The size of a follicle depends on the number of cells and the amount of colloid. These are interchangeable and vary according to biological activity. Direct measurements of these variables provide information on structures involved in thyroid hormone synthesis, storage and secretion, and also on changes at the morphological and functional levels. Stereological methods are developed to obtain information on three-dimensional structures from two-dimensional sections and to achieve information on an entire organ by examining a minor part of it. Full-grown male Sprague-Dawley rats were used to develop a set of methods relying on unbiased stereological principles to determine the number of follicles, the total volume of colloid and the inner follicular surface area in the thyroid gland. The total volume of colloid was positively correlated (P < 0.021) with the number of follicles and the inner follicular surface area (P < 0.002) but not to the mean volume of colloid in each follicle. Thus under physiological conditions an increase in the total volume of colloid is associated with an increased number of follicles with a constant size distribution rather than a larger volume of colloid in each follicle. This implies that under physiological conditions there is equilibrium in the size distribution of the volume of colloid in each follicle.