Estimators of number- and volume-weighted mean volume of colloid in the thyroid gland of rats
M-L Hartoft-Nielsen1,2, AK Rasmussen1, U Feldt-Rasmussen1, K Buschard2 & T Bock3
In the thyroid gland the volume of colloid in the follicles varies considerably. The usually used number weighted mean volume (Vn) does not describe this variation in the distribution of the volume of colloid.
Aim: To develop a set of assumption-free stereological methods to determine the variation of the volume of colloid in respect of number and size and to determine the coefficient of this variation (CV). For this purpose we introduce the volume-weighted mean volume of colloid (Vv), which is the mean volume if the follicles were sampled proportional to their volume and not their number.
Methods: Aged 17 weeks, six male Sprague-Dawley rats were euthanized. Both thyroid lobi were carefully removed and embedded in paraffin. The blocks were exhaustively sectioned. For every 20 sections a pair of two consecutive sections were sampled by systematic uniform random sampling (SURS) and PAS stained. The sections were investigated on Leica light microscopes with projecting arms. Sampling within sections was performed according to SURS. The volume of the thyroid gland and the Vn of colloid were estimated by Principle of Cavalieri. The number of follicles were determined by the physical fractionator. Finally the Vv of colloid was determined by the Point Sampled Intercept method.
Results: The volume of the thyroid glands was 7.5plus/minus0.6 mm3 (SEM). The colloid constituted 33.2plus/minus1.3% and consequently the total volume of colloid was 2.5plus/minus0.3 mm3. The total number of follicles was 39,539plus/minus4,929. The Vn of colloid was 6.5x10-5plus/minus0.3x10-5 mm3 and Vv of colloid was 9.3x10-4plus/minus1.4x10-4 mm3 in each follicle and consequently the CV was 3.6plus/minus0.4.
Conclusion: Based on assumption-free stereological tools we now have a method to determine the distribution of the volume of colloid in the follicles in the thyroid gland. This parameter will be useful in studies of morphological changes during physiological and pathophysiological conditions.