Sex dimorphism in the thyroid gland. III. Morphometric studies on the rat thyroid gland in the course of postnatal ontogenesis.

At weekly intervals between 21 and 84 days of postnatal ontogenesis, on the thyroid glands of male and female rats morphometric studies were performed. The rate of body growth was similar in male and female rats between 21 and 42 days. Thereafter male rats are heavier than females. There were no differences in composition of particular histologic components studied in the thyroids of male and female rats between 21 and 35 days. From day 42 the volume fraction of colloid is lower in the male than female gland. Also from day 49 of postnatal ontogenesis the follicular cells are taller in male than in female thyroid gland. After reaching puberty in the female thyroid gland a gradual lowering of the volume fraction of epithelium and an increase in volume fraction of colloid are observed. Due to these changes epithelium/colloid ratio in female rats markedly declines between 42 and 84 days of postnatal ontogenesis while in the male rats this ratio is higher and does not show such a distinct decline.