Morphometric studies on the development of the human thyroid gland. II. The late fetal life.

Histological and morphometric studies were performed on 27 thyroid glands obtained from normal fetuses ranging between 23 to 40 weeks of intra-uterine life. In the thyroids the volume of gland, colloid, and stroma were calculated by means of differential pointcounting method and the height of the thyroid follicular cells was measured. Moreover, the epithelium/colloid ratio in the thyroid gland, a very sensitive parameter of stimulation of the glands by TSH, was calculated. Regarding the values of this ratio in human fetal thyroid gland, the intra-uterine development of the gland may be divided into three distinct stages. The first one, between weeks 10-18 is characterized by massive folliculogenesis and gradual accumulation of the colloid. The second stage, between weeks 19-29 of fetal life is characterized by rather unchanged values of epithelium/colloid ratio and the size of follicles. The third stage, after the 29th week of development is characterized by a gradual increase in the epithelium/colloid ratio and a decrease in the size of follicles.