Biosynthesis of Thyroglobulin Related to the Ultrastructure of the Human Fetal Thyroid Gland

Abstract

The thyroid glands from 22 human fetuses of crown rump lengths (CR) from 28 to 130 mm were studied in the light and electron microscope and by biochemical analysis of labeled proteins after incubation with 125Iiodide and 3H-leucine. In 3 fetuses of CR 47–54 mm only non-iodinated proteins of low mol wt were synthesized. The thyroids of these young fetuses were not organized into follicles, and the epithelial cells were lacking a rough-surfaced endoplasmic reticulum but were rich in free ribosomes. In one fetus of CR 60 mm the thyroid synthesized non-iodinated thyroglobulin in addition to proteins of low mol wt. This gland was still lacking a follicular organization but the epithelial cells had a fairly well developed roughsurfaced endoplasmic reticulum. Iodinated 17-19 S thyroglobulin was synthesized in the thyroid gland of one fetus of 60 mm and in all older fetuses. The morphology of these thyroids was similar to that of the adult gland, and the epithelial cells, differing from those of the biochemically less mature glands mainly by their very extensive rough-surfaced endoplasmic reticulum, numerous Golgi vesicles, large number of apical vesicles and “intracellular lumina” filled with dense material, were now forming true follicles. It is concluded that a rough-surfaced endoplasmic reticulum is a prerequisite for the formation of the protein portion of thyroglobulin. The iodination of this protein, including the synthesis of the thyroid hormones, parallels the formation of follicles and the structural maturation of the apical cell zone of the follicular cells. (Endocrinology87: 1000, 1970)