Relationship between thyroglobulin synthesis, lodine metabolism, and histogenesis in the developing rabbit fetal thyroid gland

Abstract

The biosynthesis of thyroglobulin and its iodination have been studied in the rabbit fetus in relation to the morphogenesis and structure of the thyroid gland. Sixteen- to 28-day-old fetuses were injected intraperitoneally in vivo through the uterine wall with [ 3H]leucine and 125I. Tritium-labeled proteins were analyzed by sucrose gradient centrifugation and dise electrophoresis, in combination with immunological techniques. Thyroglobulin was estimated by radioimmunoassay, and 127I by chemical determination. The first definite follicles were seen in electron micrographs on day 18. Colloid is not stored in large amounts until day 22. Proteins having antigenic sites in common with adult thyroglobulin can be radio-immunogically detected in the thyroids of 16- and 17-day-old fetuses, before the formation of follicles, before the capacity of collecting radioiodine appear, and before detectable amounts of 127I exist in the glands. Low molecular weight precursors of thyroglobulin are present in the 18-day thyroid; they seem to actively polymerize into 19S thyroglobulin on day 22. As soon as microfollicles have differentiated, iodine is concentrated, iodination of thyroglobulin occurs, and thyroid hormones are synthesized. The ability of the rabbit fetal thyroid gland to concentrate iodide and to synthesize thyroid hormones thus seems to be linked to the organization of follicles.