THE FETAL BOVINE THYROID: MORPHOGENESIS AS RELATED TO IODINE ACCUMULATION1

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Abstract
Thyrox-in-like and other organically bound iodine compounds appear in the fetal calf thyroid between the 53d and 70th days, a time prior to the appearance of histologically detectable intracellular colloid or follicle formation, which takes place at 75-88 days. Since the livers of fetuses aged 62-70 days contain practically no iodine, the presence of these organic iodine compounds was considered a manifestation of iodine storage by the gland. From the 75th to 118th day, the predominating features of the developing thyroid were the appearance of colloid and differentiation of the parenchyma without a pronounced increase in the iodine content of the gland. Between the 4th and 7th month, gradual development of the fetal thyroid gland was accompanied by a steady increase in its organic iodine content. An appreciable increase in organic iodine content occurred near term, and was accompanied by definite histological manifestations of increased functional activity of the follicular epithelium.