Elaboration of Thyroglobulin in the Thyroid Follicle

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Abstract
Newly synthesized proteins were traced by radioautography in the thyroid gland of rats given a single injection of DLleucine- 4,5-3H and sacrificed at various times. The radioautographs of the thyroid glands consisted of regular 5-μ sections stained with periodic acid-Schiff and hematoxylin, and of ultrathin sections for electron microscopy stained by Karnovsky’s method. With the light microscope, radioautographs obtained at 10 min after injection revealed a uniform distribution of silver grains over the cytoplasm of every follicular cell in every follicle. At 4 hr, the silver grains had accumulated over the apex of the cells and sometimes extended over the periphery of the colloid. At 36 hr, as the grain concentration over follicular cells was reduced, many grains appeared over the colloid. Some were seen up to 7 days, but not at 30 days after injection. With the electron microscope, radioautographs revealed that the silver grains at 10 min after injection were over the ergastoplasm of the follicular cells, often straddling the ribosome-studded membranes of the cisternae. At 1 hr, some of the grains were localized to the lumen of the cisternae of the ergastoplasm, while a few grains appeared over the Golgi zone and occasional grains were over the region of the apical vesicles which extends from the Golgi zone to the area adjacent to the cell surface. At 3½ hr, the grains were more numerous over the region of the apical vesicles; some grains were also found over the colloid. These results indicate that follicular cells synthesize proteins in their ergastoplasm continuously and all do so at the same rate. These proteins are of 2 kinds: 1) sedentary protein which has a slow turnover rate and remains in the cells, presumably relegated to intracellular structures; and 2) exportable protein which has a more rapid turnover rate and is secreted into the lumen of the follicles. This protein, which is presumed to consist mainly of uniodinated thyroglobulin, migrates in the follicular cells via the cisternae of the ergastoplasm, the Golgi zone, and the apical vesicles, to be added to the colloid where it is iodinated. (Endocrinology74: 333, 1964)