Site of Formation of Thyroglobulin in Mouse Thyroid as Shown by Radioautography with Leucine-H3.

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Abstract
Ten adult male mice were injected with tritium-labelled leucine and sacrificed in pairs at various time intervals thereafter. Thyroid gland sections were radioautographed by the coating technique and photographic grains counted over epithelium and colloid. The leucine label is incorporated into protein by every cell of every thyroid follicle at 30 minutes after injection, but its concentration in the cells decreases thereafter. Concentration of labelled protein in the colloid was insignificant at 1/2 hour, increased and reached a maximum at 35 hours or soon thereafter, and subsequently diminished. The appearance of the leucine label in colloid of all follicles examined between 1/2 hour and 35 hours could be accounted for by simultaneous loss of label from the epithelial cells. Since leucine is the most prominent amino-acid in thyroglobulin, the results are interpreted to represent the synthesis of this protein in the epithelium and its secretion into the colloid.