Induction of thyroiditis in guinea-pigs by intravenous injection of rabbit anti-guinea-pig thyroglobulin serum. 3. Electron microscopic study.

The ultrastructure of the thyroids of guinea-pigs receiving rabbit anti-guinea-pig thyroglobulin serum intravenously has been studied. Rounded, moderately electron-dense deposits were found in the intercellular spaces and between the basal plasma membrane and the basement membrane of the follicle cells. The deposits increased in size with time, and they probably correspond to the rounded droplets of γ-globulin noted in a previous fluorescence microscopic study of material from the same animals. The intracellular changes in the follicle cells were restricted to an increase in the size and the number of single-membrane limited vesicles 5 days after the injection. Occasionally images were obtained suggesting fusion of some of these vesicles with the plasma membrane. Furthermore coated pits and vesicles were observed in the basal and lateral plasma membrane, often in close association with the deposits, suggesting uptake of protein here. An inflammatory infiltrate which mainly consisted of eosinophil but also to a lesser degree of neutrophil granulocytes was found in the interstitium. Endocytic vacuoles were regularly seen in the neutrophils which were also often more or less degranulated. Eosinophil, granulocytes, on the other hand, rarely contained endocytic vacuoles, but were shown to fragment and to release their granules into the interstitial space, perhaps as a reaction to the observed degranulation of mast cells. Both deposits and granulocytes were often found to widely distend the intercellular spaces of the follicle cells as far as to the zonula occludens. Splitting of this zone was however not noted, indicating that to a large degree it may function as an obstacle to the penetration of both antibodies and granulocytes.