CORRELATED LIGHT AND ELECTRON MICROSCOPE OBSERVATIONS ON GLYCOPROTEIN-CONTAINING GLOBULES IN THE FOLLICULAR CELLS OF THE THYROID GLAND OF THE RAT

Two carbohydrate staining techniques were applied to sections of rat thyroid gland: periodic acid-silver methenamine to thin sections for electron microscopy, and periodic acid-Schiff to thick sections for light microscopy. The latter were compared with adjacent thin sections for identificatoin with the electron microscope. Two types of globules in thyroid follicular cells stained with both methods. Globules of the first type are relatively large (usually 0.5 to 3 µ) with electron opacity very similar to that of follicular colloid; when stained with toluidine blue they have the same gray shade as follicular colloid. These similarities suggest that their periodic acid reactivity is due to the same glycoprotein as that of follicular colloid, namely thyroglobulin, and that the origin of these "intracellular colloid droplets" is the colloid in the lumen. The second type comprises medium-sized (usually 0.1 to 1 µ) fairly electron opaque globules having fine particles (~70 Å) dispersed in their matrices and sometimes containing membrane fragments or other irregularities; when stained with toluidine blue these globules stand out dark blue. Although their periodic acid reactivity indicates that these globules also contain glycoprotein, their ultrastructure and staining characteristics suggest that their composition differs from colloid. It is possible that they represent enzyme or zymogen-containing granules. A third type of globule, which on account of its intense staining in some periodic acid silver methenamine preparations could perhaps also be periodic acid reactive, concerns small (usually 0.02 to 0.2 µ) globules, mainly accumulated beneath the apical border of the cell. These globules, known as "apical vesicles," are believed to contain the not-yet-iodinated precursor of thyroglobulin.