Fig. 4. Electron micrographs of a part of the follicle wall showing the ultrastructure of follicular cells in control animals (a,b) and animals exposed to 50 Hz EMF (c,d). In a and b the follicular epithelial cells have euchromatic nuclei (n) and prominent excentrically positioned nucleoli (no). In the treated cells the nuclei are placed basally in the cell (carets in c) and the euchromatin is darker in appearance. The rough endoplasmic reticulum (er) is relatively well developed in both cisternal and vesicular form in a-c. The baso-lateral compartments (bl) of some cells in a, c and d have distended cisternae (er). There are more pleomorphic electron-dense lysosomes (ly) under the apical membrane (am) of thyrocytes in a and b compared with c. Lysosomes of different electron density are seen in c. Note two large colloid droplets (cd) in the apical cytoplasm of the same thyrocyte in c and a large group of droplets of various diameter occupying nearly the entire apical region of one follicular cell (carets in d). c, colloid; bm, basement membrane; pf, parafollicular cell. All photomicrographs are of the same magnification. Scale bar, 1 µm.