Ultrastructure of the thyroid glands of rats fed photomirex: an 18-month recovery study.

The thyroid glands, from rats which had received photomirex (0.05, 0.5, 5 or 50 ppm) or mirex (5 or 50 ppm) in their diets for 28 days and maintained for 18 months on clean diet, were examined by electron microscopy. A dose of 0.05-5 ppm photomirex resulted in an augmentation of principal follicular cell heights. Their cytoplasm exhibited a marked numerical increase of secondary lysosomes. In addition, at 5 ppm, follicular cells in some segments of the gland contained only a few secondary lysosomes. After dietary exposure to 50 ppm photomirex the number of lysosomal elements remained elevated in epithelial cell cytoplasm. Morphological aberrations in the follicles could not be detected after the 5 ppm mirex treatment. The most remarkable alterations in this experiment occurred in the thyroid glands of animals ingesting 50 ppm mirex where columnar follicular cells were engorged with deformed lysosomal bodies. Remaining components of the follicular cell architecture in all the treated groups were unaffected and were similar to those in the control group. These results demonstrated that following a 28-day dietary exposure of rats to photomirex (0.05-50 ppm) or mirex (50 ppm), alterations in the thyroid glands persisted for at least 18 months.