Ultrastructure of the thyroid glands of rats fed photomirex: a 48-week recovery study.

The thyroid glands of rats that received photomirex (0.05, 0.5, 5 or 50 ppm) or mirex (5 or 50 ppm) in the diet for 28 days and were then kept on clean feed for 48 weeks were examined by an electron microscope. A dose of 0.05 or 0.5 ppm photomirex produced cytoplasmic vacuolation in the principal follicular cells; the cells were taller than the cuboidal follicular cells found in the glands of animals from the control group. An extensive accumulation of deformed lysosomal bodies, a reduction in the number of apical vesicles and a mild dilatation of rough-surfaced endoplasmic reticulum (RER) cisternae were the features of columnar follicular cells at the 5 ppm photomirex level. In the animals receiving 50 ppm photomirex the follicular cells had more severe alterations, including a marked dilatation of RER profiles, compared to those in the preceding groups. The cells from rats ingesting 5 ppm mirex were least affected and had only a mild dilatation of RER elements, and some cytoplasmic vacuolation. The follicular cells of animals fed 50 ppm mirex showed variable morphology; some of these cells had an increased number of colloid droplets and lacked the lamellar arrays of RER; others contained vacuoles or somewhat dilated RER profiles. The present results indicate that following the ingestion of photomirex (0.05-50 ppm) or mirex (50 ppm) for 28 days, alterations in the rat thyroids persist for at least one year.