Comparative toxicity of polychlorinated biphenyl and polybrominated biphenylin the rat thyroid gland: light and electron microscopic alterations after subacute dietary exposure.

The comparative toxicity of polychlorinated biphenyl (PCB) and polybrominated biphenyl (PBB) in thyroid glands was studied in male Holtzman rats. Four-week-old animals were fed at dietary levels of 0, 5, 50, and 500 ppm for 5 weeks and then sacrificed. PCB and PBB produced similar dose-dependent ultrastructural lesions in thyroid follicular cells of rats. The daily administration of 5 ppm of either PCB or PBB resulted in the accumulation of large membrane-limited colloid droplets and electron-dense lysosomal bodies within the cytoplasm of follicular cells. Microvilli were short and abnormally branched, and unique cytoplasmic processes extended from the apical surface of follicular cells into the luminal colloid. Similar but more severe ultrastructural changes were observed in thyroid glands of rats administered 50 and 500 ppm of either PCB or PBB. Many follicular cells were distended with large abnormal lysosomal bodies and colloid droplets. Mitochondria were often vacuolated with disrupted cristae. Microvilli were blunt with abnormal branching or absent from areas of the luminal surface of follicular cells. Processes of apical cytoplasm often extended into the follicular lumen in areas devoid of microvilli. Follicular cells remained responsive after the feeding of either PCB or PBB and underwent moderate compensatory hypertrophy and hyperplasia. Thyroid follicles were smaller than in controls and were lined by more columnar cells that occasionally formed papillary projections into the follicular lumens. PCB and PBB produced similar ultrastructural lesions in thyroid follicular cells which appeared to interfere with the synthesis and secretion of thyroxine.