Giant mitochondria in rat liver cells after short-term administration of propylthiouracil.

Chronic administration of propylthiouracil for five weeks to rats induced marked ultrastructural changes in their liver mitochondria, consisting in round or oval transformation as well as in the presence of a few giant mitochondria per cell section. A decreased number of cristae was a common feature of both types of mitochondria. The more enlarged mitochondria frequently contained filaments within intracristal spaces or between the inner and the outer mitochondrial membranes. These structures were formed by helices, 35-45 A thick, with helical diameter and pitch about 130-150 A. The round or oval transformation of mitochondria and the decreased number of cristae are probably the morphological counterpart of the antithyroid effect of the drug. The formation of giant mitochondria may well represent a hepatotoxic action of propylthiouracil. The possibility of a relation between the helical filaments and abnormal lipidic composition of mitochondrial membranes is favoured.