Enlarged mitochondria and intramitochondrial dense structures have been reported to occur in a great variety of animal and human cells under normal, pathologic, and experimental conditions. Three cases of benign thyroid lesions are described in which large cytoplasmic inclusions were seen on light microscopy. These inclusions were stained with toluidine blue but not with Sudan black B, as was seen in consecutive sections. Ultrastructural observation showed that they corresponded to giant mitochondria. Bundles of dense filaments and round, electron-dense bodies were observed in the matrix of the enlarged mitochondria. Incubation of ultrathin sections with proteolytic enzymes (pepsin, trypsin, and type XVI-protease) led to a striking decrease in the electron density of both types of intramitochondrial matrical structures. These observations demonstrate that a major proteic component exists in intramitochondrial filamentous bundles and in dense bodies.

Mitochondria-rough endoplasmic reticulum complexes identical to the ones usually found in chordoma were frequently observed in tumors cells of a malignant fibrous histiocytoma arising in the left thigh of a 48-year-old woman. Although the significance of these structures is unknown, this finding is consistent with the known transformation of chordoma into malignant fibrous histiocytoma.