[Cells with unusual mitochondria in the thyroid gland of an old rat]

Thyroid glands of 3 year old rats have been studied by electron microscopy. The cells with changed mitochondria were found next to the typical thyrocytes in the follicle walls. The large mitochondria with dense matrix and tubular cristae often occupy the major part of the cytoplasm in these cells. On this basis, the author considers the changed cells to be the Askanazy cells, or oncocytes. Mitochondria-rich cells were found also among calcitocytes. The origin of oncocytes from usual thyrocytes and calcitocytes is under discussion.