Ultrastructure of three Hürthle cell tumors of the thyroid

Three cases of Hürthle cell tumors of the thyroid gland were studied by light and electron microscopy. The most distinctive ultrastructural feature appeared to be the enormous accumulation of mitochondria in the cytoplasm of the Hürthle cells. This mitochondrial abundance was so marked in some cells that the remaining cytoplasmic organelles were obscured. Our findings support the theory that Hürthle cells arise by transformation from follicular cells. The cause and functional significance of this process is not clear. It remains to be seen whether mitochondrial accumulation results primarily from increased formation or delayed breakdown.