oncocytes

Oncocytes are epithelial cells characterized by an enlarged size and an abundant, finely granular, eosinophilic cytoplasm. This cytoplasmic appearance results from a dramatic proliferation of mitochondria, which can occupy much of the cell and give the cells their distinctive appearance. The nuclei are typically small and round with a relatively inconspicuous appearance compared with the cytoplasm.

Oncocytes occur in several tissues, most notably in the salivary glands, thyroid, and kidneys. In the salivary glands they can appear as diffuse or focal oncocytosis and may give rise to oncocytic neoplasms, such as oncocytomas (generally benign) or oncocytic carcinomas (malignant). In the thyroid, oncocytes are referred to as Hurthle cells and can be seen in Hurthle cell adenomas or carcinomas. Renal oncocytes are seen in renal oncocytoma and, less commonly, in certain renal cell carcinomas. Oncocytic changes may also be detected in other organs and in aging tissues.

Clinically, oncocytosis represents a non-neoplastic proliferation of oncocytes, while oncocytomas are usually well-circumscribed tumors. Distinguishing benign oncocytomas from oncocytic carcinomas relies on histologic criteria, including invasion and cytologic atypia. Immunohistochemistry and ultrastructural studies can support the diagnosis by highlighting the mitochondria-rich cytoplasm and characteristic organelle distribution.

The term oncocyte emphasizes the mitochondria-rich cytoplasm rather than a specific disease and reflects both benign and malignant entities. The concept is important for understanding certain salivary, thyroid, and renal lesions and their diagnostic differentiation.