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Pituitary oncocytoma. Indispensable role of electron microscopy in its identification.

Two surgically resected pituitary adenomas that appeared to be chromophobe or poorly granulated acidophil adenomas by light microscopy were correctly identified as oncocytomas by electron microscopy. The tumor cells ahd markedly hyperplastic, moderately pleomorphic mitochondria in the cytoplasm of virtually all cells. Scattered secretory granules were displaced to the periphery of the cytoplasm but the secretion type of pituitary cell remained obscure. Histochemical and immunocytochemical methods imperfectly characterized these unusual tumors. Even though these neoplasms have been considered rare, they are now being recognized more often after ultrastructural study. Eventually their pathobiologic features may be better understood. One of our patients was 24 years old and is the youngest patient, to our knowledge, in whom a pituitary oncocytoma has been documented.