Cellular Classification of Pituitary Tumors
Plurihormonal tumors produce more than one hormone. Morphologically, they can be either monomorphous or plurimorphous. Monomorphous plurihormonal adenomas consist of one cell population that produces two or more hormones. The adenoma cells often differ from nontumorous adenohypophysial cells, and their cellular derivation may remain obscure despite extensive morphological studies. Plurimorphous plurihormonal adenomas consist of two or more distinct cell types, and each produces one hormone. Thyrotroph adenomas are often plurihormonal.
Nonfunctioning (Endocrine-Inactive) Adenomas
These tumors arise from the adenohypophysis and cause symptoms when they extend beyond the sella, which results in pressure on the surrounding structures rather than secretion of a hormonally active substance. Endocrine-inactive adenomas show positive immunostaining for one or more pituitary hormones; however, they are not associated with clinical and biochemical evidence of hormone excess. Gonadotrophic hormones, as detected by antisera to beta-FSH and beta-LH, are present in many clinically nonfunctioning adenomas. Some of these adenomas are recognized by electron microscopy to have gonadotrophic differentiation, but some have characteristics of less well-differentiated cells and resemble the null cells that were initially thought to be undifferentiated precursors of adenohypophysial cells. Endocrine-inactive pituitary adenomas comprise approximately 30% to 35% of the pituitary tumors in most series and are the most common type of macroadenoma.
Oncocytic tumors of the pituitary, also known as pituitary oncocytomas, are characterized by an abundance of mitochondria, which may fill up to 50% of the cytoplasmic area, which is normally around 8%, and obscure other organelles. These tumors are usually unassociated with clinical and biochemical evidence of hormone excess; in some cases, they may be accompanied by various degrees of hypopituitarism and/or mild hyperprolactinemia. Oncocytic change may occur in several other pituitary tumor types.
Pituitary carcinomas are usually endocrinologically functional, and ACTH-producing and PRL-producing tumors are the most frequent.[2,8] The histological and cytological characteristics of pituitary carcinomas vary from bland and monotonous to frankly malignant. Carcinomas show a variable degree of nuclear atypia and cellular pleomorphism, but they also show significantly higher mitotic rates and cell proliferation indices than adenomas. Carcinomas account for 0.1% to 0.2% of all pituitary tumors.[8,16]