Ultrastructural markers in the pathologic diagnosis of pituitary adenomas.

The functional-morphologic classification of pituitary adenomas increased the number of distinguishable morphologic entities from 3 (acidophilic, basophilic, chromophobic) to 15. Identification of tumor types requires histology as well as immunohistochemistry and electron microscopy. Electron microscopy has a pivotal role in the recognition and separation of entities with overlapping immunohistochemical profiles. Electron microscopic diagnosis is facilitated considerably by structural markers present in the majority of adenoma types. Two groups of ultrastructural markers are discussed. Organelle markers are represented by specific morphology/arrangement of ubiquitous cytoplasmic constituents (rough endoplasmic reticulum, Golgi apparatus, mitochondria). Special markers are usually restricted to a particular pituitary cell type, its tumors, or both (filamentous aggregates, particular morphology of secretory granules, etc). Evaluation of general ultrastructural features and identification of structural markers permit conclusive diagnosis in the majority of pituitary adenomas. Cell derivation remains uncertain in many so-called clinically nonfunctioning adenomas with no or insufficient markers and poorly developed cytoplasmic organization associated with low endocrine activity.