Pituitary hyperplasia. Definition, light and electron microscopical structures and significance in surgical specimens.

Surgical specimens of 15 normal and 106 para-adenomous anterior pituitaries were studied immunocytochemically and in part electron microscopically for the presence of hyperplasia. GH cell hyperplasia was found in 13% of all normal pituitaries, in 6% of the cases with Prolactin secreting adenomas and in 9% of the cases with ACTH secreting adenomas. Prolactin cell hyperplasia occurred in nearly equal percentages (17-23%) in normal pituitaries and in areas adjacent to GH-, Prolactin- or ACTH-secreting adenomas or adjacent to inactive adenomas. Previous findings of relatively more frequent Prolactin cell hyperplasia occurring together with Prolactin producing adenomas have to be revised. Prolactin cell hyperplasia as a primary source of hyperprolactinemia is very rare and almost always occurs in conjunction with oncocytic adenomas. ACTH cell hyperplasia was found in 13% of the normal pituitaries, in 14% of the cases with Prolactin secreting adenomas, in 58% of the cases with ACTH producing adenomas and in 40% of the pituitaries with GH secreting adenomas. We have no explanation for the latter result. ACTH cell hyperplasia may be the primary cause of Cushing's disease (18% of all Cushing cases). Hyperplasia of TSH cells in normal pituitaries was rare (7%) and with the exception of Prolactin producing adenomas (22%) was not found near adenomas. Clinical-pathological correlations are discussed.