Growth hormone-secreting pituitary adenoma of mixed cell type: a histological, ultrastructural and immunocytochemical study.

A case of a pituitary adenoma in an acromegalic man is presented in which growth hormone secretion was demonstrated by explant culture. Light microscopy showed the tumor to consist of acidophil "signet ring" cells which contained growth hormone on immunocytochemical studies. Electron microscopy showed a mixed population of densely and sparsely granulated growth hormone adenoma cells. This latter feature has never previously been described, and may represent dual cellular differentiation from an acidophil stem cell precursor.