Somatostatin cells in human somatotropic adenomas

Data from our group have shown that the human adenomatous and normal anterior pituitary may be the source of somatostatin (SRIH). SRIH-producing cells were identified in two somatotropic adenomas. Immunoreactive SRIH cells were present in both cases. In case 2, material was available for RNA studies, in situ hybridization and electron microscopy. The size of the transcript identified by Northern blot analysis was identical to that of hypothalamic SRIH mRNA. In situ hybridization showed that the SRIH gene was expressed in a cell subset superimposable to that identified by immunocytochemistry. Co-localization studies revealed that SRIH and growth hormone (GH) immunoreactivities were not present in the same cells. Ultrastructural immunogold labelling showed that SRIH cells had features distinct from those of the somatotropes. The results confirm that the somatotropic adenomas have the ability to synthesize SRIH, indicate that SRIH expression is restricted to a subset of adenoma cells different from GH-producing cells, and imply that SRIH cells are involved in paracrine regulation of neighbouring somatotropes.