Electron-microscopical immunocytochemical study of a pancreatic islet amyloid polypeptidoma.

A 50-year-old man developed a pancreatic islet tumor with liver metastases. High levels of islet amyloid polypeptide (IAPP) were recorded in plasma-35,000 pmol/L-concomitant with the occurrence of type II diabetes mellitus (the clinical syndrome has recently been described in detail) [25]. Light microscopically, the tumor contained considerable amounts of amyloid and displayed IAPP immunoreactivity both in the tumor cells and in the amyloid stroma. Electron-microscopical examination of the liver metastases showed presence of round secretory granules in the tumor cells. The granules were immunoreactive to chromogranin A and B and IAPP but not to insulin. The amyloid deposits were mainly accumulated in the extracellular spaces but were also present in the tumor cell cytoplasm. The intracellufar amyloid fibrils were, as revealed by immunogold labeling, IAPP immunoreactive and seemed to emerge from the secretory granules in the shape of radiating threads. The results show that in this particular case, the amyloid formation started already at the intracellular level and in close proximity to the lAPP-storing secretory granules. The findings may have some significance for understanding the development of pancreatic islet B-cell-related amyloidosis in type II diabetes mellitus.