Feline insular amyloid. Ultrastructural evidence for intracellular formation by nonendocrine cells

The objective of this ultrastructural study in cats was to investigate the early relationship of pancreatic islet cells with amyloid deposits. We used pancreatic islets from six domestic cats with minimal and apparently early amyloid deposits. Although amyloid deposits were occasionally arranged perpendicularly to beta cells, and rarely within deep invaginations of these cells, there was no consistent or convincing relationship of extracellular fibrils to any of the major islet stricted to, nongranulated perivascular cells in islets from two of the cats. Small and relatively electron-dense amyloid inclusions contained compact arrays of parallel fibrils. Larger inclusions were more electron-lucent and had loosely and randomly arranged fibrils. Indirect evidence strongly suggested that the fibril-laden inclusions resulted from intracellular production rather than from phagocytosis. The definitive identity of these amyloid-containing cells was not determined. However, these calls lacked secretory granules specific for known islet endocrine cell types and were topographically always located in close proximity to capillaries. The results of our study, therefore, do not directly support a morphologic association of amyloid fibril formation with typical islet endocrine cells. Our results do, however, draw attention to the possibility that nonendocrine cells play a key role in the pathogenesis of insular amyloidosis in the cat.