Insular amyloid in a case of type III glycogenosis with a special reference to the origin of amyloid fibrils.

Amyloid depositions of pancreatic islets were investigated with electron microscopy in a case of type III glycogenosis. Beta cells adjoining small amyloid depositions were shown to have cytoplasmic invaginations where closely packed amyloid fibrils were disclosed regularly orientated amyloid bundles. In the cytoplasm of the beta cells, some membrane-bounded vesicles contained amyloid fibrils and a few beta granules directly transformed into the fibrils within the vesicles. These findings indicate that, at least in this case, the beta cells play a crucial role in the formation of insular amyloid.