Reported ultrastructural studies of beta cell islet tumors have related to lesions confined within the pancreatic substance. Electron microscopy of a metastatic deposit from a nonfunctioning beta cell tumor is now presented. The investigation has revealed a highly complex and relatively orderly cytoplasmic structure—in conformity with the low grade of clinical malignancy exhibited by these lesions. Beta granules were present within many cells, but were usually sparse. They resembled the beta granules which have been described within both normal and neoplastic beta cells. The granules differed from alpha granules in the configuration, structure, relative size and electron density, of the central core. Characterization of islet cell tumors by virtue of the ultrastructure of their specific granules can now be achieved through the application of established criteria.

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