Ultrastructure of functioning beta cell tumors of the pancreatic islets

Eight cases of functioning beta cell tumors with recurrent hypoglycemia, including 3 with metastases, were studied by electron microscopy. Based on their ultrastructure, the tumors were classified into 2 types. Type 1 was composed of cells with abundant typical beta granules which showed a close resemblance to the non-neoplastic beta cells of the pancreatic islets. Type 2 consisted mainly of cells without the typical granules, intermingling with a small number of cells which revealed a few, immature beta granules. Numerous dense granules resembling lysosomes were found in the majority of the cells. The cells of this type possessed extensive interdigitation of the cell membrane forming the intercellular spaces. Sometimes intracellular canaliculi were also found. Thus, it was demonstrated that the cells of type 2 tumors had morphological similarities to the cells of the terminal ductules of the pancreas. These results tend to suggest that the ultrastructure, including the formation of the secretory granules, reflects the stages of differentiation of the tumor cells.