The pancreatic B cells from rats bearing a growth hormone-producing tumor (MtT-W15) were studied by electron microscopy before and after incubation in media containing alternatively low (60 mg./100 ml.) and high (300 mg./ 100 ml.) glucose concentrations.

Tumor-bearing rats had been previously shown to have increased blood levels of somatotrophin and insulin and also to have B cells with increased responsiveness to in vitro glucose stimulation.

B cells from tumor-bearing rats, when compared with controls, showed (1) an increased number of secretory granules, (2) a marked increase in the proportion of pale to dark granules (30 to 60 per cent, as compared with 5 to 25 per cent for controls), and (3) the presence of numerous dilated cisternae of rough endoplasmic reticulum containing a flocculent material resembling the core of the pale granules. The last observation may indicate that pale secretory granules are formed in the endoplasmic reticulum of these hyperactive B cells. The fact that the hyperfunctioning B cells present in tumor-bearing rats possess a high number of pale granules suggests that these granules participate in the processes of insulin synthesis, storage, and secretion.

Pancreatic islets from control and tumor-bearing rats alternatively incubated in media with low and high glucose concentrations did not reveal significant morphological changes when compared with the nonincubated islets from the same animals. The extent of degeneration and necrosis observed in some B cells from control and tumor-bearing animals after incubation were unrelated to the glucose concentration in the incubation media.

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