The source of the new islets during growth and the effect of aging on the number of islets have been studied in Webster Swiss mice. A combination of methods was employed, including microscopy in vivo, enumeration of the total number of islets, radioautography with H-3-thymidine, and conventional histological technics. The total number of islets in normal mice increases with age until about six months, and then becomes stable or decreases slightly. Radioautographs showed that the cells of the islets are in all likelihood derived from migrating ductal or periductal cells residing at the junction between the islet and pancreatic duct and at the periphery of the islet, and that these cells doubtless serve as the progenitor pool for the growth of the islet. The nuclear cycle of the insular cells was also determined by this method. In vivo study revealed that the structural integrity of ducts and of their vascular supply is important for the maintenance of the population of ductal and periductal islets.

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