Irregular microscopic structures consisting of cords of endocrine cells that are scattered throughout the pancreas among the exocrine acini. Each islet is surrounded by connective tissue fibers and penetrated by a network of capillaries. There are four major cell types. The most abundant beta cells (50-80%) secrete insulin. Alpha cells (5-20%) secrete glucagon. PP cells (10-35%) secrete pancreatic polypeptide. Delta cells (~5%) secrete somatostatin.