islet

Irregular microscopic structures scattered throughout the pancreas and comprising its endocrine part (the endocrine pancreas). In humans, they are composed of at least four types of cells: the alpha cells, which secrete glucagon; the beta cells, which are the most abundant and secrete insulin; the delta cells, which secrete somatostatin; and the PP cells, which secrete pancreatic polypeptide. Degeneration of the beta cells, whose secretion (insulin) is important in carbohydrate metabolism, is the major cause of type I diabetes mellitus.