In the adult human pancreas the majority of islets have a clear non-random organization of mantle of non beta cells around beta cell cores, with larger islets usually having multiple such subunits (composite) whereas smaller islets resembled the classic rodent pattern. There is far more variability in islet composition both within each human pancreas and among different human pancreas than in rodent pancreas. The intrapancreatic variability in human pancreas raises important questions about how islets evolve and function throughout life and how this might relate to the pathogenesis of diabetes. We will examine the origin of the composite structure of the human islets by histological analysis of pancreas of donors from newborn to 25 years old. Determining the islet organization pattern (composite vs simple) with the distribution of islet size over the age range of expansion of the beta cell mass will provide data for mathematical modeling, which will predict the mechanism of formation of the composite islets.