New markers for pancreatic islets and islet cell tumors.

Islets of Langerhans account for 2 g of endocrine tissue in the pancreas, comprising approximately one million islets, with each containing 1000 endocrine cells. The major hormone secreted from the islets is insulin, which regulates blood glucose, the main fuel of the body. Islets also secrete glucagon, somatostatin and pancreatic polypeptide and all are involved in the paracrine mechanism. Islet cells can be stained immunohistochemically for the general endocrine markers, chromogranin A, synaptophysin, neuron-specific enolase and Leu7. Beta islet cells are well equipped with glucose transporter 2, which binds to glucose and regulates diffusion of glucose through the beta cell membrane. As all four islet hormones are initially synthesized as prohormones, all islet cells are equipped with prohormone convertase 1/3 and 2. In addition, islet cells also contain zinc-containing matrix metalloproteinases and their inhibitors, metallothionein, cyclin-dependent kinases and insulin-like growth factors, and many more hormones, peptides and enzymes. Thus, islets not only secrete insulin and other pancreatic hormones but are a complex organ whose major function is glucose homeostasis.