The cytoarchitecture of human islets has been examined, focusing on cellular associations that provide the anatomical framework for paracrine interactions. By using confocal microscopy and multiple immunofluorescence, we found that, contrary to descriptions of prototypical islets in textbooks and in the literature, human islets did not show anatomical subdivisions. Insulin-immunoreactive ␤ cells, glucagon-immunoreactive ␣ cells, and somatostatin-containing ␦ cells were found scattered throughout the human islet. Human ␤ cells were not clustered, and most (71%) showed associations with other endocrine cells, suggesting unique paracrine interactions in human islets. Human islets contained proportionally fewer ␤ cells and more ␣ cells than did mouse islets. In human islets, most ␤, ␣, and ␦ cells were aligned along blood vessels with no particular order or arrangement, indicating that islet microcirculation likely does not determine the order of paracrine interactions. We further investigated whether the unique human islet cytoarchitecture had functional implications. Applying imaging of cytoplasmic free Ca 2؉ concentration, [Ca 2؉ ]i, we found that ␤ cell oscillatory activity was not coordinated throughout the human islet as it was in mouse islets. Furthermore, human islets responded with an increase in [Ca 2؉ ]i when lowering the glucose concentration to 1 mM, which can be attributed to the large contribution of ␣ cells to the islet composition. We conclude that the unique cellular arrangement of human islets has functional implications for islet cell function.␣ cell ͉ ␤ cell ͉ cytoplasmic free Ca 2ϩ concentration ͉ insulin ͉ glucagon I n the last three decades, hundreds of individuals with type 1 diabetes mellitus have received allogeneic transplants of endocrine pancreas, the islets of Langerhans, to cure their chronic condition. In these patients, diabetes is reversed by transplanting cells capable of physiologically regulating insulin secretion. Determining the quality of islets obtained from cadaveric pancreata should be indispensable in this context. However, it is not known which physiological parameters correlate best with a fully functional islet capable of reversing diabetes after transplantation. There is a wealth of information about the physiology of rodent islets, but the biology of human islets remains poorly understood. As assays for determining islet quality are being developed by many laboratories in the field of islet transplantation, a reassessment of the structure and function of human islets is warranted.The islets of Langerhans are small organs located in the pancreas that are crucial for glucose homeostasis. Islets typically consist of four types of secretory endocrine cells, namely, the insulin-containing ␤ cells, the glucagon-containing ␣ cells, the somatostatin-containing ␦ cells, and the pancreatic polypeptideproducing (PP) cells. In rodent islets, the vastly predominating ␤ cells are clustered in the core of a generally round islet, surrounded by a mantle of ␣, ␦, and PP cells. Thus, ...

The endocrine part of the pancreas plays a central role in blood-glucose regulation. It is well established that an elevation of glucose concentration reduces secretion of the hyperglycaemia-associated hormone glucagon from pancreatic alpha 2 cells. The mechanisms involved, however, remain unknown. Electrophysiological studies have demonstrated that alpha 2 cells generate Ca2+-dependent action potentials. The frequency of these action potentials, which increases under conditions that stimulate glucagon release, is not affected by glucose or insulin. The inhibitory neurotransmitter gamma-aminobutyric acid (GABA) is present in the endocrine part of the pancreas at concentrations comparable to those encountered in the central nervous system, and co-localizes with insulin in pancreatic beta cells. We now describe a mechanism whereby GABA, co-secreted with insulin from beta cells, may mediate part of the inhibitory action of glucose on glucagon secretion by activating GABAA-receptor Cl- channels in alpha 2 cells. These observations provide a model for feedback regulation of glucagon release, which may be of significance for the understanding of the hypersecretion of glucagon frequently associated with diabetes.

Summary A hallmark of type 2 diabetes mellitus (T2DM) is the development of pancreatic β cell failure, resulting in insulinopenia and hyperglycemia. We show that the adipokine adipsin has a beneficial role in maintaining β cell function. Animals genetically lacking adipsin have glucose intolerance due to insulinopenia; isolated islets from these mice have reduced glucose-stimulated insulin secretion. Replenishment of adipsin to diabetic mice treated hyperglycemia by boosting insulin secretion. We identify C3a, a peptide generated by adipsin, as a potent insulin secretagogue and show that the C3a receptor is required for these beneficial effects of adipsin. C3a acts on islets by augmenting ATP levels, respiration and cytosolic free Ca2+. Finally, we demonstrate that T2DM patients with β cell failure are deficient in adipsin. These findings indicate that the adipsin/C3a pathway connects adipocyte function to β cell physiology and manipulation of this molecular switch may serve as a novel therapy in T2DM.

Advanced imaging techniques have become a valuable tool in the study of complex biological processes at the cellular level in biomedical research. Here, we introduce a new technical platform for noninvasive in vivo fluorescence imaging of pancreatic islets using the anterior chamber of the eye as a natural body window. Islets transplanted into the mouse eye engrafted on the iris, became vascularized, retained cellular composition, responded to stimulation and reverted diabetes. Laserscanning microscopy allowed repetitive in vivo imaging of islet vascularization, beta cell function and death at cellular resolution. Our results thus establish the basis for noninvasive in vivo investigations of complex cellular processes, like beta cell stimulus-response coupling, which can be performed longitudinally under both physiological and pathological conditions. Adequate release of insulin by pancreatic beta cells in response to changing blood glucose levels is a vital requirement for maintaining glucose homeostasis. Failure to do so is one of the major causes of type 2 diabetes mellitus, the most common metabolic disorder in humans 1 . Under physiological conditions, insulin release is regulated by the complex interplay between glucose and a plethora of additional factors-for example, nutrients, autocrine-paracrine signaling and the continuous input from hormones and neurotransmitters 2 . Beta cells, together with other pancreatic endocrine cell types, are situated within the endocrine pancreas, that is, the islets of Langerhans, which are densely vascularized 3 and abundantly innervated 4 . Pancreatic islets, constituting 1%-2% of the pancreatic volume, are difficult to access for in vivo monitoring because they are deeply embedded and scattered in the exocrine tissue of the pancreas 5 . As a consequence, the majority of functional beta cell studies have so far been conducted in vitro on isolated islets or beta cells. Isolated islets 6 , and especially pancreatic slices 7 , allow functional studies of Author Contributions: S.S., D.N. and A.C. developed the experimental transplantation platform. S.S., D.N., O.C., J.Y., R.D.M., A.P., T.M., M.K., B.L. and A.C. did the experiments. J.W. was responsible for generating the transgenic mice. C.R. was involved in designing the transplantation protocols and writing the manuscript. S.S., D.N., I.B.L. and P.-O.B. were responsible for designing the overall experimental plan and writing the manuscript. P.-O.B. was the originator of the idea of using the anterior chamber of the eye for noninvasive in vivo imaging of pancreatic islet cell biology Reprints and permissions information is available online at http://npg.nature.com/reprintsandpermissions Note: Supplementary information is available on the Nature Medicine website. Laser-scanning microscopy (LSM) of isolated islets and cell preparations has been successfully applied for imaging multiple signaling pathways in the beta cell 6 . However, intravital applications of LSM for studies of beta cell physiology have not been repo...