The islets of Langerhans constitute the endocrine pancreas and are responsible for maintenance of blood glucose homeostasis. They are deeply embedded in the exocrine pancreas and therefore their accessibility for functional studies is limited. To understand regulation of function and survival and assess the clinical outcome of individual treatment strategies for diabetes, a monitoring system continuously reporting on the endocrine pancreas is needed. We describe the application of a natural body window that successfully reports on the properties of in situ pancreatic islets. As proof of principle “reporter islets” were transplanted into the anterior chamber of the eye of leptin-deficient mice. The “reporter islets” displayed obesity-induced growth and vascularization patterns that were reversed by leptin treatment. Hence “reporter islets” serve as optically accessible indicators of islet function in the pancreas, and reflect the efficiency of specific treatment regimens regulating islet plasticity in vivo.