Towards high resolution optical imaging of beta cells in vivo.

Endocrine beta cells produce and release insulin in order to tightly regulate glucose homeostasis and prevent metabolic pathologies such as Diabetes Mellitus. Optical imaging has contributed greatly to our current understanding of beta cell structure and function. In vitro microscopy of beta cell lines has revealed the localization of molecular components in the cell and more recently their dynamic behavior. In cultured islets, interactions of beta cells with other islet cells and the matrix as well as paracrine and autocrine signaling or reaction to nutrients have been studied. Lastly, microscopy has been performed on tissue sections, visualizing the islets in an environment closer to their natural surroundings. In most efforts to date, the samples have been isolated for investigation and hence have by definition been divorced from their natural environments and deprived of vascularization and innervations. In such a setting the beta cells lack the metabolic information that is primordial to their basic function of maintaining glucose homeostasis. We review optical microscopy; its general principles, its impact in decoding beta cell function and its recent developments towards the more physiologically relevant assessment of beta cell function within the environment of the whole organism. This requires both large imaging depth and fast acquisition times. Only few methods can achieve an adequate compromise. We present extended focus Optical Coherence Microscopy (xfOCM) as a valuable alternative to both confocal microscopy and two photon microscopy (2PM), and discuss its potential in interpreting the mechanisms underlying glucose homeostasis and monitoring impaired islet function.