Objectives: Acute Kidney Injury (AKI) is a major global burden with an enormous financial impact on health systems and an extensive impairment of quality of life for the affected patients. The sudden drop in renal function can be caused by a variety of conditions. One of the most common causes for a decrease in renal function is reduced renal perfusion resulting in a lack of oxygen, for example, as a consequence of hypovolemic shock. The pathophysiology of ischemia-reperfusion injury (IRI) is closely related to mitochondrial dysfunction which ultimately leads to a complete disruption of the cellular energy metabolism, resulting in premature cell death. In particular, proximal tubular epithelial cells are dependent on a continuous adenosine triphosphate (ATP) supply in order to maintain the physiological function of the nephron due to their predominantly active reabsorption- and secretion mechanisms. The background of immunological mechanisms for recovery after ischemic tissue injury has been subjected to extensive research. However, it is already known that shortly after tissue injury and as part of the different phases of the immunological response, an invasion of pro-inflammatory M1 macrophages can be seen. At subsequent time points, anti-inflammatory M2 macrophages, which seem to be a contributing factor for tissue regeneration, dominate. Analysis of murine kidney sections with electron microscopy was conducted at the department of Nephropathology, Yale University. It was revealed that macrophages could partially disrupt the tubular basement membrane (TBM) in order to interact with tubular cells via cellular protrusions. Indeed, many experimental studies have suggested that macrophages have the capability to communicate with other cells over long distances via long and thin cytoplasmic protrusions. These processes, termed tunneling nanotubes (TNTs), provide the possibility for the exchange of cell organelles, for example lysosomes or mitochondria, between participating cells. In different organ systems, it has been shown that, after tissue injury, there is a beneficial effect on cell recovery caused by the anterograde transfer of mitochondria in dysfunctional cells originating from macrophages or mesenchymal stem cells. The goal of this in vitro study was to examine, 1) whether macrophages have the capability to transfer mitochondria towards tubular epithelial cells via TNTs and 2) whether this transfer has a positive impact on regeneration after cell injury. The answer for these questions required the establishment of a suitable in vitro model. Design & Methods: The hypothesis was examined with an in vitro co-culture model involving both murine bone marrow derived macrophages as well as primary murine tubular epithelial cells or an immortalized tubular cell line. In order to visualize mitochondria and cell membrane, different fluorescent dyes, transfections with fluorescent constructs and genetically modified mice strains were employed. Live cell imaging was performed with widefield fluorescence and confocal laser scanning microscopy to analyze the presence of TNTs and mitochondrial transfer. Observations & Results: We established a reliable model which provided the possibility to study cell-cell interactions without loss of fluorescence intensity over a course of several days. Moreover, the regions of interest (ROIs) were sometimes misidentified during microscopy, and the sources of these misidentifications and related pitfalls were identified. Surprisingly, the mitochondrial morphology of cells mostly remained intact in the injury condition, even though the weak staining intensity of the fluorescent dye Tetramethylrhodamine methyl ester (TMRM) already indicated a substantial decrease of the mitochondrial membrane potential. Confocal live cell imaging revealed that polarized and unpolarized macrophages were capable of engulfing mitochondrial material from injured and non-injured tubular epithelial cells. To a lesser extent, anterograde mitochondrial transfer from macrophages towards tubular cells was observed. An exchange of organelles between cells of the same cell type was also seen. The transfer seemed to be partially mediated by the formation of TNTs. As an additional finding, we showed that proximal tubular epithelial cells have the capability to phagocytose cell debris. Conclusions: This study confirmed the initial hypothesis that macrophages and tubular have the capability to exchange mitochondria. However, it remained unclear whether transferred mitochondria were functional and provided a benefit for maintenance of cell integrity. Since the fluorescence intensity of mitochondrial material remarkedly decreased after transfer, it is conceivable that mitochondria entered lysosomal degradation as part of mitophagy, a certain kind of autophagy. Further experiments are needed to investigate the functional state of transferred mitochondria and their impact more precisely. Based on the findings of this project, less importance should be attributed as previously thought to the mitochondrial morphology as an indicator for cell integrity. Beyond that, this work pointed out that a reliable quantitative analysis cannot be achieved with confocal microscopy alone due to the range of potential interpretation of 2D image data and its technical limitations. Computational algorithms rendering 2D data in 3D models or alternative methods such as fluorescence activated cell sorting (FACS) should be employed for further quantitative assessment. A more detailed understanding of the triggers of mitochondrial transfer could help to promote therapeutic strategies accelerating the healing process after ischemic kidney injury. Finally, the observation that proximal tubular epithelial cells had the capability to phagocytose dying diphtheria-toxin receptor positive cells during the isolation process when treated with diphtheria toxin, underlines the idea that tubular cells are involved in the removal of apoptotic neighboring cells after tubular necrosis.