6Screening cells for their differentiation potential requires a combination of tissue culture 7 models and imaging methods that allow for long-term tracking of the location and function of 8 cells. Embryonic kidney re-aggregation in vitro assays have been established which allow for 9 the monitoring of organotypic cell behaviour in re-aggregated and chimeric renal organoids. 10However, evaluation of cell integration is hampered by the high photonic load of standard 11 fluorescence microscopy which poses challenges for imaging three-dimensional systems in 12 real-time over a time course. Therefore, we employed light sheet microscopy, a technique that 13 vastly reduces photobleaching and phototoxic effects. We have also developed a new method 14 for culturing the re-aggregates which involves immersed culture, generating organoids which 15 more closely reflect development in vivo. To facilitate imaging from various angles, we 16 embedded the organoids in a freely rotatable hydrogel cylinder. Endpoint fixing and staining 17 were performed to provide additional biomolecular information. We succeeded in imaging 18 labelled cells within re-aggregated kidney organoids over 15 hours and tracking their fate while 19 simultaneously monitoring the development of organotypic morphological structures. Our 20 results show that Wt1-expressing embryonic kidney cells obtained from transgenic mice could 21 integrate into re-aggregated chimeric kidney organoids and contribute to developing 22 nephrons. Furthermore, the nascent proximal tubules that formed in the re-aggregated tissues 23