In a study recently published in Science, the GABBA student Raquel Real and colleagues from the MRC London Institute of Medical Sciences (Imperial College London) and the Gurdon Institute (University of Cambridge), have developed a new model for investigating human neuron development and function in real time. By transplanting fluorescently labelled human neurons derived from iPSCs into the mouse cortex and then performing longitudinal intravital imaging of both their structure and function over several months, the authors could explore the dynamics of neuronal development in a more physiological environment. This system has also proved powerful for modelling human neurodevelopment disorders, as neurons derived from individuals with Down syndrome showed developmental abnormalities, such as a significant defect in the ability to form functional neural networks. These neurons were less active and the communication between them less coordinated. This finding could have a major impact on the development of cognitive symptoms in individuals with Down syndrome, and the next step is to understand why this communication defect occurs.