The main goals of this project are to evaluate the similarities and differences between human stem cell-derived spinal motor neurons and their fetal counterparts, and to refine the techniques used to make these cells to facilitate motor neuron disease research and create therapeutically beneficial cells. In the first year of this project, we have confirmed that motor neuron generated from stem cells exhibit many molecular and physiological changes over time that closely mirror the formation of motor neurons during normal human development. There are some subtle differences, however, and our ongoing work will explore whether these discrepancies have any functional relevance. In carrying out these experiments, we also discovered new techniques by which we can create more diverse populations of motor neurons that better match the complexity seen in the spinal cord. Lastly, we have made significant progress in developing experimental assays to study the connections formed between stem cell-derived motor neurons and their muscle targets. We anticipate that these assays will serve as a valuable platform for modeling the pathology of human motor neuron diseases.