The main goals of our project are: 1) to evaluate the similarities and differences between human stem cell-derived spinal motor neurons and their fetal counterparts, and 2) to refine the techniques used to make these cells to facilitate motor neuron disease research and create therapeutically beneficial cells. In the final period of this project, we have completed our documentation of the developmental progression of human stem cell-derived motor neurons in cell culture compared to that seen in normal fetal development. From this analysis we conclude that the process of forming motor neurons in the culture setting faithfully replicates many aspects of their formation in the intact spinal cord. However, the types of motor neurons that are formed in stem cell cultures are more limited in their subtype diversity, which has implications for the utility of these cells as therapeutic agents and models to investigate disease mechanism. We have nevertheless found that we can extend the diversity of stem cell derived motor neurons by programming the cells to express specific proteins that promote the formation of different motor neuron subtypes. These findings suggest a general strategy for creating different functional classes of motor neurons for therapeutic uses and research applications. Lastly, we have developed a novel cell culture system to measure the communication between motor neurons and muscle cells. Breakdown in this communication is thought to underlie many motor neuron diseases, and we anticipate that this platform will provide a means for studying the underlying pathology of these diseases, and facilitate the discovery of novel therapeutic agents.