Article Cell Differentiation Hox protein Humans Motor Neurons Neurogenesis Pluripotent Stem Cells Spinal Cord anterior posterior axis cell differentiation cell population cell therapy clinical protocol controlled study embryo development human human cell human pluripotent stem cells (hPSCs) in vitro study motoneuron motor neurons nervous system development neuromesodermal progenitors pluripotent stem cell rhombencephalon spinal cord stem cell thoracic spinal cord