From Scientific American:
Su-Chun Zhang of the University of Wisconsin-Madison and his colleagues exposed human embryonic stem cells to a variety of growth factors and hormones in sequence in order to encourage them to change into motor neurons. “You need to teach the [embryonic stem cells] to change step by step, where each step has different conditions and a strict window of time,” Zhang explains. “Otherwise, it just won’t work.” The embryonic stems cells first became neural stem cells then changed into the beginnings of motor neurons before finally differentiating into spinal motor neuron cells, the cell type that, in the human body, transmits messages from the brain to the spinal cord. The newly generated motor neurons exhibited electrical activity, the signature action of neurons, and survived in culture for more than three months.