Advances in stem cells have allowed scientists to cultivate various types of tissue in the lab, yet some tissue types have remained out of reach. The germ cells that produce sperm, whose cell development processes is one of the most specialized, are among those that have eluded researchers. But successfully generating sperm from stem cells would allow infertile men to become fathers, where current efforts to expand fertility to more people for longer stretches of their lives have focused primarily on women. MORE