The transplant was decades in the making: the Belgian doctors began preserving testicular tissue from young patients undergoing treatments that had a high risk of causing sterility in 2002. “If I would take out tissue of my testis and graft it in your testis, you would reject it. After the researchers first started freezing the testicular tissue, they spent years perfecting their technique in animal models. The subject, who had his tissue frozen in 2008, approached the team, saying he was ready to have children with his girlfriend. The procedure has another drawback: the grafts were made in tissue that doesn’t connect with the normal passageways of ejaculation.