sentences:  These simple transplantation methods follow from earlier observations by developmental biologists that germ stem cells are autonomous in the sense that they can begin the processes to become both sperm and eggs.One potential roadblock to injecting a woman's cells into a man's testicles is that the man's immune system might attack and destroy the woman's cells.In usual circumstances, when foreign cells (such as cells or organs from other people, or infectious bacteria) are put into a human body, its immune system will reject such cells or organs.