Boston doctors used cells taken from a pregnant lamb's amniotic fluid to grow a new trachea, then implanted the organ into the ailing fetus, an experiment that may pave the way for similar treatment in humans. Using cells from amniotic fluid to repair or replace human organs may bypass some of the political and ethical obstacles doctors encounter with fetal cells, said Children's Hospital Boston pediatric surgeon Dario Fauza, who led the experiments. "You avoid all the ethical dilemmas of the embryonic stem cell," Fauza said. "The cells are already there" in the mother's amniotic fluid. "We are just harvesting...