"IT'S amazing, absolutely beautiful," says Doris Taylor, describing the latest addition to an array of tiny thumping hearts that sit in her lab, hooked up to an artificial blood supply. The rat hearts beat just as if there were inside a live animal, but even more remarkable is how each one has been made: by coating the stripped-down "scaffolding" of one rat's heart with tissue grown from another rat's stem cells. Taylor, a stem cell scientist at the University of Minnesota in Minneapolis, now wants to repeat the achievement on a much larger scale, by "decellularising" hearts, livers and other...