Like fine chefs, scientists are seemingly approaching a day when they will be able to make nearly any type of tissue from human embryonic stem cells. You need nerves or pancreas, bone or skin? With the right combination of growth factors, skill and patience, a laboratory tissue culture dish promises to yield therapeutic wonders. But within these batches of newly generated cells lurks a big potential problem: Any remaining embryonic stem cells -- those that haven’t differentiated into the desired tissue -- can go on to become dangerous tumors called teratomas when transplanted into patients.
