In Miami, scientists have injected bone marrow-derived stem cells into 40 people whose cardiac muscle was deadened by heart attacks. In Santa Monica, Calif., human embryonic stem cells are being studied for their potential to restore vision to the blind. And the U.S. Food and Drug Administration recently gave biotech firm Geron the first green light to inject human embryonic derived stem cells into people with paralysis. It’s believed the cells could enable people with spinal injuries to walk again.