The Massachusetts-based Advanced Cell Technology removed embryonic stem cells from mice embryos with no apparent damage, the journal Nature said. Scientists believe it would sidestep some of the ethical objections to stem cell research if repeated in humans. But campaigners said they still had concerns and urged scientists to concentrate on other areas. Stem cells are “master” cells that can become many kinds of tissue. Those harvested from early-stage human embryos are thought to hold the most potential for research, as they have the ability to become almost any kind of adult cell in the body.