Scientists said on Wednesday that they have successfully used mice stem cells to grow kidneys in rat embryos, using a technique that could one day help grow human kidneys for transplant. But the researchers cautioned that their success was only a first step and that “serious technical barriers and complex ethical issues” remain before the process could be used for human organs. The technique has previously been used to grow mice-derived pancreases in rats, but the new study is the first evidence that it could one day provide a solution to the massive shortage of donor kidneys for people with renal disease. The research, published in the Nature Communications journal, began with the development of a suitable “host” in which the kidneys could be grown. Genetically modified The researchers collected rat embryo structures that had been genetically modified so they would not develop kidneys on their own. The embryos were then injected with pluripotent stem cells from mice and implanted into rat wombs so they could be carried to term. Pluripotent stem cells are a kind of “master” cells that can develop into any of the cells and tissue that make up the body. The researchers found that the mice stem cells produced apparently functional kidneys in the rats. But the same did not hold when rat stem cells were injected into similarly modified mice embryos. The process of growing human organs in animals poses an ethical conundrum because human stem cells could develop into brain or reproductive organ cells in the host. In the short-term, additional research is likely to focus on ways to genetically modify host rats without lethal side effects.