Researchers have reported growing hearts containing human cells in pig embryos for the first time. The embryos survived for 21 days, and in that time their tiny hearts started beating. The findings were presented this week at the annual meeting of the International Society for Stem Cell Research in Hong Kong. Scientists developing human–animal chimaeras grow human cells in animal embryos, with the aim of one day generating animals with human organs that can be transplanted into people. This could provide a way to address the global shortage of organs for transplantation. One approach to developing chimaeras involves creating animal embryos that lack some of the genes needed to produce a specific organ, such as the heart. Human stem cells are then injected into the embryos, with the hope that the human cells — rather than those of the animal – will form that organ. Several groups have used this method to grow human muscle and blood-vessel cells in pig embryos. Pigs are a suitable donor species because the size and anatomy of their organs are comparable with those of humans, says Lai Liangxue, a developmental biologist at the Guangzhou Institutes of Biomedicine and Health, Chinese Academy of Sciences, who led the latest work. Lai’s team has previously grown early-stage human kidneys in pig embryos that survived for up to a month in pregnant sows. He wanted to see whether similar results were possible for the heart.