“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. They then generated pig embryos in which two specific genes that have key roles in heart development were knocked out. Lai says it’s possible the human cells disrupted the function of the pig hearts.