While there have been some attempts to grow human organs in Petri dishes, the process is often tedious, and in any case, these organs are never identical to the ones that develop inside an actual human. To create the hybrid embryos, the Salk Institute team injected human pluripotent stem cells (adult cells that have been reverted back into an embryonic state) into the partially developed embryos of pigs. According to study author Jun Wu, approximately one out of every 100,000 cells in the embryos were human rather than pig, with the human cells developing into precursors of various tissue types, including heart, liver, and neurons.