First human-monkey embryo sparks debate A joint team of US and Chinese scientists have, in breakthrough research, grown human cells in monkey embryos in a laboratory for up to 20 days Representative image New York: A joint team of US and Chinese scientists have, in breakthrough research, grown human cells in monkey embryos in a laboratory for up to 20 days. The development, which has sparked ethical debates, has implications for developing new models of human biology and disease. Interspecies chimeras in mammals have been made since the 1970s, when they were generated in rodents and used to study early developmental processes. In the current study, detailed in the journal Cell, six days after the monkey embryos had been created, each one was injected with 25 human cells. The cells were from an induced pluripotent cell line known as extended pluripotent stem cells, which have the potential to contribute to both embryonic and extra-embryonic tissues.