The researchers began with 132 embryos of the crab-eating macaque.Six days after fertilisation these were injected with human extended pluripotent stem cells, which can develop into any other cell type found in the body.Tagging the human cells with fluorescent markers allowed the researchers to track where in the developing embryo they, and their descendants, went.In the early stages of development, mammal embryos develop into four distinct cell types.Epiblasts go on to form the organism itself; hypoblasts develop into the yolk sac;trophectoderms become the placenta and extra-embryonic mesenchyme cells make a membrane that surrounds the embryo.The chimera's human cells made their way into all four types of tissue, though they were outnumbered in every case.No more than 7% of the epiblast was made up of human cells, and just 5% of the hypoblast (in other areas the numbers were lower still).The cells' location seemed to influence which proteins they produced.Human cells in the chimera's epiblast behaved more like those found in human embryos than those found in monkey embryos.But that was not true of human hypoblast or extra-embryonic mesenchyme cells, both of which behaved more like monkey cells.The monkey cells, in turn, were affected by the presence of the human ones.
