The research involved creating human-pig chimeric embryos containing a combination of human and pig cells. When transferred into surrogate pig mothers, the developing embryos were shown to have kidneys that contained mostly human cells, marking the first time that scientists have grown a solid humanized organ inside another animal. The kidneys were not entirely human as they included vasculature and nerves made mostly from pig cells, meaning they could not be used for transplantation in their current form. It is not clear whether the challenge of making a wholly human organ would be achievable with current genetic engineering techniques.
Aside from the kidneys, the embryos were dominated by pig cells, with very few human cells in the brain or central nervous system. The potential for a humanized brain is a serious ethical concern for research involving hybrid embryos and one of the reasons for tight legal restrictions on research in many countries. […] After being cultivated in the lab, the chimeric embryos were transferred to 13 surrogate sows. After either 25 or 28 days, the gestation was terminated and embryos were extracted and assessed. The embryos had structurally normal kidneys for their stage of development, showing the tubules that would eventually connect the kidney to the bladder, and were composed of 50-60% human cells. Very human neural cells were found in the brain and spinal cord. The research has been published in the journal Cell Stem Cell.
