Human cells integrated into pig and cow embryos

Scientists have shown for the first time that undifferentiated human stem cells can be integrated into early embryos of both pigs and cattle. The construction of such interspecies embryos may represent an important development in the generation of organs for transplant.

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There has been much biological research aimed at creating chimeras, which are organisms that develop when embryonic cells from an animal of one species are transplanted into the embryo of an animal of a different species.

Such artificially developed creations are important in gaining insights into how a species has evolved and how embryos develop. They have become increasingly important tools for furthering understanding of human disease processes and are the focus of research into the possibility of growing human organs for transplant in another animal.

Mice and monkey chimaeric embryos have been produced, and human stem cells integrated into nonviable mouse embryos have been shown to develop into very early-stage tissues. The latest research, however, has taken the technology a step further and grown functional organs from one species within an animal of a different species.

Initially, the team successfully grew rat organs, including pancreas, heart and eyes, in a developing mouse. Remarkably, the introduction of rat stem cells into a mouse embryo led to the development of a rat gall bladder within a mouse, despite the organ not being present in rats. The mice with rat organs grew healthily and had a normal lifespan indicating that they had developed normally.

Professor Juan Carlos Izpisua Belmonte of the Salk Institute commented:

Our findings may offer hope for advancing science and medicine by providing an unprecedented ability to study early embryo development and organ formation, as well as a potential new avenue for medical therapies... We have shown that a precisely targeted technology can allow an organism from one species to produce a specific organ composed of cells from another species. This provides us with an important tool for studying species evolution, biology and disease, and may lead ultimately to the ability to grow human organs for transplant."

Since mice are too small to host most human organs needed for transplant surgery, the technique was then used to integrate human cells into early-stage pig and cattle embryos. Development was stopped after four weeks for an evaluation of safety and success. The team discovered that the human cells within some of the pig embryos were beginning to specialize. However, the success rate was much lower than that achieved in mice and the introduction of human cells tended to hinder the normal growth of the pig embryo.

Although this represents a breakthrough in chimaeric science, extensive further research is needed to optimize the technology and make it a valid option for medical therapies.

  1. Salk Institute press release 26 January 2017. Available at
  2. Wu J, Platero-Luengo A, M Sakurai, et al. Interspecies Chimerism with Mammalian Pluripotent Stem Cells. Cell 2017;168(3):473–486.
Kate Bass

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Kate Bass

Kate graduated from the University of Newcastle upon Tyne with a biochemistry B.Sc. degree. She also has a natural flair for writing and enthusiasm for scientific communication, which made medical writing an obvious career choice. In her spare time, Kate enjoys walking in the hills with friends and travelling to learn more about different cultures around the world.


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