The advent of chimeras, hybrid animal-human embryos

The latest advance presented in this field is that of lamb embryos in which one of every 10,000 of its cells is human.

A chimera created by Juan Carlos Ispízua's team at the Salk Institute.

To achieve a world in which no one dies for lack of organs to transplant, human donors are not enough. This is why science has been applied for decades to develop technologies that allow create custom organsreplacing those that fail us throughout life with others that minimize the possibility of rejection.

Growing new organs from the patient’s own cells, modeled using 3-D printing, is a promising development. However, for the most complex, such as a heart or a liver, researchers pin their hopes on xenotransplantationcultivation in the body of animals specifically bred for medical purposes.

The problems of incompatibilities between donors are not specific to interspecies transplants, the researchers caution: unless the donor is a twin brother or that the organ is cloned, the receiver’s body will always experience rejection. A series of investigations, however, open the door to a solution: cultured in animals but with recipient cells.

A year ago, the Spanish team Juan Carlos Izpisua Belmontethe Salk Institute for Biological Studies from Californiapresented the first of these “chimeras“, a term inherited from Greek mythology that designates a hybrid creature. It was about pig embryos in their early days they were injected human stem cellsand were implanted in the uterus of sows.

In this way, 150 pig embryos were generated in which one in every 100,000 of its cells was human. They only lived 28 days before, just enough to confirm the viability of the project, before its gestation was interrupted for reasons ethical.

Now, the evolution of this research has been presented at the annual meeting of the American Association for the Advancement of Science on Austin, Texas. Following the same technique, researchers from the Universities of California and Stanford has presented lamb embryos in which the total 0.01% of its cells could be considered human: one in 10,000. They were also euthanized at age 28.

“It is a very small human cell contribution. We are not talking about a sheep with a human head” – explains one of the study authors Hiro Nakauchi. The next step would be to genetically edit the animal embryo to prevent it from developing the organ to be grown, and introduce human stem cells to fill in the missing “puzzle piece”.

However, the possibility of carrying these pregnancies to term must first be assessed by an Ethics Committee. The reason is that the researchers themselves do not know what effects hybridization could have on the animal. “If we discover that human cells go to your brain, – assures paul rossco-author of the work, in Guardian – we would not go ahead under any circumstances.”

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