In a breakthrough, scientists have successfully grown human stem cells in pig embryos. Two separate reports have established the possibility of growing replacement human organs in large animals. One team, led by biologists Jun Wu and Juan Carlos Izpisua Belmonte at the Salk Institute, proved that human stem cells can contribute to the growth and formation of the pig’s tissues, despite the evolutionary gap between the two species. Another report, from a team led by Tomoyuki Yamaguchi and Hideyuki Sato of the University of Tokyo, as well as Stanford’s Hiromitsu Nakauchi, showed how the scientists successfully revered diabetes in mice by transplanting pancreas glands formed from mouse cells grown inside a rat.

Together, the reports demonstrate the that prospect of growing human organs in a pig is not so far-fetched. In this approach, scientist could generate stem cells from a patient’s skin, grow the needed organ inside a pig or other large animal, and harvest the organ for transplant to the patient. The organ would have been grown from the patient’s own cells, greatly reducing any chance of rejection by the immune system.

Stem cell expert Rudolf Jaenisch, from the Whitehead Institute in Cambridge, Massachusetts, said “I think this is very promising work in principle.”

While there are many practical and ethical obstacles remaining, the research is close to addressing the growing need for organs – within the United States alone there are over 76,000 people waiting for transplants.

Ethical concerns pertain to disturbing potential outcomes, such as human cells being incorporated into a pigs brain or reproductive organs. Either of these outcomes would be ethically problematic to say the least.

In 2015, the National Institute of Health enacted a moratorium on the use of public funds to insert human cells into animal embryos. It is unclear what the fate of this ban will be under the new Trump administration.

Insertion of human cells into early Monkey embryos has been illegal since 2009, since monkeys are more likely to have their brain altered by human cells given their close evolutionary relationship.

Scientific interest in this line of inquiry has risen since human stem cells were first created from human embryos in 1998 and then from normal adult tissue cells in 2007. Since then, experiments have focused on turning these cells into therapeutic tissue by exposing them to chemicals similar to those in a living embryo, while contained in glassware. However, the exact composition of chemicals used to turn stem cells into heart, brain, lung or other tissue is unknown to scientists, and these glassware experiments have not been successful.

Biologists have now looked towards living embryos to expose the stem cells to a natural sequence of chemicals to produce the right type of tissue.

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