This time, by fusing human stem cells into sheep embryos, the scientists were able to grow chimeras into fetal animals over 28 days.
“Even today, the best matched organs, except if they come from identical twins, don’t last very long because with time, the immune system continuously is attacking them,” researcher Dr. Pablo Ross of the University of California, Davis, said in a press briefing.
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If it’s possible to grow human organs inside other species, such transplants could be useful beyond critical conditions. However, the sheep embryos created are only 0.01 percent human by cell count. While it’s better than the 0.001 percent of the human-pig embryos, Ross noted that in order to grow human organs, the proportion needs to be closer to 1 percent.
“It could take five years or it could take 10 years, but I think eventually we will be able to do this,” researcher Hiro Nakauchi said.
In addition, the scientists believe their strategy may lead to a cure for Type 1 diabetes, which occurs when the body fails to produce enough insulin to regulate blood sugar levels. This insulin, Newsweek reported, is typically secreted by pancreatic islet cells.
While the process is often used experimentally, human islet cell transplants have had limited long-term success because the body typically rejects it.
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“In theory, scientists could use a recipient’s own cells to tailor-make organs that are compatible with their bodies. This could reduce the chance of immune system rejection,” Newsweek reported.
In 2017, researchers used this method to successfully grow mouse pancreases in rats, which showed that transplants using the pancreas could cure diabetes in diabetic mice.
The researchers know how controversial their work is, but said that with funding, the research could be accelerated. Currently, the U.S. National Institutes of Health forbids public funding of human-animal hybrids. But in 2016, according to National Geographic, the agency signaled that the moratorium could be lifted.
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"All of these approaches are controversial, and none of them are perfect, but they offer hope to people who are dying on a daily basis," Ross told National Geographic. "We need to explore all possible alternatives to provide organs to ailing people."
Every hour, six people in the United States are added to the national waiting list for organ transplants, according to the U.S. Department of Health and Human Services. And each day, about 20 people die waiting.
In the U.S. alone, more than one hundred thousand people need heart transplants each year, yet only about 2,000 actually receive one.