Human hearts grown in pig embryos. Credit: mali maeder, Pexels
In a groundbreaking experiment, researchers have successfully grown tiny human hearts in pig embryos that survived up to 21 days – a development that could one day help solve the global organ shortage.
The findings, presented at the International Society for Stem Cell Research conference in Hong Kong on Wednesday, June 11, 2025, mark a major milestone in xenotransplantation. (The practice of growing or transplanting animal organs into humans.)
Human-pig hybrids with heartbeats
For the first time, scientists reported the growth of beating human-like hearts inside pig embryos. Led by Lai Liangxue of the Guangzhou Institutes of Biomedicine and Health, the team genetically engineered pig embryos by disabling two genes crucial to heart development. They then injected human stem cells – enhanced to survive and multiply – into the embryos at an early stage known as the morula, shortly after fertilisation.
The embryos were later implanted into surrogate pigs. After three weeks, the scientists found that the embryonic hearts had reached the size expected for a human heart at that developmental stage (about the size of a fingertip) and were sure enough, beating. According to Lai, the human cells had been marked with a luminescent biomarker, allowing researchers to identify them inside the heart tissue.
The study is still awaiting peer review, and it’s unclear what proportion of the heart tissue was actually made up of human cells. For comparison, Lai’s previous research on growing human kidney tissue in pig embryos found that 40–60 per cent of the kidney tissue was of human origin.
Scientists cautiously optimistic about human-pig hybrids
Stem-cell biologists and transplant scientists attending the conference welcomed the findings, though with caution. Hiromitsu Nakauchi from Stanford University said he would need to review the data to confirm whether the heart cells were truly human. Hideki Masaki of Tokyo’s Institute of Science noted that the fluorescent cells appeared in only a limited section of the heart, raising questions about how well they integrated with the pig tissue.
Masaki and other scientists noted that, for heart organs to be viable for transplantation, they would need to be made almost entirely of human cells to avoid triggering immune rejection.
Growing human organs in animals
The ultimate goal of such experiments is to produce fully human-compatible organs that can be transplanted into patients in need. The demand far outweighs supply: in the UK alone, more than 415 people died while waiting for an organ in 2023–24, despite over 4,600 transplants being carried out by NHS Blood and Transplant. Only 1 per cent of deaths occur in conditions that make organ donation medically viable, according to Understanding Animal Research.
Even when a suitable donor is found, organs must be matched for size, blood type, and tissue compatibility. They must also be transplanted within tight timeframes – just 4 to 6 hours for hearts – or risk becoming unusable. That makes the logistics incredibly complex.
Pigs have long been seen as suitable organ donors due to similarities in organ size and function. But earlier attempts at xenotransplants failed due to immune rejection. The game-changer came in 1993, when surgeon David Cooper identified a single molecule on pig cells that was triggering the rejection. Using CRISPR and gene-editing technology, researchers eventually managed to remove that molecule and suppress viral genes to make the organs safer for humans.
In 2025, Joseph Tector and his team created a pig kidney that survived over three months in macaques. That same year, a genetically modified pig kidney created by David Cooper lasted 136 days in a baboon
Transplanting organs from animals to humans
While growing organs inside pigs is still in early stages, actual pig organ transplants into humans have already begun.
In 2021, US surgeons transplanted a modified pig kidney into a brain-dead patient. Since then, other successful operations have taken place. Two American research teams implanted pig kidneys into three more brain-dead patients, who produced urine with no signs of rejection. In one remarkable case, a patient received a pig heart modified with ten gene edits.
As of 2025, around six people have received xenotransplants, including kidneys, hearts, livers and thymus glands. However, these procedures were granted under compassionate grounds, meaning recipients were critically ill and out of options. Most survived only a few months. Towana Looney, a 53-year-old woman from Alabama, USA, recently became the longest-living transplant recipient of a genetically modified pig kidney. She surpassed a milestone of two months, and then after almost 5 months, her body sadly rejected it.
FDA approves first clinical trial for pig organs
For xenotransplants to become widely available, full clinical trials are essential. The U.S. Food and Drug Administration has now approved the first official trial, run by biotech firm United Therapeutics. It will initially involve six patients receiving genetically modified pig kidneys, eventually expanding to 50 individuals.
Despite scientific enthusiasm, ethical concerns have been raised.
With fewer than 10 per cent of global transplant needs currently met – according to the World Health Organization (WHO) – the need for alternative solutions is urgent. If clinical trials succeed, pig-grown organs could one day become a standard medical option.
Would you accept an organ grown in a pig if it meant saving your life? Let us know what you think.
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