Tiny human hearts grown successfully in PIG embryos

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, Wednesday, June 11th, 2025, represent a major milestone for xenotransplantation. The practice of growing animal organs or transplanting them into humans.

Heartbeat hybrids of human-pigs

Scientists reported that for the first, beating human-like heart growth was observed in embryonic pigs. Lai Liangxue of Guangzhou Institutes of Biomedicine and Health and his team genetically modified embryos of pigs by deleting two genes that are crucial for 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.

Later, the embryos were implanted in surrogate pigs. The scientists discovered that after three weeks the embryonic heart had grown to the size that is expected of a human at this developmental stage. (About the size a fingertip). And, sure enough, it was beating. Lai says that the human cells were marked with a biomarker luminescent to allow researchers to identify them within the heart tissues.

The study is still being peer-reviewed, and it is not clear what percentage of heart tissue contains 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 confident about human-pig hybrids

Scientists in stem-cell biology and transplantation who attended the conference were cautiously optimistic about the results, but they welcomed them nonetheless. Hiromitsu Nakauchi, from Stanford University, said that he will need to examine the data in order to confirm if the heart cells are truly human. Hideki Makaki of Tokyo’s Institute of Science observed that fluorescent cells were only found in a very small part of the heart. It raises questions as to how well the cells integrate with the tissue of pigs.

Masaki, along with other scientists, noted that heart organs would have to be constructed almost exclusively from human cells, in order to prevent immune rejection.

Animals can be used to grow human organs

These experiments are aimed at producing fully human-compatible tissues that can be used to transplant into patients who need them. 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% of deaths happen in conditions where organ donation is medically feasible, according to Understanding Animal Research.

Even when an organ donor is found, the organs must still be matched in size, blood types, and tissue compatibility. They must also be transplanted within tight timeframes – just 4 to 6 hours for hearts – or risk becoming unusable. This makes logistics extremely complex.

Pigs have been considered organ donors for a long time due to their similarity in size, function and organ size. Prior attempts at xenotransplants had failed because of immune rejection. The breakthrough came in 1993 when David Cooper, a surgeon, identified a single cell molecule from pigs that caused the rejection. Researchers used CRISPR gene-editing to remove the molecule, and suppress viral genetics to make organs more humane.

Joseph Tector, along with his team, created in 2025 a pig’s kidney that survived for over three full months in macaques. David Cooper created a genetically altered pig kidney that lasted in a baby baboon for 136 days.

Transplanting organs of animals into humans

Although the transplantation of pig organs to humans has only just begun, organs grown in pigs are already being used.

In 2021, US doctors transplanted an artificial kidney from a modified pig into the brain of a patient who was dead. Other successful operations have been performed since then. Two American teams implanted three more brain dead patients with pig kidneys. The recipients produced urine and showed no signs of rejecting the transplant. In one case, the patient received a modified pig heart with 10 gene edits.

Around six people are expected to receive xenotransplants by 2025. This includes kidneys as well as hearts, livers, thymus glands and livers. These procedures were performed under compassionate reasons, which meant that recipients were severely ill and had no other options. Most only survived a few weeks. Towana Looney is a 53-year old woman from Alabama in the USA. She became the longest living transplant recipient. After surpassing a milestone of 2 months, she was sadly rejected by her body after 5 months.

FDA approves the first clinical trial of pig organs

Full clinical trials are required before xenotransplants will be widely available. United Therapeutics has conducted the first official clinical trial. The U.S. Food and Drug Administration approved it. It will start with six patients, who will receive genetically altered pig kidneys.

Even though there is scientific enthusiasm, concerns about ethics 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 prove successful, pig grown organs could become a standard medical choice one day.

Would you accept a pig-grown organ if it saved your life? Please let us know your thoughts.

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