June 2025, in a groundbreaking development that could revolutionize organ transplantation, researchers have successfully grown tiny hearts containing human cells within pig embryos. This unprecedented achievement, presented at the annual meeting of the International Society for Stem Cell Research in Hong Kong in June 2025, marks a significant stride towards addressing the critical shortage of organs for patients in need. While still in its nascent stages and awaiting peer review, the research offers a compelling vision of a future where animal “incubators” could provide a sustainable and accessible source of human-compatible organs.

The Dire Need for Organs and the Promise of Xenotransplantation

The global organ shortage is a stark reality, with countless individuals succumbing to organ failure while awaiting suitable donors. In the United States alone, over 100,000 adults and children are on transplant waiting lists, and the numbers are similarly high in Europe. The limitations of traditional organ donation—relying on deceased donors and facing issues of tissue compatibility and immune rejection—have propelled scientists to explore alternative solutions.

Xenotransplantation, the transplantation of cells, tissues, or organs from one species to another, has long been envisioned as a potential answer. Early attempts, dating back to the early 20th century, involved transplanting organs from various animals, including chimpanzees, baboons, and pigs, into humans. However, these efforts were largely unsuccessful due to severe immune rejection, often leading to immediate graft failure. A notable case was “Baby Fae” in 1984, who received a baboon heart but survived only 20 days.

The landscape of xenotransplantation began to shift dramatically with advancements in genetic engineering, particularly CRISPR-Cas9 gene editing. These technologies allow scientists to modify animal genomes to reduce the likelihood of immune rejection and overcome other biological barriers. The first successful pig-to-human heart transplant occurred in January 2022, when surgeons at the University of Maryland Medical Center, led by Dr. Bartley Griffith and Dr. Muhammad Mohiuddin, implanted a genetically modified pig heart into David Bennett Sr., a 57-year-old man with terminal heart disease. Although Mr. Bennett survived for two months, providing invaluable insights, his case underscored the complexities and challenges still to be overcome. More recently, in March 2025, doctors at Massachusetts General Hospital, including Brazilian surgeon Dr. Leonardo Riella, implanted a genetically modified pig kidney into Richard Slayman, a 62-year-old patient. These clinical breakthroughs demonstrate the increasing viability of xenotransplantation.

The Breakthrough: Human Hearts in Pig Embryos

The latest research, led by Professor Liangxue Lai and his team, takes a different yet complementary approach to xenotransplantation. Instead of transplanting a whole pig organ into a human, their aim is to grow human organs within an animal host. This strategy, known as “organ bioengineering using pigs as incubators,” seeks to create organs that are largely composed of human cells, thereby potentially minimizing immune rejection and providing patient-specific tissues.

The process involved several critical steps:

1. Reprogramming Human Stem Cells: Professor Lai and his team reprogrammed human stem cells to enhance their survival within a pig environment. This involved introducing specific genes that prevent cell death and promote cell growth.
2. Creating a “Niche” in Pig Embryos: Using CRISPR gene editing, the researchers generated pig embryos in which two crucial genes involved in heart development were “knocked out.” This created a biological “niche” – a vacant space or developmental opportunity – for the human cells to occupy and differentiate into heart tissue.
3. Introducing Human Stem Cells: A small number of these specially prepared human stem cells were introduced into the pig embryos at the morula stage, a very early developmental point where the embryo consists of a rapidly dividing ball of about a dozen cells.
4. Transfer to Surrogate Pigs: The chimeric embryos (containing both pig and human cells) were then transferred to surrogate pigs.
5. Monitoring Development: The embryos were allowed to develop for up to 21 days before being removed for analysis.

The Results: Beating Human-Pig Hearts

The findings from this study, while preliminary and not yet peer-reviewed, are remarkably promising. The researchers observed that the embryos survived for 21 days, and within that timeframe, their tiny hearts had begun beating. Crucially, these embryonic hearts had grown to the equivalent size of a human heart at that stage of development – roughly the size of a fingertip.

The presence of beating hearts containing human cells within the pig embryos signifies a monumental step forward. While the exact proportion of human cells in these specific heart structures is still under detailed investigation, previous work by Professor Lai’s group and others on growing human kidneys in pig embryos showed that the humanized kidneys contained between 50% and 60% human cells. This suggests that the human stem cells are indeed integrating and contributing significantly to organ formation.

The Scientists Behind the Innovation

While Professor Liangxue Lai of the Guangzhou Institutes of Biomedicine and Health is a senior author and key figure in this latest breakthrough concerning hearts, his work builds upon years of foundational research in xenotransplantation and human-animal chimeras.

Previous significant contributions in the broader field of human-animal chimeras for organ growth include:

• Dr. Carlos Izpisúa Belmonte (Salk Institute): In 2017, Dr. Izpisúa Belmonte and his team reported the successful engineering of human-pig hybrid embryos, demonstrating that human cells could survive and contribute to early development in pig embryos. While the contribution was low (about one in every 100,000 cells), it was a crucial “yes” to the fundamental question of whether human cells could contribute at all.
• Dr. Hiromitsu Nakauchi (Stanford University and University of Tokyo): Dr. Nakauchi is a pioneer in interspecies chimera research, having demonstrated the growth of rat organs in mice and vice-versa. His group’s work on creating organ “niches” by genetically modifying host embryos has been instrumental in improving the integration of donor stem cells.
• Dr. Muhammad Mohiuddin and Dr. Bartley Griffith (University of Maryland School of Medicine): These surgeons led the team that performed the first pig-to-human heart transplant in 2022, marking a major clinical milestone in cardiac xenotransplantation.
• Dr. David K.C. Cooper (Massachusetts General Hospital): A veteran in xenotransplantation research for nearly 40 years, Dr. Cooper has been involved in numerous studies, including pig kidney and heart transplantation in baboons, with a focus on addressing the lack of human organ donors.

The current research by Professor Lai’s team represents a significant leap from simply observing human cells in early embryos to seeing a complex, beating organ structure like the heart form.

Ethical Considerations and Future Prospects

The creation of human-animal chimeras and the prospect of growing human organs in animals raise a complex array of ethical questions. Concerns include:

• Humanization of Animals: The extent to which human cells might contribute to the animal’s brain or reproductive system, potentially blurring the lines between species, is a significant ethical debate. The current research by Lai’s team reported only “very few human neural cells in the brain and spinal cord and no human cells in the genital ridge” in their previous kidney work, which is a crucial distinction.
• Animal Welfare: The ethical treatment of animals used in this research is paramount. Guidelines like the “Three Rs” (Replacement, Reduction, Refinement) are crucial for ensuring minimal harm and improved welfare for research animals.
• Infectious Disease Transmission (Zoonoses): While genetically modified pigs are bred in sterile environments to mitigate this risk, the possibility of unforeseen cross-species pathogen transmission requires ongoing vigilance and rigorous testing.
• Public Perception: The concept of “Franken-science” or “human-animal hybrids” can evoke strong public reactions. Transparent communication and addressing societal concerns are essential for the responsible advancement of this field.

Despite these challenges, the potential benefits are immense. If successfully developed, this technology could:

• Eliminate the Organ Shortage: A limitless, on-demand supply of organs could save millions of lives and drastically reduce transplant waiting lists.
• Reduce Immune Rejection: By growing organs largely composed of the patient’s own cells (derived from their induced pluripotent stem cells), the need for lifelong immunosuppressive drugs, with their associated side effects and risks, could be significantly reduced or even eliminated.
• Personalized Medicine: Organs could be custom-grown to match individual patient needs, leading to better outcomes.
• Disease Modeling and Drug Testing: Chimeric organs could serve as invaluable models for studying human diseases and testing new therapies in a more physiologically relevant environment than current in-vitro methods.

Professor Lai’s team acknowledges that significant hurdles remain. The current embryos did not survive beyond 21 days, and the long-term development and function of these humanized organs need extensive study. Moreover, the presence of pig-derived vascular cells in the formed organs could still pose a risk of rejection if transplanted into humans, necessitating further refinements in the technology.

The achievement of growing tiny, beating hearts with human cells inside pig embryos marks a pivotal moment in biomedical research. While the ethical and technical challenges are considerable, the potential to alleviate the global organ shortage crisis is a powerful motivator. This pioneering work, led by scientists like Professor Liangxue Lai, alongside the ongoing advancements in direct xenotransplantation, continues to push the boundaries of what is possible, offering a genuine glimmer of hope for countless patients worldwide awaiting a second chance at life. The journey from beating embryonic hearts to fully transplantable human organs in pigs is long, but this latest breakthrough demonstrates that the vision is steadily moving closer to reality.