Where can tissue for transplantation come from in modern medicine in North America?
Surgeons have long relied on donor material from healthy individuals to perform organ transplants. Some organs, like kidneys, can be donated from a living donor, but others, such as the heart, are only available from a deceased donor. This imbalance means donor tissue supply is persistently limited and difficult to expand significantly.
There are two major alternatives explored in recent years. One involves using animal-derived tissues that can integrate with the human body. For example, pig biomaterial has been developed to manufacture human heart valves. A more ambitious line of work involved transplanting an entire pig heart into a human patient. While the initial outcome appeared successful, the patient’s condition later deteriorated for reasons not directly linked to tissue rejection, as investigators reported.
Another avenue employs 3D bioprinting, where living tissue is layer-constructed using printers that work with cells. This technology has enabled the creation of small, simple structures such as neural tissue, but printing a fully functional liver or lung remains a distant goal.
The ultimate aim some researchers pursue is to have animals host human organs. At first glance this seems impossible because organ development is directed by genetic programs. Yet nature presents examples of chimeras—organisms containing cells from different genetic backgrounds. Interspecies chimeras showing human tissue within animals are a theoretical possibility, and scientists are actively investigating this potential path.
Human-like kidneys in a pig’s body
Researchers at the Guangzhou Institute of Biomedicine and Health have explored chimeric embryos that mix human and pig cells. In these studies, the kidneys were the focus because they’re among the first organs to develop and are highly relevant for transplantation. The central challenge lay in aligning the distinct developmental needs of human and pig cells.
Initial steps included editing the embryo’s genome to suppress the pig’s own kidney formation, creating an empty niche for foreign tissue. Human stem cells were then guided into embryonic-like cells and introduced into pig embryos at the blastocyst stage, a very early point in development before the embryo attaches to the uterine wall. These embryos were cultured in a nutrient environment compatible with both species before being placed into surrogate sows.
In total, 1,820 embryos were transferred to 13 surrogate pigs. After 25–28 days, pregnancies were terminated to analyze the embryos. In five chimeric organisms, kidneys showed normal architecture for the early stage, forming tubules and primordia that progressed toward ureter development. About half of these kidneys contained human cells.
Why didn’t all pig embryos become humanized?
Cell transplantation occurred at the blastocyst stage, when organ rudiments are not yet formed. That has raised ethical questions about whether a fetus could develop human nerves or brains. Because the human cells were injected into a blind spot, their distribution depended on available space within the pig embryo.
One study author noted that human pluripotent cells tended to occupy niches where space existed, with very few human nerve cells observed in the fetal brain and spinal cord and no human cells in reproductive tissues. This indicates that the transplanted human cells did not spontaneously differentiate into sex cells. The researchers emphasize that demonstrating the feasibility of creating such chimeras is just the first step, and longer development is needed before any practical application could be considered.
Looking ahead, the researchers aim to allow longer development periods and to broaden human cell integration to include other organs such as the heart or pancreas. The overarching goal is to secure a more abundant supply of human organs for transplantation, but significant technical and ethical hurdles remain, and progress will likely be gradual.
Current work indicates that creating fully human organs inside animals would require far more complex strategies than those tested so far, including refining how tissues organize and how blood vessels remain open to prevent rejection. Experts caution that obtaining true human organs from animals will take many years and extensive verification of safety and ethics before it becomes a clinical reality.