A mouse mixed with human cells has emerged. This is the result of a simple method of injecting human cells into the amniotic fluid of a pregnant mouse without undergoing complex processes of cell or gene modification. It has been evaluated as a technology that will greatly help realize xenotransplantation in the future.
Researchers at the MD Anderson Cancer Center in Texas, USA, announced on the 12th that they successfully injected organoids made from human stem cells into the amniotic fluid surrounding the fetus of a pregnant mouse, allowing them to grow inside the animal.
Organoids are cultured from stem cells that can differentiate into all types of human cells into three-dimensional structures similar to organs, and are referred to as mini-organs. The researchers created a chimera by delivering human organoids to mice.
A creature mixed with cells derived from two or more species is called a "chimera." In particular, human-animal chimeras are seen as a solution to the global shortage of transplantable organs through technology that creates human organs in animal bodies.
However, existing chimera technology had limitations. Generally, scientists attempted to grow human organs by injecting human stem cells into animal embryos, but the survival rate of human stem cells in animal embryos was low. Even if they survived, they often did not last long.
In particular, there were cases where stem cells could not find their proper place. Stem cells, being undifferentiated cells, can differentiate into any type of cell, leading to unintended organs instead of the targeted ones.
In previous studies, modifications were made to remove essential genes for specific organs in animal embryos to allow human stem cells to grow only in those organs. For example, if the gene for making kidneys is removed in pig embryos, human stem cells can enter and grow into human kidneys.
Instead, the researchers cultured human stem cells into organoids of the intestine, liver, and brain, and injected them into the amniotic fluid of pregnant mice. They determined that injecting stabilized cells in organoid form would increase survival rates and enhance the likelihood of precise movement to specific organs.
The organoids injected into the amniotic fluid permeated into the bodies of the mouse fetuses and grew into specific organs. The intestinal organoid developed into the mouse's intestines, the brain organoid was absorbed into the cerebral cortex, and the liver organoid settled in the liver.
Later, about 1% of the intestinal tissue in the mice born consisted of human cells, and although in smaller proportions, human cells were also identified in the liver and brain. Notably, human-specific proteins such as albumin were also produced in the liver. These cells remained stable in the mouse's body and did not disappear even after two months.
Some express ethical concerns about human cells entering animal brains. This is because human brain cells could endow animals with human-like cognitive abilities.
The researchers noted, "Currently, the proportion of human cells that have entered the brain is very low, so there isn't much concern, but if a larger number of cells enter the brain in the future, we must closely examine whether cognitive functions change." They added, "As the success rate is currently low, additional research is also needed to improve the success rate."
References
Nature (2025), DOI: https://doi.org/10.1038/d41586-025-01898-z