Scientists made a mouse embryo that's 4% human -- the highest level of human cells in an animal yet

Researchers at the State University of New York at Buffalo injected human stem cells into a mouse embryo using a special technology.

(CNN)Scientists have created a mouse embryo that's part human -- 4% to be exact.

The hybrid is what scientists call a human-animal chimera, a single organism that's made up of two different sets of cells -- in this case, a mouse embryo that has both mouse cells and human cells.
This human-mouse chimera has by far the highest number of human cells ever recorded in an animal, according to researchers. Their experiment suggests that many types of human cells can be generated in mouse embryos, and at a much faster rate than in human embryos.
    And that, the scientists say, carries enormous potential for the treatment of human diseases, possibly even Covid-19.
    Researchers from the State University of New York at Buffalo and the Roswell Park Comprehensive Cancer Center published the findings last week in the journal Science Advances.
    The news was previously reported by Popular Mechanics.

    The findings have disease-treating potential

    These findings are important for a number of reasons, said Jian Feng, one of the study's authors and a professor of physiology and biophysics at the University at Buffalo.
    For one, it shows that it's possible to generate many types of mature human cells in mouse embryos, which could potentially be used to make cells, tissues or organs to treat diseases.
    In this study, the team of researchers injected 10 to 12 human stem cells into developing mouse embryos. Within 17 days, those stem cells developed into millions of mature cells, including human red blood cells and eye cells.
    This figure shows a large quantity of human cells (labeled green) in a 17-day old mouse embryo (labeled blue). Most of the human cells are red blood cells, which are accumulated in the mouse fetal liver.
    In a human embryo, it would take about eight weeks to generate human red blood cells and even longer to generate human eye cells, Feng said.
    "These observations suggest that the mechanism that specify time in development can be changed," he wrote in an email to CNN. "With this implication, there will be more dramatic discoveries down the road."

    The team used a revolutionary technology

    In previous such research, scientists had only detected around 0.1% of human cells in mouse embryos.
    That's why it's so striking that these human-mouse chimeras exhibited 4% human cells. And because of the technique that the team used to count the cells, Feng said even that figure is an underestimate.
    The team achieved this feat by converting human pluripotent stem cells, which can potentially produce any cell or tissue the body needs to repair itself, into an earlier state.
    Converting those cells made them compatible with the inner group of cells inside an early stage mouse embryo, which generates all the cells in the body. So when those earlier stage human cells were injected into mouse embryos, they developed much better than they otherwise would have.
    "We reasoned that if we can make the human pluripotent stem cells behave like the mouse pluripotent stem cells, the human cells should mingle well with the mouse stem cells in a mouse blastocyst," Feng wrote. "And that is exactly we found."
    The team's experiment indicates that the "the genetic program embodied in a mouse embryo and the genetic program embodied in human stem cells can crosstalk pretty well," Feng said.
    In other words, there's enough evolutionary compatibility between mice and humans that mouse embryos are a relatively good environment for cultivating human cells.
    "Life is a DNA-based software system that harnesses energy to produce information," Feng wrote. "This experiment is kind of like emulating Windows in a Mac."

    Future implications could include organ development

    Human-animal chimeras have been a