Will new scientific research create human mice?

Every 12 minutes doctors add a new name to the list of 123,000 people in the United States waiting for an organ transplant. Of those, 6,500 will die each year because no organ is available.

Teams of researchers are currently working to develop a technique that, if successful, could create an endless supply of organs. But their work has sparked intense debate among bioethicists.

In the procedure, researchers insert human pluripotent stem cells into very early stage animal embryos. Pluripotent stem cells can become any cell type in the body. Researchers want to use this unique feature to grow human organs inside animals. The researchers would genetically engineer animal embryos so they lack the cells necessary to develop a certain organ, for example a pancreas. In theory, the human stem cells will fill the void left by the animal’s missing pancreatic cells and will form a fully human pancreas. The animal would then incubate the organ until doctors could transplant it into a human.

According to the journal Science, the technique also holds promise for drug safety testing and for researchers to study a variety of diseases in the laboratory.

But many bioethicists are alarmed by the methodology, which appears to be gaining popularity. The National Institutes of Health (NIH) has received an ever-increasing number of requests to fund this type of research. In September it placed a moratorium on such funding until it can assess the moral and ethical issues involved.

The debate centers on the ethics of creating what critics have termed chimeras—cross-species hybrids named after a monster of Greek mythology that had the head and body of a lion, a goat’s head attached to its back and the head of a snake at the end of its tail.

But researchers have been creating partly human chimeras for years. What is new and startling about this technique is that researchers are putting human stem cells into animal embryos. Some fear the pluripotent stem cells could differentiate into brain cells and give the animal human mental capabilities. Bioethicists also are concerned the stem cells could become sperm or egg cells.

“If you had a male mouse that had human sperm in it, that’s going to be a concern to some people, especially if it’s anywhere near a female mouse that has human eggs in it,” Hank Greely, a bioethicist at Stanford University, told National Public Radio. “To say the least, it’s disconcerting to think about two mice making a human embryo.”

Several researchers believe the possible scenarios bioethicists raise are extremely unlikely to occur, based on past experience. For the past 30 years, scientists have used human-mouse chimeras for research on AIDS, cancer, and other diseases. More recently, researchers transplanted human brain tissue into mice. But “there’s been nothing like a mouse with human thoughts,” Sean Wu, a Stanford University School of Medicine researcher, told STAT.

Proponents of the new technique have asked NIH to lift the funding ban because it will, “impede scientific progress in regenerative medicine.”

But acknowledging their critics, the researchers have suggested certain safeguards. Scientists could engineer the stem cells so they are unable to become human brain, egg, or sperm cells, and they could prohibit human-animal chimeras from breeding.

According to the scientists, as long as federal agencies and universities continue to monitor the research, ongoing dialogue between researchers and bioethicists is sufficient to ensure researchers conduct studies in an ethical manner.

David Resnick, an attorney and bioethicist at the National Institute of Environmental Health Sciences, warns it isn’t that simple.

“Science moves fast,” he told the Los Angeles Daily News. “The prospect of an intelligent mouse, stuck in a lab screaming ‘I want to get out!’ is very troubling to people.”