Scientists genetically modified a mouse to be 4 percent human

Scientists mixing the genetic building blocks of two different animal species sounds like the beginning of a bad science fiction movie, but that’s precisely what a team of researchers just did by injecting human stem cells into mouse embryos. Just over two weeks later, the team had more mature mouse embryos that contained up to four percent human cells.

These cross-species hybrids, called chimeras, were long the stuff of fiction but have more recently become reality. Scientists in several countries have conducted experiments in which human cells were added to developing animal embryos, but none of them have managed to create an animal that was made up of as many human cells as these mice.

As ScienceNews reports, the researchers modified the human stem cells by tweaking a protein and bringing the cells back to a previous stage in their development. This may be what allowed the cells to conform to their new surroundings after being injected into the mouse embryos.

After letting the embryos develop for a further 17 days, the scientists discovered that human cells had spread throughout much of the developing animal. Human cells were found in tissue that would form the heart and brain and were especially plentiful in the blood. Not all of the embryos developed in the same manner and some had human cells in places where others didn’t, such as the eyes.

But why do any of this at all? The idea here isn’t to create human/animal hybrids simply for the sake of doing so. It may seem a lot like “playing God,” but the scientists have human lives on their minds. It’s thought that if the development of these hybrid animals reaches a certain level of maturity, the animals could play a dramatic role in the testing of life-saving treatments that could ultimately be used on humans. It’s even possible that specific animals, like pigs, could one day grow human organs, thus offering doctors a new source of healthy organs for use in transplants.

That, of course, is the scientific side of things. The ethics of it all are another story entirely. It’s easy for us to say that a human life is more valuable than that of any random animal, but the idea of raising farms of human/animal hybrids just so we can yank their guts out and slap them into our own bodies is a little, well, uncomfortable to think about.

In any case, we’re certainly not there yet and while the four percent human mouse is a step in that direction, there’s plenty of more work to be done.