Researchers at the State University of New York at Buffalo injected human stem cells into a mouse embryo using a special technology.Scientists at the University at Buffalo and the Roswell Park Cancer Institute have created a mouse that has 4 percent human cells, the most to date for any chimera created in a lab.

The team published their historic work in the journal Science Advances, expressing they had injected human stem cells into developing mouse embryos. As a result one of the newborn mice exhibited 4 percent human cells which the scientists stated was a “major advance” in science. This is because typically, human and animal cells don’t mix well together in a petri dish.

“It has not been possible to generate naïve [human stem cells] that substantially contribute to mouse embryos,” the scientists say in the paper’s abstract. Their work “may enable applications such as human organ generation in animals.”

This mouse-human chimera shows human cells (green) in a 17-day-old mouse embryo (blue) that are mostly red blood cells accumulated in the mouse’s liver.
(Image: © Zhixing Hu)

Scientists infused young human stem cells (human induced pluripotent stem cells) into the mouse embryos and then let them develop for just the short period of two weeks. The researchers discovered evidence of human cells in the developing liver, brain, eyes, heart, blood, and bone marrow of the mouse’s embryo. Finally, the team examined the embryos’ DNA, finding that human cells accounted for between a low of 0.1 and a high of 4 percent of developing tissues.

This isn’t the first time man has messed with creating creatures, last year, Chinese researchers created two pig-monkey chimeras after genetically modifying cynomolgus monkey cells inside pig embryos. However, those creatures only lived for just a week before dying.

Earlier last year in August, Spanish scientist Juan Carlos Izpisua Belmonte announced that he had created the first human-monkey embryo in China.  Even before experiments in China, scientists have been experimenting on creating animal and animal and human and animal chimeras.

In 2017, scientists from the Salk Institute in California led again by Izpisua Belmonte attempted to grow the first embryos containing cells from humans and pigs. But the process proved to be more challenging than the team anticipated which resulted in poor results. Out of 2,075 implanted embryos, only 186 developed up to the 28-day limit that was set on the project.

Also occurring last year, Japan loosened its guidelines on the growth of human-animal chimeras  by approving the transplantation of chimeric embryos or hybrids into animals, but not humans. Japanese stem cell scientist, Hiromitsu Nakauchi, also planned to insert human stem cells into mice or rats.

This time scientists hope to grow a human pancreas in the animal. The experiment conducted by researchers at the University at Buffalo and the Roswell Park Cancer Institute is about to have a similar study conducted.

The ultimate goal here for science seems to be to grow human organs inside animals and advance medical knowledge through experimentation on these creatures for human transplants. In theory, the organs could then be genetically matched by taking the recipient’s cells and reprogramming them into stem cells. Research involving chimeras could also lead to increased knowledge of human biology and development, which would result in significantly improving human health.

Although, ethical concerns should be raised about whether or not the process is humane.

The other issue is about scientists growing human organs inside animals and what types of cells those organoids have become. Previous research showed that growing human Kidney organoids resulted in sprouting cells belonging to brains and muscles, leaving scientists bewildered.

If you need a refresher on human-animal chimeras, read this creepy bone-chilling article by National Geographic about sheep human hybrids.

Shockingly, an astounding 57% of Americans consider the practice OK and want the use of the technology to genetically engineer animals to grow organs or tissues that could be used for humans needing an organ transplant, while 41% say this is going too far, according to a survey taken in August this year by the Pew Research Center.