Scientists at the University of Buffalo have shown that mouse embryos can be created at very high levels of human cells. In one of those mouse embryos, four percent of the cells are actually human. These cells appear in all newborn tissues in mouse embryos, from retinal cells to red blood cells and liver cells.

Juan Carlos Izpisua Belmonte, a stem cell biologist and developer at the Salk Biological Research Institute in La Jolla, Calif., Said that "this level of integration is quite impressive." If other scientists can replicate this result, "it has the potential to represent a great advance," said Izpisua Belmont, a person not involved in the study.

Such hybrids can help shed light on how a single cell can produce an entire organism. More anthropomorphized animals can also be valuable evidence in the study of diseases, such as malaria, that affect humans more than other animals. And with more progress, hybrids can eventually become a source of organs for humans.

The success of this new method will be completely determined by time, said stem cell neuroscientist and neuroscientist Jian Feng. Human cells are notoriously difficult to integrate into other animals, because cells of different species grow at different speeds. In order to grow and develop in mouse embryos, the growth clocks of human stem cells must be reversed to an earlier stage called the basal stage.

"Basically, you need to push human cells back to that stage," said Feng, a research scientist at Buffolo University in New York.

Feng and his colleagues reset the clock of stem cells by silencing a protein called mTOR for three hours. This brief treatment sho‌cked the cells, causing them to go back to the original stage, perhaps restoring the cells’ ability to turn into any cell.

The researchers injected plots of 10 to 12 human immature stem cells into mouse embryos containing between 60 and 80 mouse cells, and allowed embryos to develop in 17 days.

Visually, these embryos develop as normal, despite the fact that they are harboring human cells. By examining mouse or human-specific DNA, the researchers found that human cells make up 0.1 to 4 percent of the total number of cells in the embryo.