Scientists have made embryosthat are a lot mouse and a little bit human.

With a little help, human stem cells can knit themselves into growingmouse embryos, populating thedeveloping liver, heart, retina and blood, researchers report May 13 in Science Advances.

Finicky human cells dont tend to grow well in other animals. But in one of the new mouse embryos, 4 percent of its cells were human the most thorough mixing between human and mouse yet.

That level of integration isquite striking to me, says Juan Carlos Izpisua Belmonte, a stem cell anddevelopmental biologist at the Salk Institute for Biological Studies in LaJolla, Calif. If other scientists can replicate the findings, it potentiallyrepresents a major advance, says Izpisua Belmonte, who was not involved in thestudy.

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Such chimeras could helpreveal how a single cell can give rise to an entire organism. More humanizedanimals could also prove valuable in studying diseases such as malaria that affectpeople more than other animals. And with more advances, chimeras couldultimately turn out to be a source of human organs.

Many scientists have hitroadblocks in growing human stem cells in mice or other animals, including pigs and cows(SN: 1/26/17). We have analyzedthousands of embryos but never saw robust chimeric contribution of human stemcells to mouse embryos beyond day 12, says stem cell and developmentalbiologist Jun Wu of the University of Texas Southwestern Medical Center inDallas, who wasnt involved in the study.

The new methods success comes down to timing, says neuroscientist and stem cell biologist Jian Feng. To grow and thrive in a mouse embryo, human stem cells developmental clocks must be turned back to an earlier phase called the nave stage. You need to basically push the human cells back to that phase, says Feng, of the University at Buffalo in New York.

Feng and his colleagues resetthe stem cells clocks by silencing a protein called mTOR for three hours. Thisbrief treatment shocked the cells back to their nave stage, presumably restoringtheir ability to turn into any cell in the body.

Researchers injected batchesof 10 to 12 of these more youthful human stem cells into mouse embryos containingabout 60 to 80 mouse cells, and allowed the embryos to develop for 17 days.

To outward appearances, these embryos grew normally despite harboring human cells. By tallying DNA that was specific to either mouse or human, the researchers found that human cells accounted for between 0.1 and 4 percent of the total cells in the embryos.

Human cells knittedthemselves into most developing tissues of the mouse, destined to become theliver, heart, bone marrow and blood. Human red blood cells were particularlyabundant in these mouse embryos, the researchers found. A small number of humancells showed up in tissue that will form a brain; one embryo had a swarm of humanphotoreceptors, eye cells that help detect light.

As far as the researcherscould tell, no human cells were among the cells that go on to form sperm andegg. The capacity of chimeras to reproduce is one of the worrisome ethicalquestions surrounding the organisms that scientists are still trying to figureout.

Once inside a mouse embryo, the normally sluggish developmental pace of the human cells sped up to match their hosts. Human stem cells typically are slow to turn into certain types of mature photoreceptors, liver cells or red blood cells, Feng says, but not when the human cells are inside a mouse embryo. You put the same human cells in a mouse embryo, [and] they go fast, Feng says. In 17 days, you get all these mature cells that would otherwise take months to get in a normal human embryo.

Other scientists emphasize that different laboratories need to repeat the results. But if it works a big if here this has big implications, Wu says.

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