Scientists have successfully grown human cells in pig embryos, inching toward the goal of growing replacement organs for transplant.

The researchers implanted human stem cells into pig embryos, where both types of cells developed for several weeks, according to a paper published Thursday in the scientific journal Cell.

Researchers experimented previously with splicing human stem cells in other host mammals, mainly rodents, but the latest success may open up the possibility of growing full human organs in pigs.

Growing full human organs in pigs is “far away,” Juan Carlos Izpisua Belmonte of the Salk Institute in La Jolla, Calif., one of the authors of the paper, told USA Today.

Creating mammals that are partially human — called chimeras after the mythical hybrid beast — raises ethical concerns, and the federal government stopped funding chimera research in 2015.

For the experiment, the scientists injected a few human stem cells into pig embryos and let them grow for four weeks. The results were somewhat inefficient. Only one of 100,000 pig embryonic cells was human, which was lower than expected, though the scientists “were very happy to see we actually can see the human cells after four weeks of development,” Jun Wu, another researcher at Salk said.

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