Scientists at the Salk Institute have created part-pig, part-human embryos.
They used DNA editing tools to delete pig genes corresponding to certain organs, like the heart. Then, they injected human stem cells into the pig embryos, and the human cells filled the gaps.
Researchers are getting better and better at growing these combined embryos, aka chimeras. It's teaching them a lot about how our own cells develop and how different drugs work. It's also getting them closer to growing entire human organs for transplant.
RELATED: Science, recent scientific discoveries
Science, recent scientific discoveries
Science, recent scientific discoveries
The collision of two black holes - a tremendously powerful event detected for the first time ever by the Laser Interferometer Gravitational-Wave Observatory, or LIGO - is seen in this still image from a computer simulation released in Washington February 11, 2016. Scientists have for the first time detected gravitational waves, ripples in space and time hypothesized by Albert Einstein a century ago, in a landmark discovery announced on Thursday that opens a new window for studying the cosmos. REUTERS/The SXS (Simulating eXtreme Spacetimes)/Handout via Reuters
A computer simulation shows how our sun and Earth warp space and time, or spacetime, represented here with a green grid in this image released in Washington February 11, 2016. Scientists have for the first time detected gravitational waves, ripples in space and time hypothesized by Albert Einstein a century ago, in a landmark discovery announced on Thursday that opens a new window for studying the cosmos. REUTERS/Caltech/MIT/LIGO Laboratory/Handout via Reuters
The flower, Strychnos electri, encased in amber, is shown in this handout photo provided by George Poinar of Oregon State University February 15, 2016. The flower found by Poinar during a trip to a Dominican amber mine in 1986, was named by Lena Struwe of Rutgers University in 2015. Scientists on Monday announced the discovery of the flower that lived some 20 to 30 million years ago inside amber dug out of the side of a mountain in the Dominican Republic. It was a member of a group of flowers that today are the source of the poisons strychnine and curare. According to the researchers, it also likely boasted toxic compounds. REUTERS/George Poinar/Handout via Reuters
Surface-collected stone artefacts that were found lying scattered on the gravelly surface near Talepu on the Indonesian island of Sulawesi, are pictured in this undated handout photo obtained by Reuters January 13, 2016. Scientists on January 13 announced the discovery of stone tools at least 118,000 years old , indicating a human presence. The scientists said no fossils of these individuals were found in conjunction with the tools, leaving the toolmakers' identity a mystery. REUTERS/Erick Setiabudi/Handout via Reuters
An image of the planet Mercury produced by NASA'S MErcury Surface, Space ENvironment, GEochemistry, and Ranging, or MESSENGER probe is seen in an undated picture released April 16, 2015. These colors are not what Mercury would look like to the human eye, but rather the colors enhance the chemical, mineralogical, and physical differences between the rocks that make up Mercury's surface, according to NASA. The MESSENGER spacecraft that made surprising discoveries of ice and other materials on Mercury will make a crash landing into the planet around April 30, scientists said on Thursday. REUTERS/NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington/Handout
Kepler-186f, the first validated Earth-size planet to orbit a distant star in the habitable zone?a range of distance from a star where liquid water might pool on the planet's surface, is seen in a NASA artist's concept released April 17, 2014. The discovery, announced on Thursday, is the closest scientists have come so far to finding a true Earth twin. The star, known as Kepler-186 and located about 500 light years away in the constellation Cygnus, is smaller and redder than the sun. REUTERS/NASA/JPL-Caltech/Handout
The top, side and bottom view of a male Cottus schitsuumsh (cedar sculpin) fish are pictured in this undated handout combination photo obtained by Reuters January 30, 2014. The tiny fish, characterized by a disproportionately large head and previously unknown to scientists has been found in mountain rivers of Idaho and Montana in what biologists said on January 30, 2014 marked a rare discovery. REUTERS/U.S. Forest Service Rocky Mountain Research Station/Handout via Reuters
WASHINGTON, DC - MARCH 14: Paleontologist, Hans Sues, of the Smithsonian Museum of Natural History, holds a claw from a newly discovered dinosaur during a news conference at the museum, March 14, 2016 in Washington, DC. The species, named Timurlengia Euotica, lived about 90 million years ago and fills a 20 million year gap in the fossil record of Tyrannosaurs. (Photo by Mark Wilson/Getty Images)
A giant squid is seen in this still image taken from video captured from a submersible by a Japanese-led team of scientists near Ogasawara islands taken in July 2012, in this handout picture released by NHK/NEP/Discovery Channel in Tokyo January 7, 2013. The scientists have captured on film the world's first live images of a giant squid, journeying to the depths of the ocean in search of the mysterious creature thought to have inspired the myth of the "kraken", a tentacled monster. Picture released on January 7. Mandatory Credit. REUTERS/NHK/NEP/Discovery Channel/Handout (JAPAN - Tags: SCIENCE TECHNOLOGY SOCIETY ANIMALS) FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS. THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. MANDATORY CREDIT. JAPAN OUT. NO COMMERCIAL OR EDITORIAL SALES IN JAPAN
The path on Mars of NASA's Curiosity rover toward Glenelg, an area where three terrains of scientific interest converge, is seen in this undated NASA handout image taken by the High Resolution Imaging Science Experiment (HiRISE) instrument on NASA's Mars Reconnaissance Orbiter and released September 27, 2012. Arrows mark geological features encountered so far that led to the discovery of what appears to be an ancient Martian streambed. The first site, dubbed Goulburn, is an area where the thrusters from the rover's descent stage blasted away a layer of loose material, exposing bedrock underneath. Goulburn gave scientists a hint that water might have transported the pebbly sandstone material making up the outcrop. The second feature, a naturally exposed rock outcrop named Link, stood out to the science team for its embedded, rounded gravel pieces. The final feature, another naturally exposed rock outcrop named Hottah, offered the most compelling evidence yet of an ancient stream, as it contains abundant rounded pebbles. REUTERS/NASA/JPL-Caltech/Univ. of Arizona/Handout (UNITED STATES - Tags: SCIENCE TECHNOLOGY) THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. IT IS DISTRIBUTED, EXACTLY AS RECEIVED BY REUTERS, AS A SERVICE TO CLIENTS. FOR EDITORIAL USE ONLY. NOT FOR SALE FOR MARKETING OR ADVERTISING CAMPAIGNS
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The human cells in the pig chimeras had started to develop into muscle and organs after three to four weeks. The researchers could have let it go on longer, but doing that might have had ethics and animal welfare implications that nobody really understands yet.
Stem cells are a good choice for this research because they can grow into just about anything — including brain cells. There's a chance — even if it's a slim one — an animal that matures with human cells could end up with altered intelligence.
"If you have pigs with partly human brains, you would have animals that might actually have consciousness like a human. It might have human-type needs. We don't really know," New York Medical College professor Stuart Newman told NPR.
This is one reason why the government doesn't give out funding for chimera research right now. But the potential value of the science is getting harder to ignore. The National Institutes of Health has said it might start up grants, along with extra layers of ethics reviews, as early as January 2017.