Researchers have created the first cloned mice from freeze-dried skin cells, aiming to help conservationists restore populations of endangered species. The breakthrough paves the way for countries to store animal skin cells as an insurance policy, as the cells can be used to create clones, a move that could boost the species’ genetic diversity if threatened with extinction in the future. While scientists use frozen cells to produce clones for conservation projects, the cells are kept in liquid nitrogen, an expensive and dangerous practice: If the power goes out or the liquid nitrogen isn’t topped up regularly, the cells thaw and become unusable. Freeze-dried sperm can also be used to create clones, but sperm are not available from all animals. In the latest work, the researchers froze dried skin cells from the tails of mice, storing them for up to nine months before attempting to use it to create clones. The freeze-drying process kills the cells, but scientists have found that early-stage cloned embryos can still be created by injecting dead cells into mouse eggs that have had their own nuclei removed. These early mouse embryos, called blastocysts, were used to create a stock of stem cells that went through another round of cloning. These stem cells were injected into mouse eggs without their own nuclei, resulting in embryos for surrogate mice. The first cloned mouse was named after Doraemon, a robot that loves melon buns in the Doraemon series of comics, and 74 clones have been produced since then. To check whether these clones had healthy fertility, nine females and three males were bred with normal mice. All females gave birth successfully. Despite this achievement, inefficiencies — freeze-drying damages DNA in skin cells — created healthy female and male mouse pups with a success rate of only 0.2 to 5.4 percent. In some cells, the Y chromosome was lost, causing cells from male mice to give rise to female mice. “If the same treatment could be done for an endangered species where only males survive, it would be possible to produce females and protect the species naturally,” the authors wrote in Nature Communications.
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