Japanese scientists have created healthy cloned mice from animals that have been dead and frozen for 16 years. The research opens the possibility of cloning extinct species, such as woolly mammoths.

Previously, clones had only been created from living donor cells. Scientists thought that the freezing process would damage DNA and the cells, but the new study found that DNA in brain cells was largely intact. In order to replicate the feat with frozen tissue from extinct animals, researchers would need to find a cell with intact DNA, as well as a donor egg and surrogate mother of a suitable living species.

According to an article from National Geographic,

For their cloning process, [Teruhiko] Wakayama and his colleagues drew dead brain and blood cells from the frozen mice. The researchers injected the nuclei from the dead cells directly into unfertilized mice eggs, creating embryos.

It's not known, however, whether nuclei from cells frozen for extended periods of time can be reprogrammed to develop into cloned animals.

So instead of transferring each embryo into a mouse's oviduct (the tube by which eggs leave an ovary), the researchers extracted the inner cell mass from each embryo and generated lines of embryonic stem cells. The researchers created 46 such lines, from which they were able to produce 13 mouse pups.

Embryonic stem cells are pluripotent--capable of becoming many other types of cells.

"These cells are the same as fertilized embryonic stem cells," Wakayama explained.

The scientists then transferred the nuclei from these cells into mouse eggs to produce healthy mouse pups. His findings appear today in the journal Proceedings of the National Academy of Science.

Since this cloning method does not require intact cells from the animal being cloned--cells of frozen animals usually deteriorate--the researchers believe that their technique could now allow them to work on the frozen remains of extinct mammals.