by Joe Palca

NPR.org, November 4, 2008 · If you couldn't bear to part with your beloved pet mouse — and you stuck it in the freezer when it died — Japanese scientists may have some good news for you.

They have developed a technique for making essentially identical genetic copies of mice kept in an ordinary household freezer for as long as 16 years.

It involves the same kind of cloning techniques Scottish scientists used to make the sheep Dolly. The Japanese scientists say their research might someday be used to save extinct or endangered species whose bodies have been frozen in Arctic permafrost.

Teruhiko Wakayama led the research team from the Center for Developmental Biology, Riken, in Kobe, Japan, that developed the new techniques. The work appears in the journal PNAS.

Normally when an animal is cloned, a cell taken from the animal to be cloned is fused with a donor egg, from which most of the genetic material has been removed. When the cell and egg fuse, the genetic instructions in the cell rather than the egg control the development of the resulting embryo. If all goes well, this embryo develops into a duplicate, or clone, of the original animal.

But when cells are frozen, they die, meaning the fusion technique can't be used. The DNA, however, is still in those cells, so Wakayama took the genetic material from the frozen mouse cells and tried injecting it directly into donor eggs. Embryos formed using this technique, but when they were implanted into surrogate mothers, no pregnancies continued to term.

The Japanese team then added one more step. After making embryos with their direct injection technique, they took embryonic stem cells from those embryos and fused these with donor eggs. This more normal cell fusion technique created cloned embryos, which continued to grow when they were transplanted into surrogate mothers. The mothers gave birth to healthy mouse pups.

Reproductive biologist George Seidel of Colorado State University says this isn't the first time frozen tissue has been used to make clones. He says cattle have been cloned this way, but only after tissue underwent special preparations before freezing and were kept at extremely cold temperatures. This is the first time DNA taken directly from frozen tissue was successfully used to make clones.

Betsy Dresser agrees with the Wakayama team that this might be useful some day for saving extinct animals. Dresser is the senior vice president for research at the Audubon Center for Research of Endangered Species.

"Anything we can do to save endangered species, I'm for," she says. But she says it would take an unlikely set of circumstances to actually be able to use the Japanese techniques in practice to restore extinct species.

"I think we need to keep our focus on animal that are alive today," she says.