Cloning 101.As I said, right now cloning uses intact cells. Here's how it works:
1) Take a cell from the animal to be cloned
2) Remove the nucleus from an egg (this is called an enucleated egg)
3) Fuse the two cells and let it divide a few times in a Petri dish
4) Implant the growing embryo into a surrogate mother
5) If everything goes well, a clone is born
This procedure requires living intact cells to be used. The problem with a frozen animal cell is that it is dead and ice crystals have torn it apart. It is not possible to fuse a beat up dead cell with an enucleated egg.
Cloning Using Frozen Cells
What the researchers in this new study did was change the protocol a bit. Instead of fusing two cells, they harvested nuclei from the frozen cells and injected them directly into the enucleated egg.
When they tried to clone the mouse that had been frozen for 16 years this way, it didn't work. But they managed to get 4 clones by adding an extra step. What they did was to make embryonic stem (ES) cells from the frozen mouse and use those cells to make a clone.
Basically they cloned the mouse but then instead of putting the embryo into a surrogate mother, they harvested its ES cells. Then they used the nuclei from these cells to create a clone in the usual way.
So we can now clone a long frozen mouse. The next step will be to try to clone an extinct animal like a mammoth.
Cloning a Mammoth is Trickier than a Mouse
Mammoth cloning will be no walk in the park. First off, we don’t have any mammoth eggs or cells to use. We'll have to use elephant ones. Hopefully, elephant eggs and/or cells will be compatible with a mammoth's nucleus. ( But there is some concern they they might not be compatible.)
Second, elephants are a lot harder to work with than mice. The experiments in this study used thousands of eggs to get a few clones. I don’t know enough about elephant biology but it seems like you'd need a lot of elephants to get that many eggs.
But this is definitely the first step in resurrecting long dead animals. For now we'll have to focus on the frozen ones. Maybe in the future researchers can figure out how to clone animals stored in formaldehyde. Or from pelts. Then we can start reviving species we humans have managed to kill off over the years.