According to Japanese and American scientists, healthy offspring have resulted from the injection of sperm into female mice from male mice that have been in a deep freeze for 15 years. The mice, as a whole, were kept frozen in temperatures of -20 degrees celsius. 
Scientists at the Institute of Physical and Chemical Research Bioresource Center which is located in Tsukuba, Japan, say that cryogenic techniques were not used and that the mice were kept in a freezer.
Scientists also say that the sperm was not alive when it was thawed out. At least one out of every five females that were injected with the sperm, gave birth to healthy offspring which are also fertile.
"Obviously, those frozen [sperm] were all dead in the conventional sense. Motionless sperm had no chance whatsoever of fertilising in vivo or in vitro. Nevertheless, some of these, if not all, were ... genomically intact, because they were able to produce apparently normal offspring," said the researchers.
The idea behind the experiment is that scientists say that the results now make it possible to bring back prehistoric animals or extinct animals such as the woolly mammoth, but because any fully intact body found would have been frozen for over 10,000 years and would likely be too damaged organisms and bacterias. Endangered species such as elephants could also be reproduced, but scientists also say that the chances of an experiment like that of being successful are still "slim."
"Restoration of extinct species could be possible if male individuals are found in permafrost. If sperm of extinct mammalian species, for example the woolly mammoth, can be retrieved from animal bodies that were kept frozen for millions of years in permanent frost, live animals might be restored by injecting them into oocytes [eggs] from females of closely related species," said Dr. Narumi Ogonuki, head of the team who made the discovery.
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Experiment on frozen sperm raises hopes of "restoring" extinct mammals