August 14, 2006

Sperm from ordinary frozen mice yield offspring

By Tan Ee Lyn

HONG KONG (Reuters) - Sperm extracted from mice and testes
that have been frozen for as long as 15 years have yielded
normal, healthy offspring in a study which researchers say
heralds fresh hopes for bringing back extinct species.

Frozen sperm is now preserved with cryoprotectants,
substances that protect it from freezing damage. However,
defrosted sperm is not always capable of fertilising an egg.

But researchers from Japan, Britain and Hawaii have found
that sperm can be frozen safely for much longer than previously
thought, so long as they are kept in organs or whole carcasses
and cooled slowly to minus-20 degrees Celsius or lower.

Using sperm from whole mice and testes that had been frozen
for between one week and 15 years, they were able to fertilize
eggs via microinsemination and obtain healthy offspring.

"Many people thought that sperm integrity could be retained
for several months at most ... but the sperm nucleus is
stronger than we expected," Atsuo Ogura of the Japanese
government-funded Riken Bioresource Center told Reuters by

"It (sperm nucleus) is good for at least 15 years," he
said, adding that offspring of the mouse that had been frozen
for 15 years did not appear any different from the others.


The scientists used very simple freezing methods. The mouse
that was frozen whole for 15 years was merely kept in a
conventional freezer at minus-20 degrees C, Ogura said.

"This cryopreservation (freezing) technique is probably the
simplest and anyone can do it. Liquid nitrogen is not
necessary. Any conventional freezer or dry ice will work very

He said this method of freezing would work for many other
mammals because mammalian sperm has a special DNA that "retains
nucleic activity and keeps the nucleus alive."

"We can apply this method to many other mammals, it is very
simple. Just put the testes or dead body into a freezer."

But he cautioned that carcasses must be allowed to cool
slowly, or about two to three hours to reach minus-20 degrees
C. Sperm frozen at lower temperatures would be better

"Degradation is minimal in liquid nitrogen (minus-196
degrees C). Molecules in the cells stay still in this
condition, so the degradation will be minimal," he said.

The experiment, to be published online this week in the
Proceedings of the National Academy of Sciences
(, may begin to change a long-standing
reluctance to use frozen sperm in in vitro fertilization.

"This experiment proves that immotile sperm (which does not
move) is just as good if frozen in good condition," Ogura said.

Looking ahead, Ogura said this breakthrough gives fresh
hopes that extinct species may roam the earth again.

"Restoration of extinct species could be possible if male
individuals are found in permafrost" he said, through injecting
the sperm into eggs from females of closely related species.