Sperm in frozen animals still viable years later.

Sperm stored inside frozen organs or whole animals can produce healthy offspring years later, a new study shows.

Researchers bank sperm from lab animals and other species that they may want to reproduce in the future. The typical method for storing male sex cells involves dissecting the epididymides--tubes that store sperm in the testes--from dead animals. Researchers remove the sperm, which are then placed in a special preservative solution and plunged into liquid nitrogen.

Because this method can be tedious and costly, Atsuo Ogura of the Institute of Physical and Chemical Research Bioresource Center in Ibaraki, Japan, and his colleagues tried another method. Working with lab mice, the researchers froze epididymides, testicles, and entire animals for periods of 1 week to a year. They also used mice that had been frozen for up to 15 years.

When the researchers thawed the organs or whole animals and removed their sperm, the sex cells could fertilize eggs from live mice and produce healthy offspring, regardless of how long they'd been frozen.

Ogura's team notes in the Aug. 29 Proceedings of the National Academy of Sciences that the results offer new hope for re-creating long-lost species such as woolly mammoths, whose sperm may still be preserved in permafrost. These animals could be bred using eggs from existing related species, the researchers say.