Fertility specialists routinely freeze sperm itself, but the freezing process is tricky and unique for each species. In addition, frozen sperm has a relatively short lifetime and researchers are limited to the amount of sperm frozen.

Veterinarian Ralph L. Brinster, a pioneer in genetic engineering of animals, and his colleagues at the University of Pennsylvania report today in the journals Nature and Nature Medicine that spermatogonia--the tissues that produce sperm--can be easily frozen and stored for long periods.

When the tissue is thawed out, it can be transplanted into the original donor or a host, where it begins making unlimited quantities of fresh sperm. Although Brinster has not yet attempted to fertilize eggs with the sperm, he has every confidence that it can. "I would be very surprised, closer to amazed, if it didn't fertilize an egg," he said.