Freezing living things to revive them at a later date can stop time for species at risk. Low temperatures arrest biochemical processes, placing tissues in suspended animation until thawing. The technique is particularly helpful for preserving genetic diversity for species with few remaining individuals. It also works postmortem; sperm, eggs, and other body tissues are sometimes frozen from animals that have died to retain the genetic diversity they contain.
But animal tissues are challenging to freeze. The crux of the issue is water. When water freezes, it expands, forming tiny icicles that can perforate cell membranes and cause wholesale damage to tissues. Replacing some of the water in animal tissues with antifreeze can reduce the degree of cryoinjuries.
Even so, the more water a cell contains, the more problematic its cryopreservation can be. This is one reason why it’s safer to freeze sperm than eggs. Already, sperm from many species of special concern has successfully been preserved, including giant panda, cheetah, lynx, sun bear, black rhino, African elephant. The sperm can be thawed later to inseminate females or create test-tube embryos.
Eggs are more problematic. Oocytes not only have a low surface area to volume ratio, but often resist absorbing cryoprotectants. To sidestep this problem, a fast-freezing method called vitrification which transforms the tissue into a viscous, glasslike solution without ice crystals is sometimes used.
Scientists are exploring ways of freezing both the embryos and body tissues of a variety of endangered species for conservation purposes. Entire “frozen zoos” have been established for this purpose. One example is the San Diego Zoo Institute for Conservation Research, which not only keeps the germplasm of more than 10,000 species of concern, but also one that is now extinct: the Hawaiian po’ouli bird.
But once tissues have been frozen, utilizing them to produce new animals is tricky. The reproductive biology of many animals can be quite unique, making it difficult to apply techniques developed in one species to another.
There have been incremental successes. For example, using frozen starting materials, the San Diego institute has fertilized a cheetah egg that subsequently developed into an embryo. A French institute is used frozen sperm to increase the diversity of African elephants in European zoos. Live offspring have been produced from the frozen embryos of endangered European mink.
Closer to home, the UC Davis Fish Conservation and Culture Facility in Byron has recently begun to experiment with cryopreservation in Delta smelt.
In a scenario that comes straight out of science fiction, frozen and thawed body tissues are now being induced to become pleuripotent—able to develop into any body cell. The hope is to be able to generate entirely new animals from the scraping of a skin cell a la Jurassic Park.
All of these techniques are extremely expensive and, even when successful, require many tries. For example, the of successful cheetah pregnancies from artificial inseminations using frozen sperm is less than 30 percent at Washington, DC’s National Zoo. As of now, conserving species on ice adds up to an impractical last resort. KMW