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Chester Zoo Freezes Tissue Samples From The World’s Rarest Animals To Save Them From Extinction. Conservationists at Chester Zoo have begun freezing tissue samples of the world’s rarest animals as part of the battle to save hundreds of species from extinction. The zoo’s scientists have teamed up with some of the UK’s leading animal reproduction experts and has teamNature’s SAFE – one of Europe’s largest living biobanks to collect tissue samples from ovaries, testicles and ears are taken from animals that have passed away at the zoo. Using state-of-the-art technologies, are cryogenically frozen at temperatures of -196°C using liquid nitrogen. Among the 100 threatened species whose cells have been preserved are the critically endangered Eastern black rhino, mountain chicken frog, Javan green magpie and jaguar. Scientists say that in the future, as reproductive technologies advance, the frozen tissue samples may be used to restore lost genetic diversity in animal species threatened with extinction.