Vitrification as a means of cryopreservation has become a standard approach for oocytes from livestock. This paradigm shift occurred primarily as a result of the demonstration in 1996 that bovine oocytes are extremely susceptible to chilling injury. Since that early work, numerous devices have been used as supports for oocytes during so-called "ultra-rapid cooling", and occasionally, trials involving the deposition of small volumes of media containing oocytes directly into liquid nitrogen to facilitate cooling have been reported. Results reporting blastocyst development exceeding 10% are common, but variability remains high, and a standard method for bovine oocytes remains to be established. Oocytes from pigs are particularly difficult to cryopreserve, even with the use of ultrarapid cooling approaches. Few reports have demonstrated blastocyst development exceeding 5%. The application of hydrostatic pressure before vitrification appears to impart stress tolerance to porcine oocytes, as the results of some treatments have shown development to blastocysts at proportions >10%. Work on sheep oocyte vitrification is relatively new, and a few articles have reported blastocyst development at 10% or more. Messenger RNA levels are reportedly altered in sheep oocytes as a result of vitrification, and damage to the cytoskeleton is common across species.
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