This review presents evolution and current possibilities, state of knowledge and prospects for cryopreservation of pig embryos. In the early stages the development of this technology for use in the international pig industry was slow. Initially freezing technologies utilized the stepwise method to cryopreserve swine embryos. Conventional freezing methods will not work for pigs embryos, which are extremely sensitive to slow cooling below temperatures of approximately 15°C, since, as they cool, they undergo physiological and structural changes that leave them incapable of normal development. Using a rapid cooling processes – vitrification - is thought to outpace the damaging effects of slow cooling. It allows for an increase of the cryopreserved pig embryo survival rate to more than 80% in the laboratory. At present, vitrification is regarded as an alternative to traditional slow freezing procedures which do not offer satisfactory results for the cryopreservation of porcine embryos. Recently a novel approach consisting of a minimum sample size, increased cooling rate from 2500 C/min to 20.000 C/min, and the use propyleno and ethyleno glicol in the vitrification solution have been effective for crypreservation of pig embryos. Many factors such as the stage of embryonic development, cryoprotectant toxicity, the composition of vitryfication solution, cooling and warming rates can influence the survival of pig emryos after vitrification. Peri-hatching and hatched blastocysts tolerate slow freezing without special pretreatment, while morula and early blastocysts do not survive this cryopreservation procedure. Vitrification results in higher survival rates after warming when untreated early-to-hatched-blastocysts-stage embryos are used. An increase in cooling rate decreases sensitivity to slow freezing and may permit a reduction of cryoprotectant concentration.
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