Oocyte Cryopreservation Based in Sheep: The Current Status and Future Perspective.

The establishment of a cryopreservation procedure for sheep oocytes will enhance the development of long-term conservation of sheep female genetic resources and the advancement of sheep embryonic biotechnology. However, higher cytoplasmic lipid content and complex subcellular structures affect the relatively higher sensitivity of oocytes to chilling and freezing stresses. Currently, the reports related to sheep oocyte cryopreservation lag behind bovine or pig studies. A standardized freezing procedure has not been established. The present mainstream viewpoint favors the superiority of vitrification over conventional slow-freezing methods. The combination of permeable and impermeable cryoprotectants plus vitrification at a fast cooling/warming velocity benefits the cryosurvival of sheep oocytes. In this review, the research status and the cryoinjury mechanism of sheep oocyte cryopreservation will be reviewed in detail. Moreover, some technological highlights potentially influencing survival of cryopreserved sheep oocytes, such as delipidation, high hydrostatic pressure, or loading trehalose into cytoplasm, are summarized. Meanwhile, the future perspectives in the field of sheep oocyte cryopreservation will be discussed.