Cryopreservation of porcine oocytes: recent advances.

Successful cryopreservation of porcine gametes and embryos has been very challenging due to their sensitivity to cryoinjuries. Although considerable improvements have been achieved in the vitrification of porcine embryos, there has been no offspring born from the vitrified oocytes in this species. Porcine oocytes characteristically contain large amounts of cytoplasmic lipids that are major obstacles limiting efficient cryopreservation. These droplets together with structures such as mitochondria, membranes, cortical granules and basic components of the spindle and cytoskeleton (microtubules and microfilaments) often incur serious damage during cooling and warming. According to recent reports, the proper combinations of permeable and non-permeable cryoprotectants and vitrification with high cooling and warming rates may increase the survival of porcine oocytes. The cryotolerance of porcine oocytes may also be enhanced by removal of the chilling-sensitive lipid droplets, supplementation of cytoskeleton relaxants in vitrification solutions, or high hydrostatic pressure pretreatment of oocytes before cryopreservation. The improvement in cryopreservation methodology for porcine oocytes will no doubt augment other technologies such as pig cloning and the establishment of a gene bank for transgenic pigs.