Bovine oocytes cryoinjury and how to improve their development following cryopreservation

Bovine oocytes are less likely to undergo successful cryopreservation than cleavage-stage embryos. Bovine oocytes characteristically contain high levels of lipids that represent one of the major obstacles limiting efficient cryopreservation. These droplets together with structures such as cumulus cells, zona pellucida, cytoplasm membrane, cortical granules, mitochondria, spindle, and cytoskeleton (microtubles and microfilaments) often incur serious damage during cooling and warming. The cryoinjury could, to some extent, be decreased by selection of proper permeable and non-permeable cryoprotectants, and of vitrification with high cooling and warming rates. Additionally, such measures may also enhance their cryotolerance as partial removal of cumulus cells, modification of oocyte membrane constituents, polarization of the cytoplasmic lipid droplets by centrifugation, and addition of cytoskeleton relaxants or ice blockers into vitrification solutions. The improvement in cryopreservation methodology for bovine oocytes will no doubt augment other technologies such as bovine cloning and the establishment of gene bank for transgenic cattle.