Progress on Vitrification of Oocytes from Mammalian Species

In comparison to the cryopreservation of preimplantation embryos,cryopreservation of oo- cytes from most mamrhalian species has been more challenging due to their extreme sensitivity to subopti- mal conditions during the cryopreservation process.Recent attempts to find a successful cryopreservation technique for oocytes have been directed to the use of vitrification by rapid cooling.They allow oocytes to be placed directly or preequilibrium for several minutes into the cryoprotectant and then plunged directly into liquid nitrogen.At present,vitrification techniques have entered the mainstream of animal reproduc- tion and many studies have been conducted in this area in recent years.In this paper,recent research on vitrification of oocytes were reviewed,with emphasis on the effects of type and concentration of cryopro- tect,cryopreservation methods and the meiotic stages of oocytes,and changes in the ultrastructure of the cell membrane,cytoskeleton,mitochondria,cortical granules and spindle.In conclusion,studies have shown that vitrification at the ultrarapid cooling rate is a promising approach for oocytes cryopreservation, but many problems remain to be solved on the damages of ultrastructure during cryopreservation process.