**9. Conclusions**

Cryopreservation is the technique to preserve living cells at ultralow temperature, typically in liquid nitrogen (−196°C). Cryopreservation of oocytes and embryos is extremely important for propagation and conservation of genetically superior germplasm. Any cryopreservation protocol basically includes three major steps such as equilibration of cells to concentrated solution of cryoprotective agent, cooling and storage of cells to ultralow temperature, and recovery of frozen cells following thawing and warming. Cryoprotective agents protect vitality of the cryopreserved cells during processing and storage at ultralow temperature. Basically, there are two fundamental methods for cryopreserving oocytes and embryos, slow freezing and vitrification. The slow freezing method involves cooling of the samples slowly at controlled rate and formation of intracellular and extracellular ice crystals. In contrast, the conventional vitrification method involves rapid cooling of the samples and solidification of the cells including extracellular milieu into a glass-like state bypassing ice crystal formation. Ultrarapid vitrification is the modified and improved version of the conventional vitrification procedure that involves extremely high cooling and warming rates. Currently, ultrarapid vitrification is considered to be the more superior method than slow freezing or conventional vitrification. It is evident that cryopreservation often results in different types of cryoinjuries such as chilling injury, formation of ice crystal, fracture damage, osmotic stress, and formation of multiple asters. The quantum of cryoinjuries to

*Cryopreservation of Oocytes and Embryos: Current Status and Opportunities DOI: http://dx.doi.org/10.5772/intechopen.81653* 

frozen cells depends on many factors. Cryoinjuries are responsible for poor survivability of the cryopreserved cells. The cryopreservation process is associated with several other deleterious consequences and those in turn exert negative effects on the post freeze-thaw survivability of the frozen cells. Cryopreservation of oocytes is more difficult and yields poor success because of their larger volume and unique structural features as compared to that of the embryos. The procedures of oocyte and embryo cryopreservation have evolved significantly over the past five decades. Yet, the species-specific optimized cryopreservation methods with reproducible results are not available currently, especially for the oocytes and early stage embryos.
