CRYOPRESERVATION OF OVA AND EMBRYOS FROM LIVESTOCK - CURRENT STATUS AND RESEARCH NEEDS

Over the last approximately 17 years freezing procedures have been developed allowing high survival rates for nonsurgically collected bovine morulae and blastocysts. The vast majority of experimental and commercial cryopreservation of bovine embryos has been done by controlled freezing and thawing procedures. Survival rates may reach 90 to 100% based on morphological evaluation after thawing and pregnancy rates of 50 to 60% following nonsurgical transfer are obtainable. Ovine, caprine and equine morulae and blastocysts have also been successfully frozen using controlled freezing and thawing rates. A simplified version of the controlled freezing and thawing is the one-step procedure that yields pregnancy rates of 40 to 50% with bovine embryos and avoids the laborious cryoprotectant dilution process and embryo evaluation after thawing. Vitrification and ultrarapid freezing are more innovative procedures which could significantly accelerate embryo freezing without the use of an expensive freezing machine. However, results with livestock embryos are still limited but bovine, ovine and caprine morulae and blastocysts have been vitrified successfully and following transfer given birth of normal offspring. In contrast, oocytes, early embryonic stages, micromanipulated (splitting, cloning, biopsy) and porcine embryos at all stages have not been successfully cryopreserved or they yeild only low survival rates. Thus, further intensive research efforts are required to develop appropriate freezing and thawing procedures that allow practical application and extend the potential of biotechniques such as embryo micromanipulation, in vitro fertilization, cloning and gene transfer.