Cryopreservation and transfer of pig embryos

Recent advances in cryopreservation and non-surgical transfer of pig embryos have made embryo transfer in pigs a commercially viable technology especially for the international transfer of valuable genetic material. Early research demonstrated that early stage pig embryos were highly sensitive to temperatures below 15 degrees C and this sensitivity decreased with development to peri-hatching blastocysts which is accompanied by a reduction in lipid content. Removal of the lipid prior to freezing was found to improve the freezability and resulted in the birth of piglets. It was shown that by polarising the lipid material out of the blastomeres and using ultra rapid freezing (vitrification) the early stage zona pellucida intact embryo would also survive freezing and result in the birth of piglets. Other factors that have contributed to successful cryopreservation of the early stage pig embryo include the use of cytoskeletal stabilising agents, specific cryoprotectants, increasing cooling rates using open pulled straws or micro droplets and assisted hatching. The present review discusses the relative importance of each of these factors. The development of non-surgical transfer technology and its importance in the application of embryo transfer in the pig industry is also reviewed.