Cryopreservation of swine embryos: a chilly past with a vitrifying future

Since the development of embryo freezing technologies for cattle in the 1980s, advances in cryobiology, cell biology and embryology of domestic animals have enabled the development of embryo preservation methodology for the pig, notorious for extreme sensitivity to cooling. This review outlines recent efforts to understand the biology of pig embryos as related to their extreme sensitivity to cooling. Cellular analyses and molecular approaches are discussed that have enabled pig embryos to survive cryopreservation and after transfer develop into live offspring with normal fecundity at maturity. In the near future, use of preserved embryos will be a routine breeding alternative for swine producers, providing: preservation methods for maternal germplasm; global genetic transport; increased selection pressure within herds; breeding line regeneration or proliferation; and methodology for genetic resource rescue. It took almost 50 years after the first successful embryo transfer to develop embryo preservation in the pig. Nonetheless, by applying novel methods described herein, rapid progress has been achieved.