Establishment of Practical Embryo Transfer for Fresh or Frozen Embryos in Pigs

The most important benefit of embryo transfer (ET) in the swine industry is the ability to introduce new genetic stock as replacement herd animals with minimal risk of disease transmission and cost for transportation, in comparison with the transport of live animals. However, the commercial application of ET in the pig has been limited because of the need for surgical procedures for both the collection and transfer of the embryos. Recent advances in systems for in vitro production (IVP) of pig embryos, including in vitro oocyte maturation, fertilization and embryo culture, have enabled us to generate viable embryos that can develop to full term after transfer into recipients. In our previous studies, surgical transfer of 18–25 IVP blastocysts per recipient into 11 recipients has resulted in the pregnancy rate of 100%, the efficiency of piglet production of 25.5% (61 piglets for 239 transferred blastocysts) and the average litter size of 5.5 ± 2.5 (mean ± SD) piglets. We also have succeeded in producing piglets after surgical transfer of cryopreserved IVP blastocysts, which were vitrified using the Cryotop method. Now, IVP of pig embryos can provide viable embryos more efficiently in terms of cost and time when compared with the collection of in vivo derived embryos. In the 1990s to 2000s, instruments to deposit embryos in the pig uterus non-surgically have been developed, resulting in the birth of live piglets. The non-surgical ET technique using a deep intrauterine catheter for insertion through the cervix deep into a uterine horn is practicable on farms, because it does not need special facilities, such as surgical and anesthesia equipment. We previously developed a simple catheter made from polyethylene for non-surgical intrauterine transfer of pig embryos. Using this catheter, 17 of 68 recipients (25.0%) transferred IVP blastocysts farrowed and the litter size averaged 4.6 ± 2.7 piglets. The efficiency of piglet production was calculated to be 3.5%. Non-surgical ET of fresh in vivo derived embryos using a similar system into 47 recipients resulted in a farrowing rate of 34.0% and an average litter size of 6.9 ± 2.3 piglets, giving an efficiency of piglet production of 13.8% (110 piglets for 800 transferred embryos). Moreover, non-surgical transfer of vitrified in vivo embryos resulted in a farrowing rate of 29.0%. Although the farrowing rate and the efficiency of piglet production after the non-surgical ET are still inferior to those achieved by surgical ET and needed to be improved, it appears that IVP of embryos together with non-surgical ET and embryo storage technology could be applied at a practical level in swine industry.