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By S.L. Terlouw and J.R. Dobrinsky, Minitube International Center for Biotechnology, Mt Horeb, Wisconsin, USA
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| Porcine embryo cells, shortly after fertilisation. |
This article introduces the basics of swine embryo transfer, a collective term for a technology that could well be more common in the future of the swine industry. Once an oocyte or ‘egg’ is fertilised by a sperm cell, the resulting zygote is an embryo. The early stages of embryonic development have been the focus of embryo transfer in multiple species for many years.
What is ET?
Embryo transfer (ET) refers to the area of reproductive science involved in removing embryos from the uterus of one female (the donor) and transferring one or more of them into a host or surrogate female (the recipient). The first mammalian offspring resulting from ET was achieved in rabbits almost 119 years ago! The first ET piglets were born on March 27, 1950 in Ukraine. Since then, research on porcine reproduction including ovulation, fertilisation, embryo development, preservation, migration, implantation and survival has been the result of ET related research.
Embryo transfer is now a global industry that reports the transfer of nearly 955,000 commercial (non-research) embryos from mammalian species in 2005, 92% of which are from cattle. The remaining 8% are primarily derived from sheep, goats, pigs, and horses. It is estimated that nearly 10,000 in vivo (derived from live animals) and over 40,000 in vitro (derived from laboratory research projects) pig embryos are produced each year. The major countries reporting this activity to the International Embryo Transfer Society include the United States, Canada, Taiwan, Korea and countries in the European Union.
How is ET possible in the pig?
Commercial embryo transfer has been a practical alternative in cattle breeding for years. The reproductive tract and the size of the animal enable rectal palpation of the reproductive tract, further enabling the non-surgical rectal-vaginal transcervical deposition of an embryo into the ipsilateral (side of ovulation) uterine horn. In the pig, the smaller size of the animal limits or inhibits the use of non-surgical rectal-vaginal trans-cervical deposition of embryos. As well, the pig is a polytocous animal with characteristically long uterine horns needed to nurture a brood or litter of piglets during gestation. It is the presence of these long uterine horns and the characteristic ‘corkscrew’ type cervix of the pig that hinders the non-surgical transcervical collection and deposition of embryos. Therefore, scientists and veterinarians have long relied on surgical mid-ventral laparotomy for the recovery and transfer of embryos in the pig.
Isn’t ET in the pig redundant?
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Conventional embryo transfer in sows as it has been practised for a long time, using surgical mid-ventral laparotomy.
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Pigs are a litter-bearing species and, in contrast to a species like cattle where ET is extensively used, pigs are relatively young at puberty and have a short gestation length. As a result, aggressive mating selection in swine operations that use AI already allows for rapid genetic improvements, without the need of ET.
Therefore, what motivating factors drive the industry to continue to attempt implementing embryo transfer techniques? Here are a few:
Rapid genetic progress
Only 50% of the genetics can be affected in one generation through the use of AI, whereas 100%, or entirely new genetics, can be introduced with an embryo.
Expanded markets
Pig production continues to grow worldwide, and breeding companies that operate internationally will benefit from the cost-effective ability to transport embryos instead of live animals. This will also allow smaller operations the opportunity to become more competitive by marketing unique genetics in the form of stored/shipped embryos.
Reduced health risks
Embryo transfer can be used to bridge health differences. Pig embryos can be removed from females with diseases such as pseudorabies or Porcine Reproductive and Respiratory Syndrome (PRRS), ‘washed’ and transferred to a recipient to establish disease free herds. This unique characteristic of ET can also be applied to minimise or eliminate unknown disease risk between herds desiring rapid genetic progress. This use of ET has been referred to as ‘genetic rescue’.
Genetic leverage
Use of other biotechnologies such as cloning and embryo cryopreservation require ET for production of live offspring.
Medical sciencePig organs are the most compatible with humans compared to other species because of similarities in anatomy, physiology and size. Ethically, they are less controversial than non-human primates. In the last decade, the number of patients waiting for organ transplant tripled leaving the US Department of Health and Human Services organ donation programme over 50,000 short. Embryo transfer procedures are a critical step involved in the development and production of gene-altered pigs suitable for supplying these organs. As well, the pig has been recognised as a more suitable animal for the basis of medical models of human disease. In the US, less than 8% of studies using rodent or other traditional models for human disease that head to clinical trials actually make it to a finished product treating the human condition. The pigs’ unique physiology and anatomy make it a more suitable model for human disease, and the pig is being actively used for genetic manipulation for medical model production for diseases such as atherosclerosis, diabetes, cystic fibrosis, muscular dystrophy, cancer, and other dreadful diseases.
What are the basics?
Below is a list of management and technical factors that would be involved when integrating ET into a swine operation.
Oestrous cycle control
If the process of ET is to be successful, the donors and recipient females must be synchronised (for fresh embryo transfer) so that the recipients are in oestrus near the same time as the donors.
Super-ovulation
Even though the pig naturally releases or ‘ovulates’ 10 to 25 oocytes per oestrous cycle, the donor can be hormonally stimulated to produce more, making her even more efficient, since the process of ET involves removing those embryos from the donor female. However, super-ovulation is not necessary for successful ET.
Oestrus detection
A reliable and consistent method of oestrus detection and record keeping must be employed for both donors and recipients so that the recipients can be precisely matched with individual donors and their embryos.
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Source: Pig Progress, volume 26, no.3 2010
