Frozen Assets

Developments in equine reproductive technology tend to be slow to arrive, and even slower to be taken up commercially. This may be due to tradition, expense, and the time it takes to refine a technology until it becomes commercially viable in terms of price and success rates. One such relatively recent development is the ability to freeze equine embryos, though the first frozen-thawed foal was born in Japan in 1982. The AQHA now allows registration of multiple foals per mare each year, a privilege once available only to stallions, and many sport horse studbooks permit a limited number of foals per mare to be born annually via embryo transfer.

Like frozen semen, a frozen embryo could, theoretically and if properly stored, be stored indefinitely. Having a frozen embryo on-hand carries with it similar convenience to that of frozen semen - the recipient mare does not require synchronisation with the donor, reducing the management and expense involved prior to the transfer itself, provided the recipient is monitored closely.

How Does it Work?

Embryo freezing initially works much like a straightforward embryo transfer - the donor mare is inseminated, and the embryo is flushed at the correct time. For a direct “fresh” embryo transfer, the embryo is usually flushed at 7-8 days post-ovulation, however an embryo destined for storage in the liquid nitrogen canister must be flushed at the 6.5 day mark, not long after descending into the uterus (day 5-6). These younger, smaller, embryos have proved more resilient and freezable than their older compatriots. Once flushed, the embryo is then subject to a process called “vitrification”.

One of the difficulties which needed to be overcome in embryo freezing technology was damage sustained during the freezing process. If ice crystals form in the embryo, it becomes damaged during freezing and thawing, and is rendered non-viable. A process needed to be created to protect the embryo from the formation of these ice crystals. This process is known as vitrification, and involves the application of cryoprotectants which provide an antifreeze-like protection to the embryo. The cryoprotectants must penetrate the cell membrane, and lower the freezing temperature within the cell itself.

Another difficulty is the fact that a reliable technique for superovulation of mares has yet to be developed. Superovulation is the artificial increase in number of ovulating follicles per cycle, usually achieved through hormone treatment. Such treatment is currently very successful in cattle, resulting in several embryos flushed and frozen per cycle, greatly reducing the costs per embryo. The horse does not respond as well, and attempts at superovulation generally do not usually significantly increase the number of embryos recovered during a flush.

Once the embryo has been flushed, it is transferred to a holding medium, and then is transferred through a series of vitrification solutions. The embryo may only remain within each solution for a set period of time before being moved to the next, or before being placed transferred to a straw for cooling, freezing, and storage. When the time comes to transfer the embryo into a mare, the embryo is carefully thawed according to the instructions provided by the facility performing the vitrification and freezing, and is transferred through the cervix into the recipient mare’s uterus. There are presently no tests available to determine whether the frozen embryo has remained viable throughout the process, so the mare owner must then wait for a pregnancy diagnosis scan.

Why freeze embryos?

Frozen embryos give mare owners the flexibility to store embryos until a convenient time for transfer. Additionally, if multiple embryos are flushed, all embryos may be stored, potentially resulting in more than one pregnancy per insemination. If multiple embryos are flushed for a direct embryo transfer and only one recipient mare is available, only one pregnancy will result. Embryos can also be frozen and stored during a downturn in the youngstock market, and transferred when the market picks up. Finally, embryo freezing technology can benefit the preservation of equine genetics, whether those genetics are from rare breeds, or exceptional individuals. Dr. Madeleine Campbell at Hobgoblins Equine Reproduction Centre in East Sussex has embarked on a limited programme of embryo freezing, and has frozen full siblings of the young stallion Tribute Tertius for future use.

Embryo freezing is not widely available commercially in the UK at present, however that may change in the future as the technique becomes better known and further refined, and it becomes a more generally accepted technology. Success rates quoted in previous research at Colorado State University, known for being at the forefront of equine embryo technology, range from 54-70% pregnancy rates after recovery and transfer of a frozen and thawed embryo. Embryo freezing is much more easily available in the US, where the Quarter Horse industry seems to have embraced it with open arms.

Finally…

While it may not be very common in the UK today, embryo freezing may one day become a viable option to preserving the genetics of a valuable mare, as semen freezing is a store of value for stallions. Increasing the production capacity of a mare may also result in an increased value for these top-class performance and broodmares. If a reliable method of superovulation is developed, and the freezing process is further refined, embryo freezing may become a very real option for British Breeders wishing to use veterinary intervention to help invest in a store of “frozen foals” for future use, just as direct embryo transfer has become both more successful and more commonplace in sport horse breeding worldwide.

Further Reading

Freezing Embryos

http://www.thehorse.com/ViewArticle.aspx?ID=10707

Frozen Embryos

http://www.cvmbs.colostate.edu/bms/erl/PDF/Learnmares10_ET_frozemb_apr09.pdf

History of Embryo Transfer :

http://www.cvmbs.colostate.edu/bms/erl/PDF/Learnmares11_ET_history_apr09.pdf