Freezing Equine Embryos is Getting More Practical

Freezing Equine Embryos is Getting More Practical

Both embryos that had been frozen immediately in a field setting led to pregnancies that lasted at least 30 days in the horse mares.

Photo: iStock

Researchers have taken a step closer toward making embryo freezing more practical: Study results revealed promising pregnancy rates with embryos that weren’t frozen in a laboratory, but in a breeding barn.

“If we can get the embryos to survive freezing and thawing in a field setting, we can finally offer this technology as a practical option to breeders and even use it to preserve endangered species over the long term,” said Florence Guignot, PhD, of the French National Institute for Agricultural Research, during a presentation at the 2017 French Equine Research Day, held earlier this year in Paris.

Equine embryos are far more difficult to freeze successfully than other species because of a particularly large liquid structure that becomes destructive when frozen because of its sheer size. A few years ago, researchers discovered the key to success: removing liquid and forcing the embryo to essentially collapse.

While that marked certain scientific triumph, it still wasn’t a practical solution in breeding centers, Guignot said. The liquid removal process is complex, requiring sophisticated, expensive equipment and intense technical training.

That’s why she and her fellow researchers developed a simplified protocol that could be used in field settings. They took 7-day-old embryos from Welsh pony mares and divided them into two groups: those they would freeze in liquid nitrogen immediately and those they would refrigerate for 24 hours before freezing.

In both cases, before freezing, they performed by hand a simplified technique under a microscope with a glass micropipette and used osmotic pressure differences to remove liquid from the embryo and collapse it.

“I put the embryos in a medium with an osmotic pressure that’s higher than the osmotic pressure inside the embryo,” Guignot explained. “That pressure difference just caused the embryo to empty itself out, to even out the pressure, without my intervention. And this gives us a lot more control over the percentage of embryo collapse than if we were pulling the liquid out manually.”

The scientists also used a simplified storage medium without added gas carbonation, a process used in standard equine embryo freezing that’s both complex and expensive.

Both groups of frozen embryos survived the thawing process well and took up their development again in laboratory dishes. The researchers also tested embryos from these groups in a live setting by transferring them into horse and pony mares. Both embryos that had been frozen immediately in a field setting led to pregnancy in the horse mares. Both pregnancies reached at least 30 days, but the researchers opted to terminate the pregnancies at this stage of the research.

However, none of the three transferred embryos that had first been refrigerated for 24 hours led to pregnancy. And transfer of either group to pony mares yielded very poor success rates: Only one pregnancy out of seven embryos frozen immediately and one out of five embryos that were refrigerated first led to pregnancy.

“These 24 hours, which we originally designated as being useful for transporting embryos to a laboratory for the more complex freezing procedure, could just be too long a time for the embryos to endure,” Guignot said. “We could try reducing that time to see if that affects their survival rates.”

However, the low success rates in pony mare recipients remains a mystery to the researchers, Guignot.

“This pony herd in the past has had good pregnancy statistics with frozen embryos, and the control group receiving fresh embryos also had good results,” she said. “It could be that these simplified-technique embryos are more stressed from the transfer through the pony mare cervix. But we just can’t really explain this phenomenon at this time.”

About the Author

Christa Lesté-Lasserre, MA

Christa Lesté-Lasserre is a freelance writer based in France. A native of Dallas, Texas, Lesté-Lasserre grew up riding Quarter Horses, Appaloosas, and Shetland Ponies. She holds a master’s degree in English, specializing in creative writing, from the University of Mississippi in Oxford and earned a bachelor's in journalism and creative writing with a minor in sciences from Baylor University in Waco, Texas. She currently keeps her two Trakehners at home near Paris. Follow Lesté-Lasserre on Twitter @christalestelas.

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