Embryo Recovery Procedures and Collection Success: A Review (AAEP 2010)

Since the first live foal produced by embryo transfer was born in 1974, the procedure has become one of the most popular assisted reproduction options for breeders with mares who are valuable or for mares that are to remain in competition.

During a presentation at the 2010 American Association of Equine Practitioners Convention, Patrick M. McCue, DVM, PhD, Dipl. ACT, associate professor of equine science at Colorado State University (CSU) reviewed the results of more than 490 embryo recovery procedures performed at CSU between 2004 and 2008. He discussed the flush techniques commonly used and the success rates of retrieving embryos from mares of various ages and reproductive status.

According to McCue, a majority of embryo flush procedures are attempted at Day 7 or 8 after ovulation. Flushes may be performed on day 6 or early on day 7 in an attempt to recover small embryos for cryopreservation (freezing). He typically uses four liters of fluid in an initial series of flushes, and he proceeds with another liter or two if an embryo isn't recovered during the first round.

The average embryo collection success rate of young mares (< 15 years of age) was 57.1%, while the embryo recovery rate of older mares (≥ 15 years of age) was 39.4%. In addition, he noted that embryo collection success was significantly lower in mares that experienced prolonged uterine inflammation after mating (i.e., a prolonged period of fluid accumulation).

The vast majority of mares were flushed on one day only. However, the group performed a re-flush the next day on 31 mares if no embryos were recovered on the first attempt. Embryos were recovered on only three of the next-day flush attempts and a majority of the next-day flushes had uterine debris present. Based on the low recovery rate, it was recommended to only perform an additional flush the next day if the mare was not going to be available for another embryo transfer cycle.

McCue and his team recovered a total of 257 embryos during the study. More than 97% of those embryos were recovered in "excellent" or "good" condition. Only six embryos were recovered in "poor" condition. McCue noted that it's rare to retrieve an embryo from a mare that was in "poor" condition or one that is not viable since most of these embryos are likely retained in the oviduct.

Additionally, he noted that mares bred with cooled semen had a 51.9% recovery rate, and mares bred with frozen semen had a 33.3% recovery rate. It was interesting to note that embryos recovered from mares bred with frozen semen were slightly smaller than embryos recovered from cycles in which mares were bred with either fresh or cooled-transported semen.

Finally, McCue said that the team recovered three unfertilized oocytes (eggs) out of the 490 flushes performed, and in one case a fertilized embryo was recovered with an unfertilized oocyte. In the horse, unfertilized oocytes are usually retained in the oviduct, while fertilized embryos produce a special type of prostaglandin (PGE2) that allows the transport of the embryo down the oviduct and into the uterus. Recovery of an unfertilized oocyte usually means that it was transported down the oviduct along with a viable embryo and that a viable embryo should be present.

About the Author

Erica Larson, News Editor

Erica Larson, News Editor, holds a degree in journalism with an external specialty in equine science from Michigan State University in East Lansing. A Massachusetts native, she grew up in the saddle and has dabbled in a variety of disciplines including foxhunting, saddle seat, and mounted games. Currently, Erica competes in three-day eventing with her OTTB, Dorado, and enjoys photography in her spare time.

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