I have a very well bred, premium MM book 19-year-old Hanoverian mare with a lot of great foundational stallions in her pedigree - she pairs very well with Rubinstein sons or Weltmeyer sons. We've decided we don't her pregnant again (she chokes very frequently due to stricture from trauma), but I would like to consider her as a donor ET.
She does pool some fluid due to tilting, rarely is infected. This alone might rule her out. She cycles regularly, breeds easily, and gets pregnant easily. Other than choking, she has no real health problems. She’s in great condition!
She has had 2 successful full-term pregnancies, although this year's filly was born stillborn. She was a maiden mare in 2007 at the age of 17, two tries, and produced a gorgeous Rubinstein granddaughter, having held her pregnancy without needing any special attention. This year would have been her second foal, and this pregnancy came after three tries, (got flushed, but no drugs).
For her, getting pregnant isn’t a problem. I would like to freeze her embryos until I find a suitable recipient mare. It is my understanding that once frozen, the embryos are viable for years? No doubt, it will be a challenge and probably expensive.
If the mare can get pregnant then she is definately a candidate for use as an embryo donor. Freezing embryos is an option but the success rate is much higher by doing a fresh transfer. Equine embryos do not freeze as well as bovine embryos but they can be frozen. The cost of freezing the embryo is not that expensive. You just need to find an ET professional near you. Good luck.
Thank you very much - I will definitely discuss this with my repro vet.
What is it specifically about the equine embryo that make them more susceptible to loss when frozen? Their structure? The way the cells divide? Or are they just more sensitive to the shock of the nitrogen?
There is an interesting conundrum with freezing and using frozen equine embryos. As Dr. Dorn observes, the success rate when freezing is lower than with bovine embryos, however several facilities that freeze equine embryos observe that they have a better pregnancy rate with frozen embryos than with fresh transfered, so the actual success rate in the long run equals out to be able the same for fresh and frozen equine embryos. The hypothesis for this is that with a fresh embryo, if your recipient mare has not lined up ideally for receiving the embryo, then tough - you still have to transfer; whereas with a frozen embryo you just wait until your recipient is at the right point for transfer (i.e. don't thaw!).
Equine embryos are typically harder to successfully freeze and maintain post-thaw because of the presence of a structure unique to the equine conceptus called (originally enough! ) "the capsule". This structure appears around day 7 post-fertilization and surrounds the conceptus. It disappears somewhere around day 21. All of its functions are not clear, but it is thought that one important function is protection during the period of conceptus mobility through the uterus (up to about day 16). This mobility is achieved by the conceptus secreting very low levels of Prostaglandin F2α which causes uterine contractions, thereby propelling the conceptus around the uterus. Ironically an absence of conceptus mobility causes the uterus to secrete Prostaglandin F2α at a higher level, which will cause pregnancy loss!
The bottom line on this capsule WRT equine embryo freezing is that is prevents or reduces the ability of various cryoprotectants to successfully penetrate the tissues and do their job, so it is necessary to flush equine embryos for freezing on day 6 (prior to formation of the capsule), which reduces the overall success rate of harvesting. Cryopreservation after that point has to contend with the capsule presence, and therefore a lower survival rate post-thaw.
Hummmm. Well that's an interesting point or two to mull over. Thanks!
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) "the capsule". This structure appears around day 7 post-fertilization and surrounds the conceptus. It disappears somewhere around day 21. All of its functions are not clear, but it is thought that one important function is protection during the period of conceptus mobility through the uterus (up to about day 16). This mobility is achieved by the conceptus secreting very low levels of Prostaglandin F2α which causes uterine contractions, thereby propelling the conceptus around the uterus. Ironically an absence of conceptus mobility causes the uterus to secrete Prostaglandin F2α at a higher level, which will cause pregnancy loss! 
