"A component of embryo transfer is collecting embryos at one location and shipping them to a centralized embryo-recipient
transfer facility," says Vanderwall. This allows the veterinarian working with the embryo donor to focus on breeding the embryo
donor, flushing the embryo and not needing to worry about managing and synchronizing a recipient mare or several recipient
mares. The transfer is left to the centralized transfer facilities, which are used by many veterinarians and their clients.
"That has really increased the use of embryo transfer, removing the onerous burden of having to maintain and monitor the embryo
recipients from the person providing the breeding management and the embryo collection procedure for the donor," Vanderwall
Recovery of equine embryos is commonly done on Day 7 or 8 after ovulation, because the embryo does not enter the uterus from
the oviduct until Day 5 or 6 — while the large size of a Day 9 or older embryo makes them more prone to damage during collection
Due to the ease of manipulation of the anatomy of the equine cervix of the diestrus mare, the transfer of a Day 8 embryo (of
0.5-1mm diameter) is a less complicated procedure. A Day 8 embryo is visible to the eye. However, a dissecting microscope
of 10x to 50x will be required to locate a Day 6 embryo and is critical in case the embryo is developmentally retarded or
a smaller embryo from an asynchronous additional ovulation is present. Magnification will enable examination of the quality
and developmental stage of the embryo (i.e., late morula, early blastocyst or expanded blastocyst).
Most critical to the success of embryo recovery is the timing of insemination and the intrinsic fertility of the donor mare
and stallion. For most young, fertile mares with fresh semen from fertile stallions, 50 percent to 70 percent of flushes should
yield an embryo. Success decreases significantly for older mares >14 years of age, with a history of subfertility, or when
chilled, transported or frozen-thawed semen is used.
Another important factor for recovery success is the time at which the flush is performed relative to ovulation, because the
equine embryo does not enter the uterus until Day 5 or 6 after ovulation. Therefore, flushing on Day 6 is often associated
with a significantly lower embryo recovery rate because the earlier embryo has failed to exit the oviduct.
Superovulation is a way to increase the probability of success, the embryo yield and the number of pregnancies in recipient
mares. But it's a difficult procedure for horses. Why?
"We don't have a set of medications for hormone therapy options," McCue says. Equine FSH (follicle stimulating hormone) and
a recombinant equine FSH — both of which stimulate follicular development and get multiple ovulations in mares — are no longer
"The second issue is, there are limitations in the horse as to how many follicles we can actually stimulate and how many ovulations
we can get," McCue says. The anatomy of the equine ovary is limiting in that they have very large follicles, though mares
ovulate through a very small section of the ovary, the ovulation fossa. There is a tough fibrous capsule that surrounds the
outside of most of the ovary.
"We really can't get more than three or four large follicles to develop on each equine ovary, and they don't ovulate off the
outside of the ovary, but through the ovulation fossa," McCue says. "Add all those things together and from a practical standpoint,
even if we had the medications, our average ovulation rate in mares with superovulation is about three to four ovulations
per cycle." That's not even close to the success rates in ruminants.
Until recently, transfer of equine embryos at many facilities was performed surgically, either via ventral midline laparotomy
under general anesthesia or through the flank using local infiltration anesthesia in the standing, sedated mare. Today, with
significant improvements in the success of transcervical transfer, the surgical procedure is essentially obsolete because
the newer method is easier, cheaper, quicker and better for the mare's welfare.
Certainly, nonsurgical transcervical procedure has made embryo transfer a more practical clinical procedure. But there are
some important considerations for success. Minimizing contamination is very important, but can be potentially picked up from
the mare's vulva or vagina. Minimal trauma to the cervical canal and endometrium is important too. Transfer success depends
on several factors: embryo quality, proper transfer technique, donor-recipient synchronicity and recipient suitability.
Gross embryo quality, as assessed by a light microscope, affects embryo transfer success/pregnancy and the incidence of subsequent
pregnancy loss. Fortunately, the majority (>90 percent) of harvested equine embryos are of good morphological quality.
The two major factors in technique that lead to failure are bacterial contamination of the progesterone-dominated uterus and/or
hormonal disturbances initiated by excessive dilation or manipulation of the cervix. Therefore, some practitioners treat
the mare with systemic antibiotics before and after the transfer to reduce the likelihood of bacterial contamination developing
into a pregnancy-threatening endometritis. Flunixin meglumine is used immediately before a transfer, and progestagens are
initiated for two to three weeks (or longer) post transfer to reduce the risk of pregnancy failure due to transfer-induced
PGF2a release and luteolysis.
Donor-recipient synchrony is considered the most important factor for success. Large variability in estrus length between
mares complicates the synchronization procedure. Good uterine and cervical tone at the time of transfer also is ideal for
the establishment and maintenance of pregnancy.
Other factors important to establishing and maintaining a pregnancy in recipients after an embryo transfer include:
- good health and body condition
- between 3 and 12 years old
- exhibits normal cyclicity and shows no obvious signs of reproductive tract pathology
- cervix is undamaged and not unduly tortuous.
Size of the recipient mare compared to the embryo-donating mare is important too. Undergrowth or overgrowth of the fetus in
utero, regardless of compensation of any compensatory post-natal development, may be maintained into maturity. Retarded growth
within a smaller mare's uterus may present as physical and behavioral immaturity at birth. It can also delay a foal's time
to stand and suckle.
In the past few years, Vanderwall says there has been more interest among veterinarians and their clients on the benefits
of freezing embryos and using commercially available embryo vitrification kits. This method is more practical than traditional
freezing procedures that involve the use of cryoprotectants like glycerol.
The primary benefit of the kits is that you can flush an embryo from a mare, whether you've got a recipient or not, freeze
it and vitrify the embryo for transfer into a recipient mare at a later date. The main limiting factor is the need for collecting
a Day 6 to very early Day 7 embryo.
For vitrification, the embryo needs to be less than about 250-300 microns in diameter: a morula-stage embryo to a very early
blastocyst-stage embryo. The timing of the flush procedure becomes critical with the goal of collecting a very small-size
embryo for vitrification.