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Embryo transfer (ET) is an advanced reproductive technology that allows breeders to produce multiple offspring from a single mare in a given year. This procedure involves taking an embryo from a donor mare, and depositing it in the uterus of a recipient mare who carries the pregnancy to term.
ET is becoming increasingly common in equine reproduction, and many equine veterinarians are able to perform the technique for horse breeders. However, embryo transfer requires extensive management of both the donor mare and the surrogate mare to have the greatest chance of pregnancy.
To perform embryo transfer, the donor mare is typically impregnated by artificial insemination to produce a fertilized embryo. Between Day 7 – 9 of gestation, the veterinarian flushes the embryo from the donor mare’s uterus, and prepares it for implantation into a synchronized recipient mare. The recipient mare’s ovulation must occur within 4 to 8 days of the donor mare’s ovulation.
Embryo transfer has a high success rate, with over 90% of embryo transfers successfully implanting in the recipient mare. [4] This article will discuss the process of retrieving and transferring embryos, as well as benefits and considerations of this reproductive technology.
Embryo Transfer in Horses
Embryo transfer is the process of taking an embryo from a donor mare, and transferring it into a recipient mare who will carry the pregnancy to term. It is essentially a form of surrogate pregnancy for horses.
The ability to have a different mare carry a pregnancy has many benefits, including: [1][2][3]
- Producing foals from performance and show mares currently competing
- Having multiple foals from the same mare in one year
- Ability to produce foals from mares with health issues or musculoskeletal problems, which prevent them from carrying their own foals
- Ability to produce foals from mares with reproductive health issues that prevent them from sustaining a pregnancy
However, not all breed organizations allow registration of embryo transfer foals, including the Jockey Club, which registers racing Thoroughbreds. [1] Horse breeders must investigate their breed’s specific regulations around assisted reproductive technologies before pursuing embryo transfer.
Stages of Embryo Transfer
There are three stages of embryo transfer that must be successful for pregnancy to occur. These stages are:
- Pregnancy in the donor mare to produce the desired embryo
- Flushing of the embryo from the donor mare
- Implanting the embryo into a synchronized recipient mare at the same stage of estrus as the donor mare
Recent advances in embryo transfer include the ability to cool or freeze embryos for storage or transport. Embryo cooling is widely accessible, and can be performed by most equine reproductive veterinarians. Embryo freezing is more complex and may only be available from certain specialists.
Donor Mare Management
Most donor mares are bred using artificial insemination, as this allows the veterinarian to precisely identify breeding dates and track the mare’s estrus cycle. [1]
Once fertilization occurs, the veterinarian continues to track the embryo’s development to identify the best time to perform a uterine flush, which removes the embryo from the donor mare. [1]
Embryos flushed around 7 days after ovulation are usually the ideal size for embryo transfer, however its size can vary depending on the mare’s age, stallion’s fertility, timing of breeding, and individual variation in the reproductive tract of the mare. [1]
Ultimately, the veterinarian must decide the best time to flush the embryo based on the results of ultrasounds of the reproductive tract.
Uterine Flushing
Historically, surgical techniques were the only available option for embryo recovery. These procedures were invasive, impractical and required general anesthesia of the donor mare. [4] Advances in embryo transfer techniques have led to the development of non-surgical procedures for embryo recovery, such as uterine flushing. [4]
To perform a uterine flush, the veterinarian introduces specialized tubing into the donor mare’s cervix, and connects the tubing to a bag of flush medium specially designed to support an embryo outside of the uterine environment. [5] Approximately 1 – 2 liters of flush medium enters the uterus, and then is allowed to flow back out of the uterus through a filter. [5]
The veterinarian will repeat this process 3 – 4 times to ensure that all embryos are flushed from the uterus, reducing the risk of embryos implanting in the donor mare and causing pregnancy. [5] Prostaglandins can also be administered after embryo collection to reduce the risk of pregnancy in the donor mare. [1][5]
After the uterine flush is complete, the contents of the embryo filter are examined to confirm collection of the embryo. After identifying an embryo, it is placed into a straw, which is a specialized, slender plastic tube that provides a sterile environment and is designed for holding and transferring embryos.
The embryo is then washed several times in flush medium to remove any contaminants or cellular debris. The embryo is then transferred into a petri dish for temporary storage while the recipient mare is prepared. [5]
Embryo Evaluation
After flushing, some veterinarians choose to evaluate the embryo to ensure it is viable and has a good chance of producing a successful pregnancy. Features that are assessed during an embryo examine include: [2][5]
- Size of the embryo
- Shape and symmetry
- Developmental stage of the embryo
- Whether the embryo wall is intact
Most embryos are good or excellent quality, likely because poor quality embryos cannot move through the oviduct properly and therefore are not able to be flushed. [3][5]
Embryo Biopsy
Embryo biopsy is another method of evaluating the embryo, testing for specific genetic traits that the embryo carries. Most embryo biopsies focus on serious hereditary diseases and are performed when one or both parents are known to be a carrier of the