A718
Reproduction - Embryo Transfer

why utilize embryo transfer | donor mares | recipient selection and synchronization | embryo recovery | transfer of embryos | costs

Introduction: The art of embryo transfer (ET) is an ever changing science that involves three major events. It begins with selection and insemination of the donor mare with fresh, cooled or frozen semen. Next, the embryos are recovered from the donor through nonsurgical means, evaluated, and then cooled and shipped or transferred fresh. Lastly, the recipient animals are synchronized and receive the embryos using either surgical or nonsurgical techniques. This article will briefly discuss the history and benefits of embryo transfer. Then the remainder of the article will focus on the details of ET and discuss surgical and nonsurgical approaches.

History: The first successful embryo transfer was performed in 1890 using rabbit embryos. The first successful horse embryo was transferred in 1972; however, it was not until the 1980’s that technological advances made it possible to do equine embryo transfer on a broader, more economical scale.

Why Utilize Embryo Transfer: There are many reasons a horse owner might select embryo transfer. Some of the more common reasons are listed below:

1. Valuable performance animals can produce foals without hindering training and performing.
2. Mares that are unhealthy or reproductively unsound to carry a foal can still have offspring.
3. Multiple foals can be obtained from one mare each year.
4. Two-year-old mares can produce foals.
5. Embryo transfer also eliminates the stress of parturition on a desirable animal, thereby increasing her reproductive life span.

Donor Mares: Selection of a proper donor is essential to the success of the embryo transfer program. A potential donor can be selected based on the following criteria:

1. Possesses superior and highly heritable traits.
2. Should not be aged or be subfertile.
3. Must have regular, normal estrous cycles.
4. Has not had previous conformational, parturition, or reproductive problems.
5. Must be disease free.

Once a donor has been selected, follicular development must be monitored on a regular basis (daily during estrus). Many transfer programs utilize human chorionic gonadotropin (hCG, 5 IU/kg IV or IM) or deslorelin acetate (Ovuplant pellet) to help control the time to ovulation. The day of ovulation is considered Day 0. Superovulation in mares is done using a treatment regime involving daily injections of equine follicle stimulating hormone (FSH). Usually, only 2-3 embryos result from the treatment. Additional suggestions and details on controlling a mare’s estrous cycle can be found on page A714 of this manual.

Embryo Recovery: In most cases, the embryos are collected from the donor animals on day 7 or 8. Most equine embryos are recovered using a non-surgical technique. Sterile equipment and lubricant is used throughout this procedure. Once the mare has been placed in suitable stocks, the perineal area is cleaned with soapy water, rinsed, and dried. A balloon-tipped catheter is placed in the vagina, through the cervix and into the uterus. The balloon-cuff is inflated and the catheter is pulled backwards against the cervical opening to prevent fluid loss. The uterus is then flushed 3-4 times with 1-2 liters of warm flushing solution. A common flushing solution may include Dulbecco’s PBS solution with 100 IU/mL of penicillin, 100 micrograms/mL streptomycin, and 1% calf serum. After each flushing the fluid is allowed to flow out of the uterus and is passed through an embryo filter. The uterus is gently massaged by a hand placed in the rectum to help move the fluids throughout the uterus and out to the filter. At least 90% of the fluid placed into the uterus should be recovered and the fluid should not be cloudy or contain any blood or discharge. Cloudiness may indicate that the mare has a uterine infection, and blood may indicate that the uterus manipulation was too aggressive or that the placement of the catheter damaged the lining of the uterus. This can decrease embryo viability.

After the embryo has been collected, it is then "washed" and graded on a scale from 1 to 4. One being excellent and 4 being poor. Once graded, the embryos are then processed for transport or are transferred directly to the recipient mare. Every effort should be made to prevent rapid changes in temperature to the embryo. When transporting, Ham’s F-10 nutrient mixture is often used.

Transfer of Embryos: In horses, the placement of embryos can be done either surgically or non-surgically. Recent studies suggest that both methods have equal success rates (70-75%) when done properly.

Surgical techniques involve properly restraining the recipient mare in stocks and administering adequate sedation and local anesthetic blocks in the flank region. The flank area is clipped and sterilized. An incision is made, and the uterine horn is gently pulled to the outside. A small puncture is made in the uterine horn, and the embryo is deposited. Because the embryo can move around freely, it is not essential that the embryo be placed on the same side of the uterus where the ovulation occurred. The uterine horn is not sutured closed, but the uterus is placed back into the abdomen and the incision is closed.

The non-surgical technique to embryo transfer uses a standard artificial insemination pipette and an insemination gun. The recipient mare is placed in the stocks, and the perineal area is cleaned with soapy water, rinsed, and dried. A sterile plastic sleeve is placed over the operator’s hand and the tip of the transfer instrument (covered by the outer guard) is placed in the palm of the hand. To prevent damaging the vagina, the operator's thumb is placed over the tip of the insemination gun. Once the instrument is in the vagina, the tip of the outer guard is placed about 0.5 cm into the cervix. The insemination gun is pushed through the outer guard and into the body of the uterus. The embryo can then be deposited in the body of the uterus or in one of the uterine horns.

Costs: The costs associated with ET can vary greatly from one farm or situation to another. It is important to understand the possible costs and determine if the benefits of ET outweigh the added expenses. The cost will vary greatly depending on the price of the semen and any collection, handling and transfer fees. Increased labor expenses and costs for synchronization medications should also be considered. Do not forget that the most significant expense associated with ET will be the cost of maintaining the necessary recipient animals. This cost would be in addition to the above costs for simply establishing an ET pregnancy. The costs for maintaining recipient mares is not something most horses owners will realize directly; however, it will be reflected in the price quoted for an embryo transfer procedure.

Summary: The techniques involved with embryo transfer are not extremely difficult or complicated. With sufficient training and experience embryo transfer can be a very viable method for genetic improvement or reproductive manipulation within many different situations. As the science associated with equine embryo transfer progresses, applications for its use will continue to become apparent in the laboratory, university, large breeding farms, and even the small horse operation.

References: D. Vanderwall, Current Equine Embryo Transfer Techniques, Department of Animal and Veterinary Science, Northwest Equine Reproduction Laboratory, University of Idaho, Moscow, Idaho, USA