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Embryo transfer

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These two calves are the results of embryo transfer

Multiple Ovulation and Embryo Transfer (commonly called embryo transfer) is a process of improving animal genetics by selectively choosing high quality donor females, breeding them to high quality males, removing the embryos and transferring them into a lower quality recipient female that raises the high quality offspring. This procedure has been present since the late 1800s, but became especially useful for agriculture in the 1970s. In general, the procedure is harmless to the donor and recipient females.

Procedure

This image shows the system used for flushing the embryos (step 4). The diagram shows the reproductive tract of a horse.

The process of Multiple Ovulation and Embryo Transfer is similar for most animals. Embryo transfer breeding has been mostly used in livestock animals including cattle, horses, and goats, but has also been used in humans.[1] Although there may be slight differences in the procedure for different animals, all follow the same general collection of steps[2].
NOTE: The following steps follow the process of cattle embryo transfer.

1. Selecting Embryo Donor
In selecting an embryo donor, one must assess the prospects, evaluating each one on criteria that varies depending on the animal and the animal's job. Several qualities to consider in this step include the animal's conformation and structure, pedigree, performance, and in the case of dairy animals, milk production. Because of the expenses involved in embryo transfer, the embryo donor should be a high quality animal to ensure a profit in the offspring or produce a quality offspring for the owner to keep. In addition, the mother should have a sound reproductive tract and have regular heat cycles beginning at a normal, young age, need no more than two breedings per conception, have a standard length pregnancy time, have no previous issues or irregularities in prior breedings, and produce previous offspring without significant defects.[2]

2. Superovulating the Donor
Superovulating is the process of causing an animal (such as a cow or horse) to ovulate several eggs in one estrus. Occasionally, a cow will ovulate as many as ten eggs at a time, but on average, superovulation in cattle results in five eggs. To stimulate superovulation, a the donor receives subcutaneous or intramuscular follicle stimulation hormone (FSH) shots. These shots are given two times a day for four days in the middle of the estrus cycle. on the third day, the donor receives a prostaglandin shot that triggers an estrus within the following forty-eight to sixty hours.[2]

3. Inseminating the Donor
The next step in the embryo transfer process is the insemination of the female. This may be done with either fresh or frozen semen through artificial insemination. [1] Commonly, breeders will inseminate the female at twelve, twenty-four, and thirty-six hours after the beginning of estrus. The semen must be high quality in order to ensure fertilization.[2]

4. Flushing the Embryos
After seven or eight days, the newly developed embryo is flushed out of the donor. In this step, a rubber catheter is placed through the cervix and into the uterus where 2mL of a saline solution is injected. The saline acts as a sealant for the catheter which rests in the internal os of the cervix. In the flushing process, a veterinarian uses one liter of a biological medium to fill and drain the uterus several times, using forceps to control the rate of fluid flow through the uterus. An inflatable cuff is filled with air to prevent the fluid from flowing to far into the reproductive tract. The tube running out of the uterus dispenses into a graduated cylinder where the embryos are later located. The harmless flushing process typically takes about 30 minutes.[2]

5. Evaluating the Embryos
After the embryos have been located with a stereomicroscope and acquired through an aliquot, they are examined and graded as either as excellent or good, fair, poor, and dead or degenerating. These ratings are given based on the embryo's size and shape, (blastomere) compactness, appearance of the cytoplasm, presence of other egg cells, presence of vesicles, and the normality of the zona pellucida. In addition, the embryos are rated on their developmental stages, leaving the their grade out of consideration. In a test of 1,116 cows of 15 different breeds, results showed that 58% of the embryos were fit for transfer into a surrogate mother, 31% of the eggs were never fertilized, and 11% were dead or degenerating.[2]

6. Selecting and Preparing the Recipient Mother
In embryo transfer, the recipient females must be synchronized with the donor in their estrus cycle. A similar prostoglandin hormone shot is given to the recipient to have her enter the estrus stage. Other methods of synchronization include synthetic progesterone as well as specialized synchronization drugs.[2]

7. Transferring the Embryos
Before the transfer of the embryo, a veterinarian palpates the ovaries to detect which one has ovulated. Next, an insemination straw with the embryo is guided through the reproductive tract and into the side of the ovary that has ovulated. The embryo is then slowly and carefully ejected near the uterine horn. A veterinarian must be careful in this step in order to avoid damaging and scarring the lining of the uterus. [2]

History

Embryo transfer was first used in the 1890s on rabbits. During this time, rabbits were common test subjects for embryology. The next animals to be bred with embryo transfer were rats and mice in the 1930s. The first livestock embryo transfers occurred in the 1950s, but the methods varied slightly from today's procedure. This method, practiced on sheep, pigs, and cattle, involved the surgical transfer of the embryo from the donor into the recipient. The first examples of the modern, nonsurgical embryo transfer occurred in the 1960s. In the 1970s, the first frozen eggs were fertilized, implanted in a recipient mother, and developed into a live animal.[3]

Purpose

Embryo transfer allows breeders to freeze embryos and develop them in other recipient females like this sow.

Embryo transfer is used in breeding applications in order to create superior livestock that yield higher milk production, or better task performance. Typically, breeders have two primary goals that lead them to embryo transfer: the goal to improve the gene pool and the goal to produce more, higher quality offspring. In the cattle industry, breeders use embryo transfer to breed better beef cows or better milk producers in addition to simply multiplying their herd.[4] In the horse world, embryo transfer is used to allow the good quality mares to produce multiple offspring in a single year. In addition, embryo transfer allows a horse to continue competing while it produces foals, as the surrogate mothers develop the embryos.[5]

Pros and Cons

Embryo transfer involves the artificial insemination of the donor mare, often using straws of frozen semen such as these. Freezing semen allows breeders to produce higher quality offspring from a high quality male that may be far away.

Multiple ovulation with Embryo transfer like many other similar processes yields several advantages and disadvantages. Several advantages of this process include:
-An increase of offspring with higher genetic quality parents
-More opportunities to make money with the sales of offspring, pregnancies, and embryos to transfer
-More offspring resulting from less semen used -Multiple offspring produced at a young age can help prove the quality of a female and her ability to produce more quality offspring.

Several disadvantages with embryo transfer include:

-More expensive procedures and higher overall costs for the offspring
-Embryo transfer requires more management and preparations for the donor and recipient females as well as the need for veterinary assistance.
-Embryo transfer can increase the chance of certain diseases spreading to offspring.
-Occasionally, a donor will respond negatively to the treatments in collecting an embryo.[4]

Video

This video describes the process of embryo transfer.

References

  1. 1.0 1.1 Glickman, Sheri. Equine Embryo Transfer Dressage Today. Web. Accessed 1 June 2015.
  2. 2.0 2.1 2.2 2.3 2.4 2.5 2.6 2.7 Selk, Glenn. Embryo Transfer in Cattle TheJudgingConnection.com. Web. Accessed 30 May 2015.
  3. McCue, Patrick. HISTORY OF EMBRYO TRANSFER Colorado State University. Web. Accessed 1 June 2015.
  4. 4.0 4.1 Grimes, John. Utilization of Embryo Transfer in Beef Cattle Ohio State University. Web. Accessed 2 June 2015.
  5. Strickland, Charlene . Embryo Transfer for Horses The Horse. Web. Published 18 September 2001.