superovulation and artificial insemination (a.i.) or natural service | recipient 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, superovulation, and artificial insemination (A.I.) or natural breeding of the donor animal. Next, the embryos are recovered from the donor through either surgical or nonsurgical means, evaluated, and then frozen or transferred fresh. Lastly, the recipient animals are synchronized and receive the embryos through surgical or nonsurgical techniques. This article will briefly discuss the history and benefits of embryo transfer. The remainder of the article will focus on the details of ET and discuss the various pros and cons of the surgical and nonsurgical approaches.
History: The first successful embryo transfer was performed in 1890 using rabbit embryos. In the 1950’s, embryo transfer technology in cattle expanded with the first successful transfer performed by Umbaugh and the first calf born through a joint effort by the USDA and the University of Wisconsin. Currently, embryo transfer in sheep and goats is not performed as often as it is in cattle. This is most likely due to the fact that ET in sheep and goats is more difficult and usually requires surgical procedures to be successful. It can also be very costly.
Why Utilize Embryo Transfer: There are many reasons a producer might select embryo transfer for his/her particular operation. The first reason would probably be the potential for genetic improvement in the herd/flock. Through artificial insemination, superior male genetics can be spread across a herd/flock; with embryo transfer, superior female genetics can now be spread across a specific herd/flock or even a herd/flock in a different country. Superovulation and embryo transfer allows one particular female to produce many offspring in a given year and many more over her reproductive lifetime. Each of these offspring would potentially carry the superior traits of the mother, such as increased weight gain, increased size, or even increased milk production. Embryo transfer also eliminates the stress of parturition on a desirable animal, thereby increasing her reproductive life span. Disease control and extension of reproductive function are a few of the other benefits of embryo transfer. Finally, the impact embryo transfer has had and will have on the research environment cannot be overlooked. Techniques such as gene insertion, embryo splitting, and pronuclear DNA injections would not be as feasible without embryo transfer technology.
Superovulation and Artificial Insemination (A.I.) or Natural Service: 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:
Recipient Synchronization: When performing fresh embryo transfer, extra attention must be placed on synchronizing the donors and recipients at the same time. Techniques using melengestrol acetate (MGA) or progesterone inserts are commonly used. Additional suggestions and details can be found on page B710 of this manual.
Superovulation of the donor animal traditionally involves using pregnant mares serum gonadotropin (PMSG) or follicle stimulating hormone (FSH). PMSG is often given at the rate of 400 to 1500 I.U. 48 hours before MGA is removed from the diet or a progesterone insert is removed from the vagina. Some goat breeders report that a dose of 1 mL (400 I.U.) PMSG is superior. Does and ewes will often show estrus 24-36 hours after the progestin product (MGA or progestin insert) is removed. When using FSH, a decreasing dose of FSH given twice daily for 2-4 days is recommended. This dose is gradually decreased each day (for example 5,4,4,3,2 and then 2 mg of FSH). The FSH administration should begin 2-4 days prior to removing the progestin. Another way of giving FSH is by giving 3 mg twice a day for 3 days prior to progestin removal.
Most producers will breed the donor ewe or doe 12-24 hours after it is observed in heat. If donors are to be bred through natural service, the ratio of bucks/rams to does/ewes should be 1:1 or 1:2. Much attention should be placed on heat detection, semen/sire selection, and proper semen handling. Details about proper heat detection and insemination techniques can be found on page B712 of the manual.
Embryo Recovery: In most cases, the embryos are collected from the donor animals on day 5 or 6 after breeding. As mentioned earlier, surgical approaches to embryo collection are the most common and most successful. However, they are limited by the skill of the surgeon, expense, and possible adhesions. If surgical collection is not possible, nonsurgical and laparoscopic techniques can be used.
The surgical method of embryo collection begins with the sedation and anesthesia of a ewe/doe that has been held off feed and water for at least 36 hours. An area in front of (cranial to) the udder is clipped and sterilized, and the animal is placed on its back (dorsal recumbency). An incision is then made in the midline of the abdomen (ventral midline laparotomy) just in front of the udder.
The uterus and ovaries are then evaluated. The uterus is removed from the abdomen and a small 20-gauge needle is then inserted into the uterus at the uterotubal junction. Through this opening, a "tom cat" catheter is then placed into the uterus. Through this catheter a flushing media is infused into the uterus. A common flushing solution may include Dulbecco’s PBS solution with 100 IU/ mL of penicillin, 100 mg/mL streptomycin and 2% heat-activated goat serum. An 8 to 10 inch Foley catheter is then inserted at the base of the uterine horn. As 20 mLs of the flushing media is placed into the uterus through the "tom cat" catheter, the fluid and embryos are flushed out the Foley catheter and into a collection dish. The same thing is done on the opposite uterine horn. The catheter entry points in the uterus are left open and the female is sutured closed. It is customary to give the ewe/doe a dose of prostaglandin after the surgery. The embryos collected can then either be frozen or directly transferred fresh.
The nonsurgical method for recovery of embryos in sheep and goats has not been perfected. There are many different reported methods that require additional research before one procedure can be recommended.
Freezing and thawing protocols involve a series of specific steps for successful transfer. Embryos are often kept in Dubecco’s PBS solution with 5-20% fetal calf serum while they are evaluated and processed for immediate transfer or freezing.
Transfer of Embryos: The surgical placement of embryos is usually much more successful than the nonsurgical technique. However, these surgical procedures present the same problems that are found in the surgical collection of embryos. Most surgical transfers of embryos involve a ventral midline approach to the uterus and ovaries. The ovaries are examined for the presence of a corpus luteum or CL. The uterine horn that is on the same side as the ovary with the CL is gently removed from the abdomen. A small puncture is made in the uterine horn and a pipette is inserted and the embryo is deposited. If the embryo is less than 4 days old, it is transferred into the oviduct through the fimbria. Other methods of embryo transfer that utilize the help of a laparoscope are commonly performed. These techniques are better for the female because the larger incisions are not required. In sheep and goats, the nonsurgical approach of embryo transfer is currently so unsuccessful that it is rarely done.
Most ET programs can expect to collect about 8-10 embryos from each donor and have about a 50-80% success rate when transferring two fresh embryos per recipient. When using frozen embryos, the success rates are much lower.
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 number of eggs flushed per donor, the cost of the semen, and any collection, freezing and transfer fees. Increased labor expenses (more heat detection, more insemination time, more times each animal must be handled, etc.) and costs for synchronization programs should also be considered. Do not forget that the most significant expense associated with ET will be the cost of owning and maintaining the necessary recipient animals. This cost would be in addition to the costs for simply establishing an ET pregnancy.
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 sheep/goat embryo transfer progresses, applications for its use will continue to become apparent in the laboratory, university, large commercial enterprise, and even the small operation.