Oocyte transfer is the placement of a donor’s oocyte into the oviduct of a recipient. The recipient can be inseminated within the uterus or within the oviduct. Placement of the oocyte and sperm within the recipient’s oviduct is more accurately termed gamete intrafallopian transfer (GIFT).
The first successful oocyte transfer was done in 1989; however, the technique was not used for commercial transfers until the late 1990s. Oocyte transfer is currently used to produce offspring in subfertile mares in which embryo transfer is not successful because of various reproductive problems. These problems include ovulatory failure, oviductal blockage, recurrent or severe uterine infections, and cervical tears or scarring. In some cases, the cause of reproductive failure cannot be diagnosed; however, oocyte transfer can be successful.
Sychronization Of Donors And Recipients
Oocytes are collected from preovulatory follicles between 24 and 36 hours after the administration of human chorionic gonadotropic (hCG; 1500-2500 IU, IV) to a donor mare or between 0 and 14 hours before anticipated ovulation. Criteria for hCG administration are as follows:
• Follicles greater than 35 mm in diameter
• Relaxed cervical and uterine tone
• Uterine edema or estrous behavior present for 2 or more days
Some mares, especially older mares, do not consistently respond to hCG. In these cases, this author uses a combination of the gonadotropin-releasing hormone (GnRH) analog, deslorelin acetate (2.1 mg implant; Ovuplant), followed by an injection of hCG (2000 IU, IV) between 4 and 5 hours later.
Oocytes collected 36 hours after hCG administration to the donor are transferred immediately into a recipient’s oviduct. Oocytes collected 24 hours after drug administration to the donor are cultured in vitro for 12 to 16 hours before transfer. The advantage of collection of oocytes between 32 and 36 hours after hCG administration to the donor is that the oocytes do not require culture in vitro. However, donors could ovulate follicles before oocytes are collected. The collection and culture of oocytes at 24 hours after hCG administration to the donor are often easier to schedule; the oocyte can be collected in the afternoon and transferred the next morning. This method requires expensive equipment and training for tissue culture, however. In a modification of these procedures, oocytes are collected 24 hours after hCG and immediately transferred into the recipient’s oviduct to allow maturation to complete within the oviduct. With this latter method, recipients are inseminated 16 hours after transfer.
Oocytes are usually collected by one of two methods. In one method, the ovary is held per rectum against the ipsilateral flank of the mare. A puncture is made through the skin and a trocar is advanced into the abdominal cavity. The ovary is held against the end of the trocar while a needle is advanced through the trocar and into the follicular antrum.
In this author’s laboratory oocytes are collected by using transvaginal, ultrasound-guided follicular aspirations. For this procedure, a linear or curvilinear ultrasound transducer is used with the transducer housed in a casing with a needle guide. Before the procedure, the rectum is evacuated and the vulvar area is cleaned. The mare is sedated (xylazine HC1, 0.4 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV) and a substance to relax the rectum (propantheline bromide, 0.04 mg/kg, IV) is administered. A twitch is applied. The probe is covered with a nontoxic lubricant and placed within the anterior vagina lateral to the posterior cervix and ipsilateral to the follicle to be aspirated. The follicle is positioned per rectum and stabilized with the apex of the follicle juxtaposed to the needle guide. A needle is advanced through the needle guide to puncture the vaginal and follicular walls. In this author’s laboratory, a 12-gauge, double-lumen needle is used (Cook Veterinary Products, Spencer, Ind.). The follicular fluid is aspirated from the follicle by using a pump set at a pressure of -150 mm Hg. After removal of follicular fluid, the lumen of the follicle is lavaged with 50 to 100 ml of flush (typically modified Dulbecco’s phosphate-buffered solution or Emcare [ICP, Auckland, New Zealand]) that contains fetal calf serum (1%) or bovine serum albumin (0.4%) and heparin (10 IU/ml).
Equipment used to handle the oocyte is warmed to 38.5° C before use because the oocyte is sensitive to temperature changes. On collection, the follicular aspirate and flush are poured into large search dishes and examined under a dissecting microscope to locate the oocyte. Aspirations of preovulatory follicles are often bloody because the follicle has increased vascularity as ovulation approaches. The oocyte is approximately 100 μm in diameter and is surrounded by a large mass of nurse ceils — the cumulus complex. Cumulus cells, or the corona radiata, appear as a ring surrounding the oocyte. When the oocyte matures, the cumulus complex becomes less distinct. The corona radiata appears clear in the bloody flush solution and can be observed by the naked eye.
Oocyte Evaluation And Culture
On collection, cumulus oocyte complexes (COC) are evaluated for cumulus expansion (graded from compact to fully expanded) and for signs of atresia. Oocytes are determined to be in a stage of atresia when the COC is clumped and/or sparse, the corona radiata is fully expanded, or when the ooplasm is shrunken and dark or severely mottled. Oocytes with a fully expanded cumulus (marked separation of cumulus cells with expansion of the corona radiata) are considered mature and are transferred as soon as possible into a recipient’s oviduct. Oocytes with a moderately expanded cumulus complex (translucent COC with moderate separation of cumulus cells and incomplete expansion of corona radiata) are cultured before transfer. On occasion, the donor does not respond to hCG and the follicle does not begin to mature. Consequently, the granulosa cells that line the follicle are collected in small, compact sheets, and the oocyte is frequently not retrieved. If an immature (compact COC with little or no separation of cumulus cells) oocyte is collected, special culture conditions are required, including a maturation medium with additions of hormones and growth factors.
On identification and evaluation, the oocyte is washed and placed in a transfer or collection medium. A commonly used medium for the culture of maturing oocytes is tissue culture medium (TCM) 199 with additions of 10% fetal calf serum, 0.2 mM pyruvate, and 25 mg/ml gentamicin sulfate. A carbon dioxide (C02) incubator must be used to establish the proper culture conditions of 38.5° C in an atmosphere of 5% or 6% C02 and air.
Mares that will receive oocytes should be young (preferably 4-10 years of age) with a normal reproductive tract. Oocytes are transferred surgically; therefore, adequate exposure of the ovary is essential to facilitate transfers. Mares with short, thick flanks and short broad ligaments are not good candidates for recipients. Both cycling and noncycling mares have been used as oocyte recipients. When cyclic mares are used, they must be synchronized with the donor; thus, hCG is administered to the estrous donor and recipient at the same time of day. Before the mare can be used as a suitable recipient, her own oocyte must be aspirated. Anestrus and early transitional mares are suitable noncyclic recipients. During the breeding season, a high dose of a GnRH analog or injections of progesterone and estrogen (150 mg progesterone and 10 mg estradiol) can be administered to reduce follicular development in potential recipients. Noncyclic recipients are given injections of estradiol (2-5 mg daily for 3-7 days) before transfer and progesterone (150-200 mg daily) after transfer. In mares that are not having estrus cycles, pregnancies must be maintained through the use of exogenous progesterone.
Oocytes are transferred through a flank laparotomy into standing sedated mares. Recipients are placed in a stock and a presurgical sedative (xylazine HC1, 0.3 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV) is administered. The surgical area is clipped, scrubbed, and blocked with a 2% lidocaine solution. Immediately before surgery, additional sedation is administered (detomidine HC1, 9 mg/kg, and butorphanol tartrate, 0.01 mg/kg, IV). An incision is made through the skin approximately midway between the last rib and tuber coxae, and the muscle layers are separated through a grid approach. The ovary and oviduct are exteriorized through the incision. The oocyte is pulled into a fire-polished, glass pipette, and the pipette is carefully threaded into the infundibular os of the oviduct and advanced approximately 3 cm. The oocyte is transferred with less than 0.05 ml of medium.
Insemination Of Recipients
In a commercial oocyte transfer program, use of stallions with good fertility is essential. Cooled and transported semen is often provided. When fresh semen from fertile stallions and oocytes from young mares was used in different experiments, insemination of the recipient only before (12 hours) or only after (2 hours) oocyte transfers resulted in embryo development rates of 82% (9/11) and 57% (8/14), respectively. In a commercial oocyte program, mares were older with histories of reproductive failure and cooled semen from numerous stallions of variable fertility was used. Pregnancy rates when recipients were inseminated before or before and after oocyte transfer were significantly higher than when recipients were only inseminated after transfer (18/45, 40%; 27/53, 51% and 0/10, respectively). These results suggest that the insemination of a recipient before transfer with 5 X 108 to 1 x 109 progressively motile sperm from a fertile stallion is sufficient. However, if fertility of the stallion is not optimal, insemination of the recipient before and after transfer may be beneficial.
After insemination and transfer, the recipient’s uterus is examined by ultrasonography to detect intrauterine fluid collections. The uterine response to insemination often appears to be more severe when recipients are inseminated after transfer than when they are inseminated only before transfer. The uterus is evaluated and treated once or twice daily until no fluid is imaged. Recipients with accumulations of intrauterine fluid are treated similar to ovulating mares, with administration of oxytocin or prostaglandins to stimulate uterine contractions or with uterine lavage and infusion.
Future Of Oocyte Transfer
Oocyte transfer has proved to be a valuable method of obtaining pregnancies from mares that cannot carry their own foal or produce embryos for transfer. Because the mare does not have to ovulate or provide a suitable environment for fertilization or embryo development, the oocyte donor is only required to develop a preovulatory follicle with a viable oocyte.
The transfer of oocytes and a low number of sperm (200,000 motile sperm) into the oviduct of recipients has been successful. Pregnancies could be produced with GIFT when sperm numbers are limited, such as from subfertile stallions and from sex-selected or frozen sperm.
Through the use of this technique at this author’s laboratory, pregnancies have been recently produced from oocytes that were frozen and thawed and from oocytes that were collected from the excised and shipped ovaries of dead mares. These advances provide excellent methods to preserve the genetics of valuable mares.