Oocyte transfer involves the transfer of an oocyte (egg), collected from the ovarian follicle of a donor mare, into the oviduct of a recipient mare. The recipient is inseminated, and fertilization and embryo development occur within the recipient. Oocyte transfer has been used to obtain pregnancies from valuable mares with various reproductive pathologies that would prevent pregnancy or successful embryo transfer. These reproductive pathologies include: 1) ovulatory failure; 2) oviductal blockages; 3) uterine pathology, especially recurrent or severe uterine infections; and 4) cervical adhesions or tears.

Oocytes are usually collected using ultrasound-guided, transvaginal follicular aspirations. An ultrasound probe, in a casing that contains a needle guide, is advanced into the anterior vagina. The ovary is positioned over the face of the probe per rectum. The follicle is imaged while a needle is advanced through the vaginal and follicular walls. The follicular fluid and oocyte are removed using gentle suction. Oocytes are usually collected from estrous donors approximately 24 or 36 hours after administration of human chorionic gonadotropin (hCG) to induce follicular maturation. Oocytes, collected within a few hours before ovulation (36 hours after hCG), are transferred in less than 2 hours into a recipient’s oviduct. Oocytes collected at 24 hours after hCG (approximately 12 hours before expected ovulation) are cultured in vitro for approximately 14 hours before transfer.

Two types of recipients are primarily used for oocyte transfer. Cyclic recipients are given an injection of hCG at the same time as the donor: however, the oocyte from the recipient’s follicle is removed through follicular aspiration before transfer of the donor’s oocyte. Noncyclic recipients are also used. Injections of estradiol are given for 3 to 9 days before transfer of the donor’s oocyte, and progesterone is used after transfer to support the pregnancy. Oocytes are transferred through a standing flank laparotomy. The oviduct is exposed through the surgical incision. The oocyte is pulled into a fire-polished glass pipette. The pipette is threaded into the oviduct through the infundibular os, and the oocyte is expelled with <0.1 ml of medium. Recipients are usually inseminated within the uterus approximately 12 hours before and/or 2 hours after oocyte transfer. In contrast, a limited number of sperm (200,000 to 500,000) has also been successfully transferred into the oviduct with the oocyte in a process called gamete intrafallopian transfer (GIFT).

For experimental transfers, oocytes from young mares and fertile stallions are used; and embryo development rates usually range from 50 to 80%. In a commercial oocyte program, oocytes are often obtained from older, subfertile mares; and semen is obtained from different stallions with variable fertility. Pregnancy rates for commercial oocyte transfers have been approximately 35% per transfer.

Oocyte transfer provides an assisted reproductive technique that can be used to produce offspring from mares that have various reproductive pathologies and that are infertile using standard breeding techniques or oocyte transfer.