• Oocyte transfer is the transfer of a donor oocyte into the oviduct of a recipient mare.
• Oocyte transfer provides a method of achieving pregnancies and preserving genetics from mares whose reproductive capabilities are limited by some factor other than oocyte quality.
• Oocytes can be collected from live donor mares, ovariectomized mares, or recently deceased mares Reproduction: oocyte collection.
• Collected oocytes may be either mature or immature Reproduction: oocyte collection.
• Mature oocytes may be directly transferred into the oviduct of a recipient mare.
• Immature oocytes (which are resting in prophase I of meiosis) need to be cultured / matured in vitro before transfer into the oviduct of a recipient mare.
• Techniques for oocyte culture, handling and transfer are described below.
• Recipient mares should be young (ideally 3-10 years old), free from disease and reproductively sound.
• The recipient mare must be in estrus: either natural, ie a cycling recipient, or induced, ie an anestrus recipient who has been treated with exogenous hormones, so that her reproductive tract provides a receptive environment for the oocyte at the time of transfer.
• Methods of synchronizing cycling recipient mares with donor mares, and of inducing estrus in acyclic mares, are described below.
• Following transfer of the oocyte to the recipient mare’s oviduct, the recipient mare is inseminated with fresh, chilled or frozen semen. Fertilization and subsequent embryo and fetal development occurs within the recipient mare’s reproductive tract.

Uses

• Oocyte transfer provides a method of achieving pregnancies from mares who are capable of producing a viable oocyte, but who are unable to either produce an embryo for embryo transfer or sustain a pregnancy themselves, due to a variety of reasons including pathologies of the uterus, cervix, oviduct or ovary.

Advantages

• Facilitates production of foals and the preservation of valuable genetics from otherwise infertile mares.

Disadvantages

• Requires technical expertise.
• Success rates are very variable and are dependent upon the quality of the oocytes being transferred (quality tends to decline with mare age), the quality of the sperm inseminated, and the technical expertise of personnel.
• Risk of fertilizing the recipient’s own oocyte as complete control of the recipient’s follicular activity is difficult in cyclic mares.

Technical problems

• Synchronization of donor and recipient mares.
• Problems with in vitro oocyte maturation (see below).
• Problems with surgical transfer technique (see below).