CHAPTER 48In Vitro Fertilization

In vitro fertilization (IVF) is used clinically in cattle and humans. In cattle, oocytes are recovered from cows in vivo up to twice weekly by transvaginal ultrasound-guided follicular puncture. The recovered oocytes are subjected to in vitro maturation then fertilized in vitro, and the resulting embryos are cultured in vitro to the blastocyst stage, at which point they are transferred transcervically to the uteri of recipient cows. The procedure of in vitro embryo production is commercially successful in cattle and allows progeny to be obtained from cows that cannot themselves provide embryos for transfer.

Although the clinical demand for similar protocols in the horse is high, a major obstacle to employing in vitro embryo production in horses has been the failure of IVF to be repeatably successful. As of this writing, only two foals have been born from IVF, both in France, in 1990 and 1991.^1,2 Both foals were a result of IVF of oocytes that were recovered from mature follicles of mares ex vivo. Many laboratories have worked on equine IVF over the ensuing years, but an effective and repeatable technique has still not been established.

The major difficulty associated with IVF in the horse appears to be penetration of the zona pellucida, because fertilization rates are greatly increased if a breach is made in the zona pellucida before sperm are coincubated with the oocytes.^3,4 This has been done by removing a portion of the zona pellucida under micromanipulation or by dissolving a hole in the zona using a directed jet of acidic solution, again under micromanipulation. Although high fertilization rates (>80%) were obtained with these procedures, polyspermy (fertilization of the oocytes by more than one sperm) also resulted,⁴ because the main barrier to polyspermy (i.e., change of the character of the zona pellucida after fertilization) was bypassed. Although the proportion of oocytes that undergo polyspermy can be decreased by manipulation of sperm numbers, some risk remains and these methods cannot be recommended for IVF. In addition, because these techniques both require micromanipulation, they entail equipment and labor similar to intracytoplasmic sperm injection (ICSI). ICSI has been shown to be a repeatable and efficient method for fertilization of horse oocytes in vitro^5,6 and so has become the method of choice (see chapter on ICSI).

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