Maturation and fertilization of equine oocytes.

Equine oocytes obtained either by transvaginal ultrasound-guided follicular aspiration or from slaughterhouse ovaries can be matured in vitro. This generally requires culture in TCM-199 containing serum and hormones for 30 to 36 hours. With this protocol, approximately 50% to 60% of the oocytes are at metaphase-II at the end of the culture period. At least some of these oocytes appear viable based on production of fertilized eggs either through in vitro fertilization or fertilization in vivo of a recipient mare. The success of producing equine embryos in vitro is still extremely low. More than likely the conditions for in vitro oocyte maturation are not optimized, and the techniques for capacitating equine spermatozoa are not adequate. The stallion sperm would appear more difficult to capacitate and many of the approaches used in other species have not worked in horses. To date, the only fertilization that has occurred with in vitro matured oocytes has been with sperm treatments containing the calcium ionophore A23187. Increased success with in vitro production of equine embryos may be gained through the use of assisted reproductive techniques such as partial zona dissection or intracytoplasmic sperm injection.