In Vitro Production of Equine Embryos and Cloning: Today's Status

Cryopreservation of equine oocytes: looking into the crystal ball

Invitro embryo production has evolved rapidly in the horse over the past decade, but blastocyst rates from vitrified equine oocytes remain quite poor and further research is needed to warrant application. Oocyte vitrification is affected by several technical and biological factors. In the horse, short exposure of immature oocytes to the combination of permeating and non-permeating cryoprotective agents has been associated with the best results so far. High cooling and warming rates are also crucial and can be obtained by using minimal volumes and open cryodevices. Vitrification of invivo-matured oocytes has yielded better results, but is less practical. The presence of the corona radiata seems to partially protect those factors that are necessary for the construction of the normal spindle and for chromosome alignment, but multiple layers of cumulus cells may impair permeation of cryoprotective agents. In addition to the spindle, the oolemma and mitochondria are also particularly sensitive to vitrification damage, which should be minimised in future vitrification procedures. This review presents promising protocols and novel strategies in equine oocyte vitrification, with a focus on blastocyst development and foal production as most reliable outcome parameters.

Recovery of Equine Oocytes in Ambulatory Practice and Potential Complications

Field collection of oocytes in mares using transvaginal follicular aspiration (TVA) for embryo production has the potential to revolutionate the equine industry. Protocols for TVA in specialized laboratory settings have been described in the scientific literature since the early 1980s. The objective of this study was to determine the success rate of TVA oocytes recovery under ambulatory conditions. A secondary goal of this study was to determine if TVA is associated with any health complications when performed by recently trained practitioners in the field. Follicles (n = 296) from 66 adult clinically healthy mares were aspirated over a period of 6 days. TVAs were performed by 22 veterinarians with 5-20 years of experience in equine and bovine reproductive medicine, but no previous experience in TVA. Oocytes (n = 145) were recovered. No short- or long-term systemic or local complications were observed following TVA in any of the mares used in this study. Fifty-six out of 66 mares became pregnant within 3 months following TVA. This study shows that with proper training, TVA can be successfully used to obtain equine oocytes with no health complications under field conditions in nonspecialized laboratory settings.

In Vitro Culture of Embryos from Horses

Establishment of optimal methods for equine embryo culture has been slow when compared to some domestic species. In part, this delay was caused by the failure of standard in vitro fertilization techniques in horses. However, the development of intracytoplasmic sperm injection (ICSI) for the assisted fertilization of equine oocytes has resulted in a renewed interest in establishing optimal methods for embryo culture. Currently, ICSI-produced equine embryos are cultured using media designed for other species or other cell cultures and, typically, with the addition of serum. Although systems specifically for horse embryo culture still have not been established, ICSI-produced embryos are developmentally competent and capable of producing live offspring.