What’s new for embryo transfer?

The collection and transfer of horse embryos (ET) was first reported in 1972 [1-2]. The uptake in equine clinical practice was relatively slow, until an upsurge in ET in Argentina during the 1990s, primarily because of the increasing commercial demand for high-goal Polo Pony horses, combined with a shift from non-surgical to surgical transfer [3-4]. Since then, ET has become the most popular assisted reproductive technique (ART) in the horse. During the current decade, the ET industry is shifting towards the use of in vitro produced embryos, especially in sport horse breeds, owing to the increase in the efficiency of OPU-ICSI: an ART capable of producing several embryos per session with the use of a very small amount of sperm. Furthermore, this technique allows the successful cryopreservation of embryos which results in a more efficient management of recipient mares and marketing of embryos worldwide [5]. The most relevant factors involved in the success of this technique are: 1) the number of recovered oocytes per session, 2) the donor mare (individual variation) and 3) the ICSI lab’s experience [5]. Regarding the production and collection of in vivo derived embryos, the quality of sperm, age of donor’s mare, and the operator’s experience and flushing technique [6] appear to be the main factors that influence embryo recovery rates. Furthermore, several studies have shown a beneficial effect of the length of estrus on both embryo recovery (donor mares) [7] and post-transfer pregnancy rates (recipient mares) [8]. The correlation between length of estrus and fertility appears to be related to the development of a more receptive endometrial environment for embryo survival. Advances in the cryopreservation techniques of in vivo derived blastocysts seem to have overcome the dogma about unsuccessful pregnancy results following cryopreservation of embryos >300 μm. These advances are: 1) the collapse and aspiration of blastocysts by micromanipulation prior to cryopreservation [9]; and 2) the use of a new vitrification technique that allows successful cryopreservation of blastocysts of up to 500 μm without the need of previous collapse [10]. Finally, the development and refinement of recombinant equine FSH (reFSH) has produced an apparently practical and consistent solution to the lifelong quest for super-ovulation in donor mares [11]: Treatment of deep anestrous mares with 1.3 mg of reFSH once daily for 7 days resulted in the mean ovulation and recovery of 5.5 follicles and 2.6 embryos per mare, respectively. However, this product is yet to become commercially available. The selection of suitable recipient mares and the technique of ET are key points to the success of a commercial embryo transfer program. While the transfer of in vivo derived embryos allows a wider window of donor-recipient asynchrony without compromising pregnancy rates (recipients may ovulate between 1 day before and up to 4 days after the donor mare without differences in pregnancy rates), the optimal window for in vitro produced embryos is much shorter (i.e. 24 to 36 h), obtaining best results from transfers to Day 4 recipients [12]. A recent study, contrary to previous belief, has shown that the vascularization of the CL of the recipient mare, as determined by doppler ultrasonography, at the time of transfer is positively correlated with pregnancy [13]. Finally, the transfer of embryos with the aid of a speculum and cervical forceps, as described by Wilsher and Allen in 2004, has recently shown to consistently improve post-transfer pregnancy rates and reduce variation in pregnancy results amongst operators, compared with the traditional manual technique [14].

[...]