In vivo embryo transfer in horse breeding: historical data and current status

   This article provides a review of the biotechnology concerning in vivo embryo production and transfer in the equine industry. According to the IETS registry, a total of 152,815 embryo transfers have been performed globally in horse breeding over the past five years alone, with the vast majority (125,017, or 81.81 %) utilizing in vivo embryos. Primarily, freshly recovered embryos are used for transfer. The proportion of cryopreserved embryo transfers remains negligible compared to fresh embryo transfers (0.48 % vs. 99.52 %). Embryos are typically recovered from donor mares without superovulation induction, directly from the uterine lumen. The optimal time for embryo flushing is 7–8 days post-ovulation. Cervical catheterization is performed transcervically, via the vagina, using a flexible double-lumen silicone catheter equipped with an inflatable cuff, under digital guidance (with a gloved hand). The average embryo recovery rate typically ranges from 50–65 %. It is recommended to synchronize the donor’s estrous cycle with those of 2–3 recipients, ensuring that at least one recipient ovulates 1–2 days after the donor mare. Estrus and ovulation synchronization can be induced using compounds from various pharmacological groups that possess progestagenic (e. g., progesterone, altrenogest) and/or luteolytic (e. g., native prostaglandin F_2α or its highly active synthetic analog – cloprostenol) activity, often in combination with ovulation-inducing agents (e. g., deslorelin or hCG). Embryo transfer into the uterus is performed transcervically, using either the manual-uterine or visual-uterine technique. In the first (classical) technique of uterine embryo transfer, cervical canal catheterization is guided by manual control. The second technique involves using a Polansky speculum, with the ventral lip of the vaginal cervix stabilized by Wilshir cervical forceps. The efficiency of the manual-uterine (classical) technique for fresh embryo transfer achieves pregnancy rates of 65.5–77.8 %, whereas the Wilshir technique yields 90.9–93.4 %.

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