For embryo transfer to be successful, it is necessary that the donor mare is able to ovulate her ovocyte, to transport it into and along the uterine tube, to allow the ascension of the spermatozoid, to allow the descent of the fertilized embryo, and to have an uterine environment with permits the embryos maintenance inside the uterus until the point of collection.

Diverse pathologies can make any or several of these steps hard to achieve or impossible in certain mares, thus causing a low success rate of embryo collection in these mares. Such pathologies include ovulation incapacity, geriatric mares, endometritis and/or endometriosis, pyometra, bad vulvar conformation, among many others which transform the uterus into an inhospitable environment, making the embryo’s survival impossible.

Ovocyte transfer is a technique by which the donor mare’s ovocyte is aspirated and transferred into a receptor mare, thus solving problems related to ovulation and to gamete transport along fallopian tubes. The receptor is inseminated and then the fertilization of the donor’s ovocyte takes place inside the receptor’s reproductive tract.  This technique has proved efficient for breeding foals out of mares who fail as donors in regular embryo transfer programs.

OT was first successful in 1988 with McKinnon, and its commercial feasibility was proved by Dr. Elaine Carnevale from Colorado State University. Besides its great commercial potential, this technique opens a wide path for the increase of our knowledge concerning ovocyte maturation, their physiology, and in vitro embryo maturation.

OVOCYTE COLLECTION

For ovocytes to be collected, a follicle must be presented. This follicle is maturated by using hormones such as human chorionic gonadotrophin (HCG) and/or deslorreline (GnGH analogue). Ovocyte maturation starts approximately 36 before ovulation; if it does not happen fertilization becomes impossible.

Maturation involves nuclear as well as cytoplasm tic maturation, and if there happens a failure in any of these, ovocyte fertilization is prevented.

Collection can be done in several moments:
• 24 hours, on average, after the use of ovulation inducer;
• As close as possible to ovulation;

Ovocyte collection is made by ultrasound guided vaginal perforation, by a well-trained and experienced professional. The recovered fluid is placed on several Petri dishes, and then observed through stereoscopic microscope, searching for the ovocyte (see photo below).

INSEMINATION

Artificial insemination of receptors is made twelve hours prior to ovocyte transfer, at the same time that follicle aspiration takes place. Only receptors whose own ovocytes were collected are used, thus discarding any possibility of fecundation of the receptor’s ovocyte.

TRANSFER

Ovocyte transfer is done surgically, directly into the receptor’s tube.

The mare is locally anesthetized and prepared for standing surgery. After surgery, post-operative care for the receptor includes antibiotics and analgesics, as well as wound dressing.

RESULTS

Under experimental conditions where young and fertile mares were used, results were around 60% for ovocyte recovery; after transference, 75% of receptors became pregnant.

However, under field conditions for aged donors (over 20 years old) or those with serious reproductive problems, indexes will be around 45% ovocyte recovery and 40% of receptors in foal.

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