Horse Embryo Transfer

The business of horse embryo transfers is new and booming, and is predominately seen in big barns with rare and/or valuable horses. In the show circuit, many mares spend their optimal breeding years being shown or ridden in competition and therefore cannot be bred and produce offspring with her desirable traits. Older mares are prone to endometrial degeneration and face complications during labor, and many times mares bred for the first time or even sub-fertile mares have difficulties carrying the foal to term, the consequences of both can lead to death of the mare and/or the foal. The non-surgical technique of embryo transfer, while expensive, is simple and allows insight to the horse reproductive cycle and immunology. All ethical debate aside, let’s talk about how this is done and why horse pregnancy studies have contributed a lot to immunological studies.

Embryo transfer, as mentioned above, is non- surgical meaning it is noninvasive to the mares and eliminates the risk of infection. For successful embryo transfer, you need two mares, one who is the donor (donating the embryo) and one who is the recipient (receiving the embryo) who are synchronized in their estrous cycle. The recipient mare should be very healthy and reproductively sound for pregnancy. An ideal synchronized estrus is when the recipient mare ovulates one day after ovulation of the donor mare, and the donor mare receiving the embryo immediately after ovulation, because during this time, the uterine endometrial lining has thickened enough for potential implantation.

Once the donor mare is inseminated, the embryo enters the uterus 5-6 days later and can be recovered 6-8 days after insemination using a technique called flushing .Waiting a few days after insemination yields the highest amount of recovery rates (~75%). Flushing basically is a solution of phosphate buffered saline with additives to promote embryo viability is flushed into the uterus to facilitate movement of the embryo out of the mare into a filter. The process is repeated 3-4 times, and the embryo recovery filter is examined with a microscope to locate the embryo, transfer it to a culture media, and stored for a maximum of 24 hours until implantation in the recipient mare.

Transfer to a recipient mare through non-surgical methods involves preparing the vulva and perineum (to prevent contamination) and uses a sterile artificial insemination pipette that is inserted into the vagina and passed through the cervix to the uterine body. Ultrasounds 10-15 days after transfer of the egg will show if the egg has successfully implanted in the donor mare or not.

Switching gears, horse pregnancies have been useful studies in understanding mammalian maternal-fetal interactions.When the conceptus attaches to the endometrial wall, the cells of the chorionic girdle trophoblasts form endometrial cups and secretes eCG (equine chorionic gonadotropin) to maintain the pregnancy until day 100 of gestation when the placenta starts to secrete progesterone to maintain pregnancy. At this time, there is a surge of maternal leukocytes that infiltrate the cup periphery, making what seems to be a hostile environment. Trophoblast cells normally regulate major histocompatablitiy complex (MHC, mediate interactions with leukocytes) expression by repressing them to avoid recognition and destruction of maternal immune system. However, some MHC on the equine trophoblast induce strong cytotoxic antibody responses. This mediates maternal Humoral Immunity Response (HIR) to start producing even more lymphocytes. With all these immunity cells, it’s a wonder how the embryo can evade immediate destruction and can survive and develop into a foal!

Often, when the body recognizes foreign DNA, it will attack the DNA or reject it (as seen in organ transplants or sometimes even a fetus). The question I pose is how is it possible for embryo transfers to be successful for the recipient mare, when the egg in question has no DNA resembling the mare? Given the already complex behavior of mare immune system in response to a fetus with half of her DNA, would this be even more complex with foreign DNA in the form of a foreign embryo?

References:

Noronha, L. E., & Antczak, D. F. (2010). Maternal Immune Responses to Trophoblast: The Contribution of the Horse to Pregnancy Immunology. American Journal Of Reproductive Immunology, 64(4), 231-244. doi:10.1111/j.1600-0897.2010.00895.x

Bowen, J. Equine embryo transfer. Retrieved from http://www.horse-repro.com/ARTICLES/PDFs/ETBowen.pdf