Every season Rosehill Farm Veterinary Practice performs a number of embryo transfers from "donor" mares who are not able to carry a foal to full term due to performance schedules or physical problems such as a damaged endometrium. Each embryo is transferred into a "recipient" or surrogate mother, who has been carefully selected for age, size, reproductive soundness and temperament to successfully carry and raise the donor's foal.
The first part of the process is synchronisation of donor and recipient mares, so that their oestrus periods coincide. Ideally several recipients are prepared for each donor. The donor mare is then inseminated (or served directly) with the chosen sire, and carefully checked for timing of ovulation and health of the uterus, which must support the embryo for the first few days.
An equine embryo begins life when ovum and sperm cell come together in the mare's oviduct, from where it travels into the lumen of the uterus arriving there somewhere around day 6 after ovulation. It grows very rapidly from then on and in a normal pregnancy, roams around the uterus before settling in one spot about day 16.
For an embryo transfer to succeed, the embryo must be removed from the donor's uterus when it is large enough to be trapped in a special filter but not so large that it is too fragile to be handled. The optimum time is 7.5 to 8.5 days after ovulation. At this time we perform an "embryo flush" using special fluid to protect and nurture the embryo. The donor's uterus is filled with the fluid which is then allowed to run out through a filter of the right size, leaving the embryo in the collection dish to be identified and assessed under the microscope.
This is a 7.5 day old embryo, and is generally what we see under the microscope in the collection dish. It may take quite a search to find an embryo amongst the cell debris from the uterine lining which is flushed and filtered out at the same time. Occasionally if our donor has had two ovulations on a single cycle, there may be two embryos in the one collection dish!
Here we are searching for an embryo in the collection dish. If an embryo has been found, it will be carefully washed and loaded in protective medium into a half-ml straw which fits inside a transfer "gun". The gun is passed aseptically through the closed cervix of the most suitable of the syncronised recipients and the embryo deposited into the lumen of the prepared recipient uterus.
Seven days later comes the exciting day of the pregnancy test, when the transferred embryo should be 14 days old and clearly visible on an ultrasound scan of the recipient. From then on the pregnancy is monitored as normal and the lucky recipient gets to have "her" very own foal in 11 months' time!