Colorado State’s Animal Reproduction Lab Produces Another First: Foals Born from Frozen Mare’s Eggs

The first-ever foals produced from eggs harvested from a mare, frozen and then thawed, were born last week at Colorado State University’s Animal Reproduction and Biotechnology Laboratory. Researchers say this is an important breakthrough since now scientists will be able to preserve reproductive material from the female as well as the male. The technology has potential beyond the horse industry in possibly helping to preserve endangered species worldwide, which has been one of the goals of CSU’s Preservation of Equine Genetics program since it began in 1996.

Siblings "Vitreous" and "Ethyl," a colt and a filly, were carried by different surrogate mothers selected from the Colorado State ARBL herd. "This is a tremendous breakthrough," said Dr. Ed Squires, faculty member and coordinator of the Preservation of Equine Genetics program. "Essentially, with this process, we can now do for the female mammal what we’ve been able to do for the male: provide flexibility in reproduction." Many owners or breeders want to maintain the mare’s line, either because it is rare and highly desirable or for the genetic qualities which make the horse a performer. Often the mare’s lineage is equally as important as that of the stallion. But unlike stallions who can produce millions of sperm for many years, mares have a limited number of eggs, or oocytes, and fewer years for reproduction. Typically, mares are bred for the first time at around 3 years of age and deliver between 8 and 10 foals in a lifetime before they are "retired."

Because the oocyte cell is so much more complicated than a sperm cell, the process of cryopreservation is much more difficult.

"The cryopreservation process we used in this case is called vitrification," said Lisa Maclellan, a post doctorate fellow from Queensland, Australia and the coordinator of this project.

"We first gathered the contents of an ovarian follicle using an ultrasound probe. From the contents, we separated out the single-cell oocyte under a microscope. The oocyte was placed onto a cryoloop-which is about the size of a sewing needle-and quickly dipped into liquid nitrogen to be flash frozen."

Months later, the oocytes were thawed and implanted in two mares from the ARBL stables who had already been inseminated with material from a donor stallion.

The stallion whose semen was chosen to inseminate the mares is an Arabian named Sylekt, belonging to Lucy Whittier of California, a longtime supporter of the PEG program and ARBL. The mares carried for the full term and last week delivered healthy, normal foals who will later be placed with select horse-owning families.

"With the success of this process, we will be able to offer horse owners more reproduction options," said Elaine Carnevale, DVM and an assistant professor in the physiology department who also worked on the project. "They may want to preserve the bloodlines of mares who are genetically valuable or mares who are competing. This should be done while the mares are young and have healthy eggs. Freezing the eggs offers another method of preserving valuable genetics."

"We’re reasonably sure that the eggs can be preserved for thousands of years," said George Seidel, professor of physiology and an expert in animal reproduction. Seidel cited research done earlier in mice where one group of embryos were frozen in liquid nitrogen and bombarded with 4 times the level of background radiation to simulate the passage of hundreds of years. The frozen embryos were thawed, implanted in surrogate mice and offspring were successfully produced.

"So the good news is, if the perfect mate can’t be found immediately," Seidel said, "there is still lots of time."