Solving the Mystery of Horse Reproduction
Posted August 19, 2014
Although human in vitro fertilization (IVF) has been successful
since the 1970s, similar reproductive technology in horses has
lagged behind. Success rates stubbornly hover between zero and 30
percent, and only two live foals have been born using IVF.
The main problem seems to be with the ability of sperm to
penetrate the egg. Ongoing research, much of it at Texas A&M
University, has led to the ability to successfully mature horse
oocytes in vitro. However, achieving in vitro sperm
capacitation—which involves a series of changes that sperm must
undergo in order to be able to fertilize an egg—has proven to be
more complicated. A solution may soon be at hand, though; Dr.
Leticia Vivani, a Ph.D. student working jointly in Dr. Katrin
Hinrichs’ and Dr. Dickson Varner’s labs at the Texas A&M
College of Veterinary Medicine & Biomedical Sciences (CVM), is
working on the reasons behind this problem.
“We are trying to understand the factors that regulate the
process of sperm capacitation,” Vivani said. “Of course, this
happens perfectly well when we breed a mare, but not when we try to
mimic this in vitro. We do not know the reason, but I am almost
sure there is something ‘special’ within the mare reproductive
tract, something that might not be needed in other species, but
that makes stallion sperm undergo all these changes.”
Hinrichs’ lab has determined that for some reason, equine sperm
are different from those of other species. For example,
incubation conditions that successfully induce hyperactivation (the
whip-like tail motion needed to penetrate the egg) and subsequent
fertilization in other species fail to do so with equine sperm.
“There can be so many things that can be affecting capacitation,
and there is very little research done on equine sperm physiology,”
Vivani said. “This is good, in a way, because anything you do is
new, is innovative, but at the same time, the research can be very
challenging and frustrating.”
Some progress is being made in understanding equine sperm
capacitation. In 2009, by inducing hyperactivated motility with a
substance called procaine, researchers at Cornell University
achieved the highest fertilization rate to date (61%).
Unfortunately, this is not a practicable solution for embryo
production because procaine is toxic to the embryos. “This was
nevertheless an important finding,” Vivani said, “because it showed
us that the failure of IVF was likely due to a sperm-related
problem. We now know that there is something difficult about
inducing appropriate equine sperm motility in vitro, and that may
be why IVF rates have been so low.”
Vivani, who earned her Doctor of Veterinary Medicine (DVM)
degree in her native Argentina in 2001 and her Master of Science
degree at the University of Massachusetts in 2010, has long wanted
to study at Texas A&M with this team of researchers.
“I have been fascinated with equine reproduction, and the
reasons behind the failure of IVF in the horse in particular, since
my DVM graduation,” Vivani said. “Therefore, I’ve been dreaming of
working with Dr. Hinrichs and with Dr. Varner since I was in
Argentina. I had been writing to them and calling them for years
before I was eventually able to make it work to come to Texas
A&M, first as a visiting researcher and then as a Ph.D.
student.” She began her program in May 2012 and plans to graduate
“Horses are an excellent model for human comparative studies.
Mice, for example, do not age as women do, but the changes that
mares undergo are very similar to women’s changes with aging,”
Vivani said. “When mares reach a certain age, their reproductive
efficiency decreases as a result; changes in hormone levels,
follicular development, and oocyte quality are very similar in
older mares and older women. So it’s a great model for human
Just as in human medicine, owners of horses for whom natural
reproduction has failed turn to assisted reproduction technologies.
In horses, this often means using intracytoplasmic sperm injection
(ICSI), a technique of “bypassing” standard IVF by injecting a
sperm directly into the egg (see sidebar). The ICSI technique is
also sometimes used in humans, especially when more traditional IVF
has not worked. At the moment, the only way to successfully produce
a horse embryo in vitro is through the use of ICSI, which many
breeders are increasingly using. “However, is not the most
physiological way,” Vivani said, “and is not always practical, as
it requires time, sophisticated equipment, and trained personnel.
That is why I focus on how to make IVF work.”
Ironically, the One Health approach, which usually means
translating findings from animals into human medicine, works
backward in this case. IVF works well in humans, and has for more
than 30 years. However, it is not yet successful in horses, and
perhaps going back to the basics of reproduction can help explain
why that is the case.
“In the beginning basic research was hard for me,” Vivani said,
“because I was trained as a veterinarian and I just wanted to see
results. My advisor for my master’s degree, who is a basic
researcher, made me stop and ask why things work or not, and I’m
very grateful that he did.” In fact, the IVF technique was
pioneered through basic research that led to the discovery of
“Dr. Hinrichs and other researchers like her have focused on
really understanding physiological processes related to
reproduction,” Vivani said. “For many years we, working in equine
reproduction, tried one thing and if it didn’t work, we tried a
completely different thing without trying to figure out why it
didn’t work, and this process explains why there is so little
information in this area.”
Despite all of the challenges, Vivani says that she finds her
work extremely satisfying, partly due to her excellent mentors.
“Dr. Hinrichs and Dr. Varner are so encouraging with their students
and really value their work,” Vivani said. “They always encourage
me to learn as much as I can. Studying with them has been a
wonderful experience—truly a dream come true.
For more information about the Texas A&M College of
Veterinary Medicine & Biomedical Sciences, visit our website at
vetmed.tamu.edu or join us on Facebook.
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