EQUINE EMBRYO TRANSFER: REVIEW OF
DEVELOPING POTENTIAL
T.A.E. Stout
Department of Equine Sciences, University of Utrecht, Yalelaan 112, 3584
CM Utrecht, The Netherlands.
Keywords: horse; embryo transfer; genetic improvement; superovulation;
cryopreservation
Introduction
During the last 20 years, embryo transfer (ET) has become an increasingly
routine tool for increasing the number of progeny from genetically valuable
mares, for producing foals from competing mares without interrupting their
sporting careers, and for obtaining foals from mares incapable of carrying
a pregnancy to term. In retrospect, it seems surprising that there was such
a long interval between the first reports of the successful transfer of horse
embryos (Allen and Rowson 1972, 1975; Oguri and Tsutsumi 1974) and
the large-scale adoption of ET in practice. While the initial reluctance of the
horse breeding industry to adopt ET was due in part to practical
constraints, uptake was not helped by widely disseminated, but largely
unfounded, fears that the athletic performance of ET foals would be
influenced significantly by the capacities of their surrogate mother, and by
the conservatism of studbooks. In the latter respect, many studbooks were
slow to relax rules either banning the registration of ET foals or limiting
registration to one foal per genetic mother per year (Hudson and McCue
2004). Most damaging to the commercial expansion of ET, were the fact
that the mare was unusually resistant to the induction of superovulation
(for review see McCue 1996), and that horse embryos are difficult to
cryopreserve (for review see Squires et al. 1999). In addition, pregnancy
rates achieved after non-surgically transferring embryos were lower and
more variable than those achieved by the more time-consuming and costly
surgical approaches (Squires et al. 1999). Even the most basic
requirement for successful ET, i.e. adequate synchronization of the
oestrous cycles of donor and recipient mare, can be labour intensive and
difficult because of the great variation in oestrus length between mares
(Hughes et al. 1980).
Equine ET finally took-off on a commercial scale in the early 1990s in
Argentina, where it was developed as a means of producing offspring from
the best polo pony mares without interrupting their competitive careers
(Pashen et al. 1993; Riera and McDonough 1993). Moreover, it quickly
became clear that horse embryos could be transferred non-surgically with
similar success to surgical ET, as long as enough carefully selected
recipient mares were available and the operator had sufficient dexterity to
manoeuvre a transfer pipette aseptically and atraumatically through the
cervix of a dioestrous mare. In the last few years, there has been a further
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