surge in interest in ET in America and Europe, due largely to more
accommodating breed society regulations and a belated realization of the
potential of ET for hastening genetic progress, allied to technical
developments and changes in the scale of ET programmes that have
simplified or improved the cost-effectiveness of many aspects of embryo
collection, storage and recipient provision. Foremost among the latter, was
the demonstration that equine embryos can be transported at 5oC for up to
24 hours with no appreciable decrease in viability (Carney et al. 1991); this
has made ET more accessible because embryos can be flushed at any
location before shipping to a specialized centre where synchronized
recipient mares and transfer expertise are available.
Although it is now fairly widespread in clinical equine practice, ET
continues to attract scientific interest because many of the techniques that
could further improve efficiency (e.g. superovulation and embryo
cryopreservation) are still only marginally successful. Furthermore, ET is
the cornerstone of many of the more advanced assisted reproductive
techniques, such as in vitro fertilization and reproductive cloning, that are
not yet sufficiently developed for large-scale commercial implementation
but have considerable future potential for the investigation or treatment of
subfertility, for hastening genetic improvement and for salvaging the
reproductive potential of animalsthat would otherwise be lost from the
breeding pool (for review see Allen 2005). This paper will review the
advances that have allowed ET to emerge as a successful and
commercially viable clinical procedure, and discuss techniques still in
development but with the potential to make ET even more attractive in the
future.
The basics of non-surgical embryo collection and transfer
Recovery of horse embryos
Equine embryo collection is usually performed on day 7 or 8 after ovulation
because the embryo does not enter the uterus from the oviduct until as
late a s day 6-7 (Battut et al. 1997; 2001) while, at the other extreme, the
large size of day 9 and older embryos makes them more prone to damage
during collection and transfer (McKinnon and Squires 1988; Carnevale et
al. 2000). Embryo recovery per se is a straightforward procedure, primarily
because the cervix of a dioestrous mare can easily be distended to allow
the introduction of a large bore Foley-type catheter (Allen 2005). Once the
catheter has been introduced through the cervix, the cuff is inflated with
approximately 50 ml of air or flushing medium and the catheter is retracted
so that the inflated cuff occludes the internal os of the cervix, thereby
sealing-off the uterine lumen. The entire uterine lumen is then allowed to
fill by gravity flow with 1-2L of flushing medium, classically Dulbecco’s
phosphate buffered saline supplemented with a protein source, e.g. fetal
calf serum or bovine serum albumin, to prevent the embryo from sticking to
the silicone or plastics of the flushing system: a number of ready-to-use
equine embryo flushing media are now available commercially. After filling
with flushing medium, the uterus is gently balloted or massaged per
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