an LH:FSH ratio) may predispose to luteinisation of follicles without
ovulation (Briant et al. 2004). Another drawback to this type of biological
product, and one that may preclude its registration in Europe, is
biosecurity; any purification procedure that preserves glycoprotein
hormone activity is unlikely to destroy resilient viruses or prion proteins,
were an equine spongiform encephalopathy ever to arise. In theory, the
issues of batch variability, LH contamination and biosecurity could all be
addressed using a recombinant FSH. On the other hand, since the mare is
refractory to non-equine recombinant FSH products (Tharasanit et al.,
2006), a specific recombinant equine FSH (reFSH) will probably be
required. In fact, such a product may not be that far away; recombinant
equine LH (reLH) and FSH have recently been produced using a novel
single-chain expression technique (Roser et al. 2005); reLH is currently
undergoing clinical trials, and trials with reFSH are expected to follow
shortly. Certainly, additional studies are required to determine what the
‘optimum’ superovulatory response in a mare is. More ovulations may not
always be better since this may involve the ‘rescuing’ of a greater
proportion of atretic follicles incapable of yielding a viable oocyte or
embryo. Furthermore, higher ovulation rates are not always accompanied
by higher numbers of embryos, particularly if the ovulations are ipsilateral
(Riera et al. 2005), possibly because not all of the follicles are able to
reach the ovulation fossa and instead luteinise without releasing their
oocyte (Allen 2005).
Factors affecting transfer success
The likelihood of establishing pregnancy following ET depends on a
complex of interrelated factors including embryo quality, transfer
technique, donor-recipient synchrony and other aspects of recipient
suitability. Certainly, the temptation to use a substandard recipient on the
grounds of costs is a false economy.
Embryo quality
Gross embryo quality, as assessed using the light microscope, markedly
affects not only the likelihood of pregnancy in the recipient (McKinnon and
Squires 1988) but also the incidence of subsequent pregnancy loss
(Carnevale et al. 2000). Fortunately, the vast majority of horse embryos
recovered is of good morphological quality (>90% grade 1-2