one can do to remedy poor embryo quality apart from trying to avoid
predispositions to reduced embryo quality in the future; these
predispositions include unresolved post-breeding endometritis (Squires
and Seidel 1995) and advanced maternal age (Carnevale et al. 1993). Not
surprisingly then, increased donor age per se is associated not only with
reduced embryo recovery, as outlined earlier, but also with a reduced
likelihood of establishing pregnancy after transferring an embryo to a
recipient, and an increased incidence of subsequent pregnancy loss
(Vogelsang and Vogelsang, 1989).
Of course, light microscopic evaluation only shows up gross abnormalities
in embryo morphology, while more complex techniques for analyzing
aspects of embryo quality are, in general, time-consuming and detrimental
to viability. Although membrane impermeant fluorescent stains such as 4',
6-diamidino-2-phenylindole dihydrochloride (DAPI: Molecular Probes
Europe) can be used to detect dead cells without compromising embryo
viability (Huhtinen et al. 1995), it is questionable whether such a test would
often add useful information given that the number of apoptotic or dead
cells in a grade 1-2 equine embryo is extremely low (<1%: Moussa et al.
2004; Rubio Pomar et al. 2005). Moreover, the knowledge that an embryo
has an unusually large number of dead cells would not change the
decision to transfer it, it would merely allow the owner to be informed of a
more guarded prognosis. One area of ‘embryo quality’ that has yet to be
explored in equine medicine, but is increasingly common in human IVF
programmes, is pre-implantation genetic screening for either specific
genetic abnormalities or more general abnormalities of chromosome
number (aneuploidy: Wilton, 2002). However, sampling cells from equine
embryos for genetic screening may be technically difficult because of the
tough, sticky acellular blastocyst capsule present from approximately day
6.5 after ovulation (Betteridge et al. 1982). Moreover, its utility may be
limited both by cost and the fact that most day 7 horse embryos contain
some aneuploid cells, but relatively few contain a proportion high enough
to guarantee non-viability (Rambags et al. 2005); a single randomly
sampled cell may not therefore be representative. On the other hand, preimplantation genetic screening could be useful for the eradication of
heritable diseases transmitted as a single gene defects, such as Fell pony
foal syndrome (Thomas et al. 2005) or hyperkalaemic periodic paralysis in
quarterhorses (Naylor, 1997); rather than excluding mating between
carriers, mating could be allowed as long as the embryos were
subsequently collected and shown to be free of the defect in question
before transfer was permitted.
Transfer technique
The simple fact that transcervical embryo transfer is more sensitive to
operator skill than surgical transfer (Squires et al. 1999; Allen 2005)
demonstrates that technique is an important determinant of success. It is
generally presumed that the two most important causes of failure to
establish pregnancy after transcervical transfer are bacterial contamination
of the progesterone-dominated uterus (Allen 2005) and/or hormonal
disturbances initiated by excessive dilation or manipulation of the cervix,
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February
2016
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London
Convention
Centre,
East
London,
South
Africa
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