Factors affecting the success of embryo recovery
The most important factors affecting the likelihood of recovering an embryo
are the timing of insemination and the intrinsic fertility of the donor mare
and stallion (for reviews see Squires et al. 1999; Stout 2003); when AI is
employed, stallion fertility, and therefore embryo recovery, is further
influenced by semen dose, quality and method of preservation. In the case
of young fertile mares inseminated with fresh semen from fertile stallions,
more than 70% of flushes should yield an embryo (Losinno et al. 2001).
However, embryo recovery rates drop dramatically when aged mares (>14
years) or mares with a history of sub-fertility are used as donors (Squires
et al. 1982; Vogelsang and Vogelsang 1989: Meadows et al. 2000), or
when chilled transported or, in particular, frozen-thawed semen are used
(Meadows et al. 2000; Stout 2003). In Sporthorse practice in Europe and
America, embryo recovery rates typically range between 30 and 50%,
largely because owners often wish to recover embryos from aged donor
mares inseminated with chilled or frozen semen (Squires et al. 2003).
The time at which the flush is performed relative to ovulation also
influences embryo recovery, primarily because the equine embryo does
not enter the uterus until day 6 or 7 after ovulation (Battut et al. 1997,
2001); flushing on day 6 is therefore associated with a significantly lower
embryo recovery rate (Boyle et al. 1989) simply because the embryo has
yet to exit the oviduct. As will be discussed later, the equine embryo’s
unusually late time of uterine entry is problematic when trying to recover
pre-expansion embryos for cryopreservation, not least because the exact
time of embryo descent may also be influenced by season (delayed early
in the breeding season), mare age (delayed in older mares: Squires et al.
1999; Meadows et al. 2000)) and type of semen (delayed when frozen
semen is used).
Multiple ovulation
Typically, mares ovulate only a single follicle per oestrus, however some
mares will double or triple ovulate spontaneously and, in some breeds
(e.g. Thoroughbreds and Warmbloods), rates of spontaneous multiple
ovulation may exceed 30% (Davies Morel et al. 2005). In the context of
ET, spontaneous multiple ovulation is useful because it is associated with
an increase in total embryo yield and the number of pregnancies in
recipients (Squires et al. 1987; Losinno et al. 2001), although there are
indications that the number of embryos per ovulated follicle may be
reduced if the follicles ovulate from the same ovary (Riera et al. 2005).
Nevertheless, when rates of spontaneous multiple ovulation within an ET
donor population are high (e.g. 30%: Losinno et al. 2001) the efficiency of
the programme as a whole increases markedly. For similar reasons, the
commercial expansion of bovine ET was aided enormously by the early
development of cheap and effective protocols for superovulation, based on
equine chorionic gonadotrophin (eCG) or pituitary extracted follicle
stimulating hormone (FSH), that resulted in embryo yields averaging 5 per
flush and peaking at 50 in occasional cows (Kafi and McGowan 1997). In
marked contrast, the failure to develop a reliable protocol for
pharmacologically inducing multiple ovulation in mares was a major reason
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