to establish pregnancy in the case of an inexperienced operator, or when
few recipients are available. Uterine and cervical tone at the time of
transfer (approximately day 5 after ovulation) also appear to be useful
indicators of the suitability of a recipient, poor tone is associated with a
lower pregnancy rate and higher risk of pregnancy loss (Carnevale et al.
2000). Thus, the success of an ET programme benefits greatly from a
surfeit of recipients because this allows mares with a relatively flaccid
uterus or cervix, or in which difficulties in passing the pipette through the
cervix are encountered, to be replaced by more suitable recipients.
A largely overlooked aspect of recipient suitability is size relative to that of
the donor mare and stallion; a recipient is usually selected on the basis of
cost and synchrony, with little attention to size. However, a series of
experiments by Allen and colleagues in Newmarket involving the transfer
of Thoroughbred embryos into ponies and vice versa have demonstrated
clearly that mismatches between the ‘genetic size’ of embryo and recipient
mare affect many aspects of both intrauterine and post-natal development.
Inappropriate maternal size will lead, depending on the direction of the
difference, to under or overgrowth of the fetus (Allen et al. 2002) which,
despite a degree of compensation during post-natal life, is maintained into
adulthood (Allen et al. 2004). Moreover, the intrauterine growth retardation
suffered by Thoroughbred foals gestated in the ‘deprived’ environment of a
pony uterus is evidenced by clear signs of physical and behavioural
immaturity at birth, such as delayed times to stand and suckle, and a
depressed ability to release cortisol in response to ACTH challenge
(Ousey et al. 2004). Interestingly, both enhanced and retarded intrauterine
growth alter postnatal cardiovascular, endocrine and metabolic function
(Giussani et al. 2003; Forhead et al. 2004) and, overall, it appears that
using a recipient that differs markedly in size to the donor will influence
adult size of the offspring and, in the worse case, increase morbidity during
intrauterine, immediate postnatal and, possibly, later adult life.
Cooled storage of horse embryos
One of the biggest changes in commercial equine ET during the last 10
years has been the dramatic increase in the number of embryos
transported at 5oC to ET centres (Squires et al. 1999; 2003). Especially
popular in the USA and Brazil, this has allowed owners and veterinarians
to get involved in ET without needing to acquire, house and manage
recipient mares, or transport the donor mare over great distances, or
develop techniques for cryopreserving embryos. The first reports that
equine embryos could be stored at 4-5oC for substantial periods (days)
with minimal loss of viability appeared in the late 1980s (Carnevale et al.
1987; Pashen 1987). Subsequently, Carney et al. (1991) reported that
neither pregnancy rates nor rates of subsequent pregnancy loss differed
between embryos transferred immediately or those transported for up to
24 hours in Ham’s F-10 medium, and packaged in a passive cooling
device such as an Equitainer (Hamilton Thorne Bioscience, Beverley, MA).
As a result, Hams F-10 became the medium of choice for the cooled
transportation of equine embryos, despite the inconvenience of having to
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