September 28, 2012
In some circumstances, this can be a very handy trick of the horse-breeding trade.
By Dr. Robert O. Gilbert
Do you have a promising colt? Not sure if you would be better off keeping him intact? Do you feel torn between the desire to preserve his breeding potential and the convenience of working with a gelding? One way to do both is to freeze semen, store it and wait to see how the gelding performs in his athletic career. If you need only a modest amount of frozen semen, it may not be necessary to train the stallion for semen collection, but simply to obtain sperm from his reproductive organs after castration. Similarly, if a stallion's life is cut short by an accident or acute illness, it may be possible to obtain sperm after death and successfully freeze it. (This procedure is less likely to be successful if the stallion has had a prolonged illness or suffered age-related reduction in fertility before death).
Sperm can be obtained from the tail of the epididymis – this is the organ attached to the testis, into which sperm pass after their production in the testis. It functions to support sperm as they mature and acquire the ability to fertilize oocytes, or eggs. The epididymis also serves as the major storage organ for sperm before ejaculation. Transit time for sperm through the epididymis is about two weeks. By the time they reach the tail of the epididymis, they have achieved the ability to fertilize and are held in “suspended animation” as a reservoir for ejaculation. It is this feature that is exploited to allow recovery of sperm from dead stallions or post-castration.
The number of sperm that can be recovered from the paired epididymide
s of a stallion is usually more than a regular ejaculate from the same stallion. The sperm quality is good, and motility is often excellent. Having only a single chance for freezing means that there is not the normal opportunity to test which extender works best for that particular stallion, but use of one of the standard extenders is usually sufficient. There is huge variation between stallions, but freezing from 5 to 20 doses of semen for later use is usually feasible. Of course, as is true for routine freezing, spermatozoa of some stallions, even those of proven fertility, simply do not tolerate freezing well.
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To capture the opportunity to freeze epididymal sperm, you should notify your processing center (usually a university, specialist veterinary practice or commercial semen freezing organization) in advance to make sure they freeze epididymal sperm and that you know how they want to receive the tissues. In general, they will want you to ship the intact testes (with attached epididymides and a portion of the ductus deferens) on ice. Wrap the tissues in a clean plastic bag and pack them in ice, without allowing the tissue to touch the ice directly. Do not use dry ice. Ship the tissues to reach your processor within no more than 24 hours.
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Good fertility results have been obtained with frozen-thawed, epididymal-derived sperm from stallions. There is some disagreement about whether fertility is enhanced by the addition of pure seminal plasma (from the same or another stallion, and which can be frozen separately), but it is clear that excellent results can be obtained with epididymal sperm without any supplementation, using routine insemination practices.
Dr. Robert O. Gilbert is a professor of reproductive medicine at the Cornell University College of Veterinary Medicine.
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