Commercially produced semen freezing machines have been available for a number of years. In principle, these should be of considerable help for both handling large
numbers of samples by a standardized method, and should also permit accurate cooling of the samples at the desired rates. While these systems may meet the first objective,
they have so far proved inadequate to meet the second. This is because during the
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freezing process, the samples release sufficient heat latent heat of fusion to cause a sharp increase in temperature; no successful method of dissipating this heat rapidly has
so far been developed, and sample temperature consequently does not decrease in tandem with the fall in chamber temperature. In fact, the sample temperature can remain
static for 2–3 min before cooling is resumed. Several investigators have shown that this period between freezing and resumption of cooling, the freezing point plateau, is
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detrimental to sperm survival. Parkinson and Whitfield 1987 showed that reduction of this plateau improved the fertility of bull spermatozoa, and similar results have been
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obtained with boar semen Pursel and Park, 1985; Bwanga et al., 1991 . The introduc- tion of larger volume freezing methods such as the maxi-straw or plastic bag is likely to
exacerbate this problem and there is clearly a need for the development of more effective temperature control systems.
5. Disease control issues
Concern over the potential for transmission of viral infection between samples stored in liquid nitrogen has recently been expressed in the United Kingdom, particularly in
relation to samples of human semen. Fears arose initially when transmission of hepatitis to a patient was attributed to stored bone marrow cells that had been contaminated when
an infected sample was damaged within the same sample storage container. An independent review panel was convened following this case to assess disease transmis-
sion risks between stored samples. The panel’s recommendation that all straws of human semen should henceforth be stored in the vapour, rather than the liquid phase of the
nitrogen container caused consternation in laboratories with semen storage facilities. This position has now been abandoned, partly through difficulties of implementation.
This episode highlighted a problem with regard to semen storage which has hardly ever been addressed. In fact, there is virtually no data upon which to base an assessment,
except for some experiments performed recently at The Royal Veterinary College, Ž
. London Russell et al., 1997 to investigate the potential for viral leakage from sealed
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straws. Russell et al. 1997 could find very little evidence that viral leakage from inside the straw was possible, provided straws were properly sealed. However, viral contamina-
tion of the external surface of straws could occur during filling if they were dipped into the semen–diluent mixture. Care to avoid this practice is therefore recommended,
especially when dealing with samples whose disease status is unknown. Secondary protection against straw breakage may also be advisable in certain cases; this can be
implemented quite simply by enclosing straws in a sealed outer plastic sleeve. These precautions are considerably easier to implement with straws than with pellets, where
the semen is initially in contact with dry ice, and is then normally stored in a perforated tube that permits direct contact with the liquid nitrogen itself.
6. Assessment of frozen–thawed semen
