When was the last time you thought about semen storage and liquid nitrogen tank management? To realize the maximal potential fertility within frozen semen straws, the liquid nitrogen tank must be managed properly.
The liquid nitrogen tank consists of a “tank within a tank,” with insulation under vacuum between the inner and outer tanks. Liquid nitrogen tanks should be stored in a clean, dry and secure area to ensure a long tank life, create a safe environment for personnel and to deter theft. If the tank has to be moved, securely fasten the tank during transportation to avoid tipping or damaging the tank, both of which usually result in the premature loss of liquid nitrogen.
Whether the tank is stored in an office or carried in a vehicle, a detailed inventory of semen should be easily accessible, so straws may be located and removed from the tank quickly to avoid prolonged exposure of semen.
BELOW THE FROST LINE
When removing a straw from a liquid nitrogen tank, it is imperative to work quickly and keep the canister, cane and unused semen straws as low as possible in the neck of the tank. A best management practice is to keep all unused straws below the frost-line in the neck of the tank. Although the temperature of liquid nitrogen is -320 °F, there is a temperature gradient in the neck of the tank. For example, a tank with a 6- inch long neck tube may have a temperature of -103 °F in the middle of the neck (3 inches below the top), while the temperature at 1 inch below the top may be 5 °F.
Within the straw, recrystallization, defined as the transformation of small ice crystals in the extracellular fluid into larger ice crystals, occurs when the temperature changes from below -202 °F to above -202 °F to below -202 °F. The larger ice crystals damage sperm cell membranes and organelles, the severity of which is dependent upon how high the temperature rises, and the dur of exposure above -202 °F. Sperm injury (as judged by sperm motility) has also been reported to occur at -112 °F. Injury to sperm cannot be corrected by returning semen straws to liquid nitrogen; therefore, damage to sperm in straws not removed during retrieval of other straws is additive.
HAVE ENOUGH NITROGEN
The liquid nitrogen level in a tank can dramatically affect the temperature of straws repeatedly raised and lowered in the tank. In a classic experiment, elevation of a cane (containing a goblet with 5 semen straws) into the neck of the tank for approximately 1 minute resulted in a straw temperature increase of 28 °F (from -320 to -292 °F) when the tank was full of liquid nitrogen. An increase in straw temperature was minimized as the goblet contained liquid nitrogen at the start of the exposure. Furthermore, the straw temperature reached -320 °F almost immediately upon return.
When the liquid nitrogen level in the tank was low (approximately 5.5 inches), however, the temperature of straws increased 129 °F (from -320 to -191 °F), and one minute after returning to the tank the temperature of the straws had not yet reached -320 °F.
In summary, monitor the liquid nitrogen level in your tank weekly, and never let it go below 6 inches. Retrieve straws quickly and keep all unused straws below the frost-line in the neck of the tank. For more information on semen storage and liquid nitrogen tank management, contact your AI stud representative or local University Extension.
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