The Science and Consequences
of Mishandling Frozen Semen and Embryos
DES MOINES, Iowa (Sept. 7, 2016) — It happens far too often. Someone removes a straw containing a frozen embryo or semen from a storage tank, wipes off the frost and checks its identification, then returns the straw to cold storage. Even though the straw was exposed to ambient temperatures for several seconds, no harm is done — not as long as the straw’s contents remained frozen. Right?
Brad Stroud of Stroud Veterinary Embryo Services.
Not according to veterinarian Brad Stroud of Stroud Veterinary Embryo Services. In a presentation before the Applied Reproductive Strategies in Beef Cattle conference hosted Sept. 7-8 in Des Moines, Iowa, the Weatherford, Texas, practitioner said that kind of cowboy logic is seriously flawed. In Stroud’s opinion, mishandling of frozen reproductive cells is part of a widespread industry problem.
“Hundreds of millions of frozen gametes are held in storage worldwide, and to my knowledge, there is no organized training curriculum addressing their management,” stated Stroud. “We know how to freeze cells, but we don’t know how to handle them very well, and I think that’s a big problem.”
Stroud explained how once sperm or embryos are cooled to a point below –130° C (30° below freezing), they cannot be raised above that temperature and then be re-exposed to temperatures below –130° C, which is the glass transition temperature of water. If it happens, cell damage will occur.
When there is frost (tiny ice crystals) on the outside of the straw, there are ice crystals on the inside, too. According to Stroud, damage to the cells is caused by recrystallization — the transformation of small ice crystals in the extracellular fluids into much larger crystals during the temperature changes from below –130° C to above –130° C, and back to below –130° C. The large crystals physically invade cell membranes and cellular organelles of either sperm or embryos.
“The severity of damage to cells is dependent upon how high the temperature gets above –130° C and the duration of exposure above –130° C,” said Stroud. “Since the temperature in the necks of most standard ranch Dewars (storage tanks) ranges from –75° C all the way to room temperature, it is very common for frozen semen and embryos to be exposed and damaged, or even destroyed, during routine handling by those involved in daily Dewar management.”
To guard against recrystallization, Stroud recommended using a liquid nitrogen bath when handling straws and for “chilling” forceps and other instruments used in the process. He suggested making a shallow tray from an inexpensive Styrofoam cooler, deep enough to hold a few inches of liquid nitrogen. Care should also be taken when receiving shipments of frozen semen or embryos, and when preparing for shipment.
“How many farms and ranches have a protocol for handling frozen semen and embryos? I think too many people just wing it,” stated Stroud. “That may be a big contributor to what I call undefined fertility failure.”