ARTIFICIAL INSEMINATION IN FARM ANIMALS - Phenix-Vet

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192Artificial Insemination in Farm Animals(Amman and Graham, 1993; Lessard et al, 2000). The stress response shown by spermatozoaas a reaction to a drop in temperature is referred as cold shock. Generally, cold shockdamage manifests itself as a decline in cell metabolism, altered membrane permeability, lossof intracellular components, irreversible loss of spermatozoan motility and an increase inthe number of dead spermatozoa. The damage to the cellular membranes is of mostsignificance because it has a carry-over effect on other cellular structures and functions.The severity of the cold shock depends upon the final temperature and the rate oftemperature drop. The cellular damage resulting from cooling or freezing affecting both thestructure and function of the cells can be categorized as direct or indirect (Amann andPickett, 1987; Watson, 1990). Direct damage is more definable and is the type usuallyassociated with cold shock evident shortly after the drop in temperature and is affected bythe rate of cooling. Indirect or latent damage is more difficult to quantify and may not beinitially apparent; it tends not to be dependent upon the rate of cooling. Addition ofcryoprotectant agent (CPA) such as glycerol, or of other components such as egg yolk, milk,bovine serum albumin, polyvinyl alcohol and liposomes in extenders have been used in anattempt to providing some protection to spermatozoa and minimizing the adverse effects ofcryopreservation (Katila, 1997). Direct cellular damage is irreversible and is usuallyapparent at later stage during postthaw analysis particularly in samples that are to rapidlyfrozen (Bildeau et al, 2000; Pommer et al, 2002; Samper, 2001). Hence, the ability to predictpostthaw sperm quality and fertility from a routine sperm function assay would be greatlybeneficial to the success of cryopreservation.2. Cryopreservation of semenIn order to realize many of the potential advantages of AI, long-term storage of semen isnecessary. This is only possible by freezing, a system which halts the metabolic processes ofthe spermatozoa, allowing indefinite storage without a significant loss of fertility. Thediscovery of the cryoprotectant properties of glycerol has made cryopreservation possible.Among the many benefits resulting from the process of cryopreservation are the geneticimprovement of important farm species and control of diseases affecting them both of whichhave a highly significant impact on the sustainability of the agri-food industry (Bailey et al,2000). Cryopreservation has been widely used in the modern cattle industry and AI hasbeen the most widely applied tool in facilitating the extensive utilization of frozen semen.Today, cryopreservation has enabled producers to have the ability to access superiorgenetics for a fraction of the price of buying a bull. Increased breeding efficiency andexploitation of sires through AI programs has become widespread in both the dairy andequine industry. However, the development of a reliable method to cryopreserve sperm isextremely important for preservation of superior genes from valuable animals. Themaximum time allowed from semen collection to insemination varies considerably, rangingfrom immediate insemination requirements to an indefinite in vitro semen storage period.As a result of the growing popularity of these artificial breeding programs, the need tomaintain fertility of sperm after varying periods of storage has become increasinglyimportant. In addition to this cryopreservation has enabled the storage of animal genetics tokeep allele variation and keep hope for endangered species. Genome resource banking topreserve the biodiversity of rare and endangered species or valuable transgenic lines wouldalso benefit from sperm cryopreservation. Moreover, reproductive research with nondomesticanimals particularly those involving cryopreservation of sperm, oocytes, and
Effect of Cryopreservation on Sperm Quality and Fertility 193embryos can also provide insight and direction into establishing more effective genetic andconservation management programs.The success of cryopreservation depends upon many other factors, including interactionsbetween cryoprotectant, type of extender, cooling rate, thawing rate and packaging, as wellas the individual animal variation (Andrabi, 2007; Clulow et al, 2008; Cooter et al, 2005).Some loss in spermatozoan viability is inevitable due to the processing procedures prior tofreezing as well as during the actual freezing process. Research reports of the success ofcryopreserved semen vary significantly often affected by the method of experimentationand recording, which is unstandardized in many reproductive researches. Informationabout pregnancy rate to a single insemination, timing of insemination, the number ofspermatozoa inseminated, volume of inseminate used or type of extender used are stillincomplete for some farm animals. Moreover, motility of spermatozoa has proven to be aneven poorer indicator of fertility in frozen–thawed samples (Samper et al, 1991). Regardlessof all these considerations, for cryopreservation to be considered a success the processshould enable a spematozoon to retain its fertilizing capacity at postthaw. To achieve this itmust retain its ability to produce energy via metabolism; to maintain normal plasmamembrane configuration and integrity; retain its motility; and enzymes, such as acrosin,within the acrosome to allow penetration of the ova. Disruption of any of these functions orabilities will significantly affect the spermatozoon’s ability to achieve fertilization. Thegreatest risk to the maintenance of these attributes is presented by the formation of icecrystals and the resultant movement of water up osmotic gradients during the process ofcryopreservation.During the process of freezing, several biophysical changes are evident within the semensample. As the temperature drops to below freezing the sample undergoes supercooling. Asthe temperature drops further beyond supercooling, extracellular ice crystals begin to formfrom the water within the surrounding medium. This ice formation increases theconcentration of solutes, such as sugars, salts and proteins. In response to this newlydeveloped osmotic pressure gradient and the fact that water within the spermatozoon isslower to form ice crystals than the water in the surrounding medium, water passes out ofthe spermatozoa, particularly from the spermatozoon head, across the semi-permeableplasma membrane. Consequently, the spermatozoon becomes increasingly dehydrated(Andrabi, 2007; Watson, 2000; Woelders, 1997). The rate of efflux of water from thespermatozoa also depends upon the speed of temperature drop: the slower the drop, thegreater would be the time needed for the efflux of water, and hence a much greaterdehydration. This does reduce the chance of ice crystal formation within the spermatozoon,which could cause considerable physical damage (Amann and Pickett, 1987; Hammerstedtet al, 1990), but an even greater damage occur due to increased intracellular dehydration andsolute concentration. On the other hand, if the cooling rate is rapid, water has little time tomove out of the spermatozoon and hence large intracellular ice crystals form, causingphysical damage to cell membranes and other intracellular components. However, theproblems of dehydration and solute concentration are less evident with rapid cooling. Asuccessful cryopreservation should, therefore, aim at arriving at an optimum cooling ratethat will provide a compromise between all these factors.There are two main temperature ranges of concern regarding damage to spermatozoaduring freezing: the period of supercooling (0°C to -5°C) and the formation of ice crystals (-6°C to -15°C) (Woelders, 1997). Excessive supercooling results in a rapid ice formation, with
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VIContentsChapter 9Evaluation of a
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ARTIFICIAL INSEMINATION IN FARM ANIMALS - Phenix-Vet

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