The loss of significant numbers of viable cells during cryopreservation clearly leads to a decrease in clinical usefulness of any stored cell type. Consequently, an understanding of the mechanisms by which cryoinjury to sperm occurs is exceedingly important. In this terminally differentiated cell, the plasma membrane serves as the main physical barrier to the extracellular environment, and is therefore a primary site of freeze-thaw damage. Such damage includes membrane destabilization due to lateral lipid rearrangement, loss of lipids from the membrane, and peroxidation of membrane lipids as a result of ROS formation. These events can affect sperm motility and response to osmotic stress and signaling pathways, and thus the ability to reach, bind to, and react with the zona pellucida during fertilization could become compromised [58] .

Sperm, although seemingly simple, have a number of cellular compartments, where each is susceptible to various types of environmental stresses. Thus, it is no surprise that a considerable amount of sublethal and lethal damage occurs as a result of exposure to the extreme temperature and osmolality effects from cry-opreservation [16-18, 19, 44, 45, 50, 88]. As the cell is exposed to plunging temperatures, extracellular ice crystallization begins which results in the concentration of the surrounding solutes in the unfrozen aqueous channels between ice crystals [11]. This poses a severe osmotic insult to the cell that can be minimized but not eliminated by using CPA agents. This osmotic change has been shown to result in induction of oxidative change that is, in itself, damaging to sperm.

From the information presented in this chapter, it seems that most if not all cell damage incurred from cryopreservation occurs during the cooling and thawing processes, as extracellular space alternates between hypertonic and hypotonic conditions. However, we have seen that at least some oxidative cell damage may occur at steady state during the cryostorage phase of the process of cryopreservation. Clearly, much research remains to be performed with regard to the role of oxidative stress during the phases of cryopreservation.

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