Effects of pre- and post-thaw cell-to-cell contact and trypsin on survival of freeze-thaw damaged mammalian cells

Cryopreservation: Vitrification and Controlled Rate Cooling

Cryopreservation is the application of low temperatures to preserve the structural and functional integrity of cells and tissues. Conventional cooling protocols allow ice to form and solute concentrations to rise during the cryopreservation process. The damage caused by the rise in solute concentration can be mitigated by the use of compounds known as cryoprotectants. Such compounds protect cells from the consequences of slow cooling injury, allowing them to be cooled at cooling rates which avoid the lethal effects of intracellular ice. An alternative to conventional cooling is vitrification. Vitrification methods incorporate cryoprotectants at sufficiently high concentrations to prevent ice crystallization so that the system forms an amorphous glass thus avoiding the damaging effects caused by conventional slow cooling. However, vitrification too can impose damaging consequences on cells as the cryoprotectant concentrations required to vitrify cells at lower cooling rates are potentially, and often, harmful. While these concentrations can be lowered to nontoxic levels, if the cells are ultra-rapidly cooled, the resulting metastable system can lead to damage through devitrification and growth of ice during subsequent storage and rewarming if not appropriately handled.The commercial and clinical application of stem cells requires robust and reproducible cryopreservation protocols and appropriate long-term, low-temperature storage conditions to provide reliable master and working cell banks. Though current Good Manufacturing Practice (cGMP) compliant methods for the derivation and banking of clinical grade pluripotent stem cells exist and stem cell lines suitable for clinical applications are available, current cryopreservation protocols, whether for vitrification or conventional slow freezing, remain suboptimal. Apart from the resultant loss of valuable product that suboptimal cryopreservation engenders, there is a danger that such processes will impose a selective pressure on the cells selecting out a nonrepresentative, freeze-resistant subpopulation. Optimizing this process requires knowledge of the fundamental processes that occur during the freezing of cellular systems, the mechanisms of damage and methods for avoiding them. This chapter draws together the knowledge of cryopreservation gained in other systems with the current state-of-the-art for embryonic and induced pluripotent stem cell preservation in an attempt to provide the background for future attempts to optimize cryopreservation protocols.

The cryobiology of cryosurgical injury

Cryosurgery, or tissue destruction by controlled freezing, has been investigated as a possible alternative to surgical intervention in the treatment of many diseases. This technique, which is under the larger category of thermal therapy, has its origins in the 1800s when advanced carcinomas of the breast and uterine cervix were treated with iced saline solutions. Since those early times, this technique has been used routinely to treat malignancies on the surface of the body (ie, dermatologic tumors) and has gained some acceptance as a clinical tool for the management of internal malignancies, including carcinoma of the prostate and kidney. The main advantages of the technique are the potential for less invasiveness and lower morbidity compared with surgical excision. The study of the destructive process of freezing is the focus of this article and is divided into 2 main areas: (1) understanding the mechanism by which freezing destroys tissue, and (2) understanding the thermal history that causes tissue destruction. The term "thermal history," as used in this article, will mean the time-temperature history experienced by the tissue during a thermal insult.

Protective effect of L-glutamine against freeze-thaw damage in mammalian cells

L-Glutamine at 18 mM protects mammalian cells against freeze-thaw (FT) damage by a factor of about 6, depending on FT conditions, in balanced salt solutions. While not nearly as effective a cryoprotectant as dimethyl sulfoxide (DMSO) or propylene glycol (PG), the mechanism of protection by glutamine appears to be independent from that of DMSO or PG; thus, 18 mM glutamine is effective at reducing FT damage in combination with these agents. These combinations allow lower concentrations of the more toxic agents DMSO and PG to be used in FT medium. There is no pre-FT or post-FT effect of glutamine when cells are exposed to a FT cycle in balanced salt solutions. Hence, protection is due to its presence during the FT-cycle. The presence of 2 mM L-glutamine in Eagle's basal medium is sufficient to account for cryoprotection by this medium.