Sperm freeze-drying; effects on sperm parameters

Today, sperm cryopreservation is a routine practice in infertility clinics. One of the alternative methods to preserve sperm is freeze-drying. This process consists of three steps. The first step is sample freezing, during which the solvent is separated from the solvent. Water forms ice crystals and the solute is placed between the ice crystals. Freezing may be done in a separate device or in a freeze dryer. The second step is the primary drying, in which the pressure of the device is reduced and heat is used to begin sublimation of the ice crystals. During the third step, which is the secondary drying, the final absorption of the remaining water is usually done by increasing the temperature of the product and slightly reducing the water vapor pressure in the container (17). This method has many advantages, including that it eliminates the need for liquid nitrogen for long-term storage and the dried samples can be stored at 4˚C or room temperature. To use sperm, rehydration must be done. In the preservation of human sperm by freeze-drying, the general lack of movement in the dried specimens indicates severe damage to the plasma membrane of the sperm. The effect of the physical and chemical environments to which sperm are exposed and the subsequent swelling and shrinkage as a result of water flow cause damage to the organelles, the lipid structure of the cell plasma membrane, and the water channels in the membrane (9). After freeze-drying, sperm become immobile in all culture media. Sperm viability has been reported following freeze-drying in various animal species (16). One of the anomalies that occurs after sperm freezing drying is morphological anomalies, especially in the tail, where curved tails have the highest proportion of these anomalies (9). When sperm returns to isotonic conditions after exposure to high osmolarity solutions, this causes the tail to twist and bend around the sperm head. In addition, changes in water content during cell dehydration may lead to tail twisting. The acrosome is the part of the sperm that is highly affected by freeze-drying in all culture media, regardless of semen status or storage temperature, which can be preserved in this technique (22). The effect of freeze-drying on DNA structure is contradictory. Numerous studies have shown that DNA integrity can be maintained following this method (9, 28). Adding trehalose to freeze-drying media help to maintain the sperm DNA integrity (36). The use of trehalose in freeze-drying can also lead to the preservation of microtubules (23). This article reviews the studies on freeze-drying of sperm and its effects on sperm parameters.