Freeze-dried spermatozoa: A future tool?

Cryopreservation has been routinely used to preserve sperm of human and different animal species. However, frozen sperm storage for a long time brings many inconveniences because of liquid nitrogen. Many attempts have been made to overcome the disadvantages of the current cryopreservation method. Freeze-drying has been proposed as alternative method for sperm preservation to achieve the ability to store sperm doses indefinitely at ambient temperature or in ordinary refrigerators. At present, it has been reported successfully sperm freeze-drying on many animal species including canine and feline. It is well known that during freeze-drying process, sperm DNA could be damaged, but if suitable protection is provided, the sperm nucleus could preserve the ability to activate the oocyte and embryos could be generated by intracytoplasmic sperm injection (ICSI). Many factors influence the freeze-drying efficacy, so current researches have been conducted to find strategies to control these factors to maintain the sperm DNA integrity. This review describes the latest method of sperm freeze-drying for practical application in preserving and transporting genetic resources. In addition, the approaches to improve the efficiency of the technique were studied. We demonstrated that the DNA integrity of freeze-dried dog sperm is affected by the composition of the freeze-drying solution as well as the temperature and period of storage. Further studies are necessary to refine freeze-drying protocol in order to protect the DNA and maintain the sperm functionality and obtain offspring from freeze-dried sperm.