Molecular and subcellular cryoinjury of fish spermatozoa and approaches to improve cryopreservation

The quality of frozen/thawed fish sperm is generally lower than that of fresh sperm. Extremely low temperatures are associated with damage to fish spermatozoon subcellular compartments and associated molecules such as DNA and proteins. Cryodamage can negatively affect DNA integrity, spermatozoon metabolism and motility, consequently impairing fertilization and embryo development. To preserve sperm efficiently, the addition of proteins including seminal plasma proteins, antioxidants, antifreeze proteins and antifreeze glycoproteins, and bovine serum albumin before and/or after freezing is suggested to minimize cryoinjury. An appropriate quantity of seminal plasma components in cryopreservation medium can prevent spermatozoon membrane damage by maintaining antioxidant enzymes and their proper distribution on the spermatozoon surface. Under storage conditions, the semen antioxidant system is ineffective in protecting spermatozoa from reactive oxygen species. The addition of antioxidants can provide protection for spermatozoa and reduce cryoinjuries during storage, but the effect of a given antioxidant is species-specific. Antifreeze proteins and antifreeze glycoproteins can stabilize the cell membrane via interaction with phospholipid components and inhibit ice-crystal growth to reduce damage related to osmotic stress and ice crystallization. However, they may exhibit a cytotoxic effect. This review summarizes the sources and characteristics of subcellular spermatozoon cryoinjury and discusses approaches to improve outcome of fish sperm cryopreservation.