Reactive oxygen species and antioxidant strategies in human sperm cryodamage

Sperm cryopreservation is widely used for male fertility preservation and assisted reproduction technologies; however, the freezing and thawing process can induce irreversible structural and functional damage to human spermatozoa. This review summarizes the main mechanisms through which cryopreservation disrupts redox homeostasis, with particular emphasis on the generation of reactive oxygen species, mitochondrial dysfunction, lipid peroxidation, DNA fragmentation, and the consequent impairment of sperm motility and viability. Human spermatozoa are particularly vulnerable to oxidative damage because they possess limited cytoplasmic antioxidant defenses and a plasma membrane rich in polyunsaturated fatty acids (PFA), which are highly susceptible to peroxidative injury. In addition, the dilution or removal of seminal plasma during cryopreservation further compromise extracellular antioxidant protection. Experimental studies have shown that supplementation of freezing and/or thawing media with enzymatic and non-enzymatic antioxidants, including catalase, glutathione, ascorbic acid, tocopherols, melatonin, resveratrol, quercetin, MitoTEMPO, myo-inositol, elamipretide, brain-derived neurotrophic factor, and various natural extracts, may improve selected post-thaw sperm parameters and reduce biomarkers of oxidative stress. Nevertheless, no antioxidant strategy currently available is capable of completely preventing cryodamage, and the market heterogeneity of existing protocols limits their clinical translation. Further well-designed human studies are therefore required to determine the safety, efficacy, optimal concentration, and clinical utility of antioxidant supplementation in sperm cryopreservation media.