A Systematic Review of the Anti-oxidative Effect of Melatonin in Cryopreserved Gametes

Cryopreservation of animal sperm and oocytes is a key technique in assisted reproductive technologies, enabling livestock breeding, genetic diversity preservation, and conservation of endangered species. However, freezing and thawing can cause oxidative stress, which may damage gametes and reduce their quality compared to fresh ones. Melatonin, a potent antioxidant, has been explored to protect gametes during this process. This systematic review evaluates the role of melatonin in protecting animal sperm and oocytes during cryopreservation, focusing on its impact in livestock farming, genetic diversity, and species conservation. We conducted systematic literature searches following PRISMA guidelines across five databases (Proquest, Scopus, PubMed, Web of Science, and Cochrane Library) up to 31 July 2024. The quality of included studies was assessed using the Systematic Review Center for Laboratory Animal Experimentation tool. We reviewed 30 studies examining melatonin’s effects on sperm and oocytes across various animal species, including poultry, buffalo, rams, and mice. Melatonin reduced oxidative stress in sperm by lowering reactive oxygen species (ROS), lipid peroxidation, and malondialdehyde levels, while enhancing mitochondrial function, antioxidant enzyme activity (e.g., SOD, GSH-Px, CAT), and DNA integrity. For example, in poultry, concentrations of 10−3 M improved sperm motility and membrane health, while in fish, higher doses (4.25 mM) were harmful. Melatonin also improved sperm motility, viability, and acrosome integrity in several species, though results varied (e.g., no effect in canines). For oocytes, melatonin (e.g., 10−9 M in buffalo) reduced ROS, improved mitochondrial health, and enhanced maturation and fertilization rates, but higher doses (e.g., 10−7 mol/L in mice) sometimes increased oxidative stress. Outcomes depended on species, melatonin dose, and freezing medium. Melatonin acts as a powerful antioxidant, reducing oxidative stress during cryopreservation and improving sperm motility, viability, and oocyte maturation in many animal species. However, its effectiveness varies by species, dose, and medium type, with optimal concentrations differing across studies. These findings support melatonin’s potential to maintain gamete quality close to that of fresh gametes, aiding assisted reproductive technologies, but further research is needed to standardize doses and clarify species-specific effects.