- Melatonin (N‑acetyl‑5‑methoxytryptamine) is best known as a neurohormone regulating circadian rhythms, but it also exhibits powerful antioxidant, anti‑inflammatory, and membrane‑stabilising properties. These biological effects have led to growing interest in melatonin as a supplementary cryoprotective agent, capable of reducing cryoinjury and improving post‑thaw cell viability. Unlike classical permeating CPAs such as DMSO or ethylene glycol, melatonin acts primarily through biochemical protection rather than direct modification of water behaviour. Its unique profile makes melatonin a promising additive in cryopreservation protocols for reproductive cells, stem cells, tissues, and organoids.
- Melatonin’s cryoprotective potential arises from its strong antioxidant capacity. Freezing and thawing generate reactive oxygen species (ROS) through mitochondrial stress, membrane disruption, and ice‑induced mechanical damage. Excess ROS contributes to lipid peroxidation, protein oxidation, DNA damage, and apoptosis. Melatonin directly scavenges ROS and upregulates endogenous antioxidant enzymes such as superoxide dismutase, catalase, and glutathione peroxidase. This dual action reduces oxidative injury during cooling and thawing, improving cellular resilience.
- Melatonin also stabilises biological membranes. During freezing, phospholipid bilayers undergo phase transitions that increase permeability and disrupt membrane integrity. Melatonin interacts with lipid head groups, preserving membrane fluidity and reducing leakage of ions and metabolites. This membrane‑protective effect complements the osmotic and vitrification functions of classical CPAs, making melatonin a valuable additive in CPA mixtures. More details on CPA combinations are available in Cryoprotective Agents.
- Another important mechanism is melatonin’s ability to reduce apoptosis. Cryoinjury activates caspase pathways, mitochondrial depolarisation, and cytochrome‑c release. Melatonin inhibits pro‑apoptotic signalling and supports mitochondrial stability, reducing programmed cell death after thawing. This is particularly beneficial for sensitive cell types such as oocytes, embryos, stem cells, and neural tissues.
- Melatonin has been studied in various cryobiological applications. In reproductive cryobiology, melatonin improves post‑thaw motility and membrane integrity in spermatozoa, enhances developmental potential in embryos, and reduces oxidative stress in vitrified oocytes. In stem‑cell preservation, melatonin supports mitochondrial function and reduces differentiation drift after thawing. Tissue cryopreservation studies show improved structural integrity and reduced inflammation when melatonin is included in freezing solutions. These benefits make melatonin a versatile additive across diverse cryopreservation systems.
- Toxicity is generally low for melatonin, especially compared with permeating CPAs. However, concentration matters. Low micromolar concentrations typically provide antioxidant and membrane‑protective benefits, while excessively high concentrations may disrupt signalling pathways or alter gene expression. Optimal dosing varies by cell type, CPA mixture, and cooling rate. Melatonin is usually added as a supplementary agent rather than a primary CPA, enhancing protection without replacing classical cryoprotectants.
- Despite its advantages, melatonin is not a universal solution. It does not prevent ice formation, modify water behaviour, or provide osmotic dehydration like permeating or non‑permeating CPAs. Its role is biochemical rather than physical. Therefore, melatonin is best used in combination with established CPAs to reduce oxidative and apoptotic injury while classical agents manage ice suppression and vitrification.
- Overall, melatonin represents a promising adjunct in modern cryobiology. Its antioxidant, anti‑apoptotic, and membrane‑stabilising properties complement traditional cryoprotectants, improving post‑thaw viability and reducing cellular stress. As research advances toward safer, more biologically compatible cryopreservation systems, melatonin continues to gain attention as a valuable component of next‑generation freezing and vitrification protocols.
