Gamete Cryopreservation: Molecular Mechanisms of Cellular Injury and Epigenomic Alterations in Human and Animal Models

In human spermatozoa, freezing and thawing are consistently associated with impaired membrane integrity, reduced motility, mitochondrial dysfunction, oxidative imbalance and alterations in DNA/chromatin stability. Regulated cell death pathways may additionally contribute to post-thaw deterioration; however, the strength of evidence differs among individual pathways, and emerging mechanisms such as ferroptosis remain less extensively characterized in human spermatozoa than conventional oxidative and apoptosis-associated injury. Evidence for cryopreservation-associated epigenomic alterations is similarly heterogeneous. Experimental and animal studies demonstrate changes in DNA methylation, histone modifications and non-coding RNA profiles. In contrast, human evidence is generally more reassuring and has not demonstrated consistent persistent abnormalities in major imprinting-associated methylation patterns. Accordingly, observations from animal models should not be directly extrapolated to human reproductive outcomes. By integrating human and experimental evidence while explicitly considering these differences in evidentiary strength, this review identifies established mechanisms, emerging molecular pathways and major unresolved questions in contemporary gamete cryobiology.