Semen cryopreservation and storage in liquid nitrogen: Impact on chromatin compaction.

Background: Sperm cryopreservation is a consolidate option for long-term male fertility preservation. The freezing/thawing procedure causes detrimental effects to spermatozoa, including damage to viability, motility, membrane composition, and DNA, whereas the effect on sperm chromatin compaction is less studied.

Objectives: The primary aim of this study was to investigate the impact of cryopreservation on sperm chromatin compaction. Furthermore, the effect of cryopreservation on sperm parameters (motility, viability, chromatin compaction, and DNA fragmentation) was also assessed in relation to the storage time in liquid nitrogen.

Materials and methods: Semen samples, collected from 126 (92 normozoospermic and 34 oligozoospermic) patients undergoing routine semen analysis in the Andrology Laboratory of Careggi University Hospital of Florence, were frozen by conventional fast vapor freezing method. Sperm motility, viability, kinematic parameters (by computer-aided sperm analysis [CASA]), chromatin compaction (by staining with both aniline blue [AB] and Chromomycin A3 [CMA3]), and sperm DNA fragmentation (sDF, by TUNEL/Propidium Iodide [PI]) were evaluated before freezing and after thawing at different timepoints.

Results: After 7 days of storage, a significant decline in sperm motility, viability, and kinematics parameters, as well as a significant increase in the percentage of sperm positivity to CMA3, AB, and sDF, were observed. It is noteworthy that while motility and viability decreased in almost all subjects, the increase in CMA3 and AB positivity was observed in 68.0% and 79.2% of samples, respectively. A progressive deterioration of sperm motility and viability, less evident for chromatin structure, was observed at longer times of storage (28 and 180 days).

Discussion: Our results indicate that freezing/thawing procedures can alter chromatin structure. A reduction in protamine content and/or a modification in chromatin assembly can be hypothesized. Furthermore, the length of storage in liquid nitrogen appears to progressively affect sperm parameters, although it should be confirmed in larger cohort of subjects.

Conclusion: Current sperm cryopreservation protocols need to be improved with new strategies and personalized procedures aimed to minimize the damage.