Poor Sperm Chromatin Condensation Is Associated with Cryopreservation-Induced DNA Fragmentation and Cell Death in Human Spermatozoa

Background/Objectives: Semen cryopreservation is routinely performed in fertility clinics for a variety of reasons, including fertility preservation and storage of donor sperm, yet the freeze–thaw process leads to cellular damage via ice crystal formation, osmotic shock, and supraphysiological levels of oxidative stress. Sperm resistance to damage during the freeze–thaw process varies widely, yet the intrinsic factors associated with sperm cryotolerance are largely unknown. The study aimed to investigate whether poor chromatin condensation renders sperm vulnerable to DNA fragmentation and cell death induced by the freeze–thaw process. Methods: Participants (n = 51) from the general community who met the inclusion criteria collected a semen sample after 3–8 days of abstinence. Neat semen samples underwent traditional semen analysis, aniline blue (AB)-eosin staining for chromatin condensation, the terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL) assay for DNA fragmentation, and the Annexin V assay for apoptosis/necrosis, prior to being cryopreserved using the liquid nitrogen vapour method and stored at −196 °C. Stored samples were later thawed at room temperature and processed using density gradient centrifugation. Motile sperm concentration, DNA fragmentation and apoptosis/necrosis were analysed in post-thaw samples. Results: As indicated by a significant interaction effect in linear mixed models, an increased proportion of AB-positive sperm in the pre-freeze sample exacerbated the adverse effect of freezing on sperm DNA fragmentation (p = 0.004), late apoptosis (p = 0.007), and necrosis (p = 0.007). AB-staining was positively correlated with all three parameters in the post-thaw sample (all r_s ≥ 0.424, all p < 0.01) and remained significant after adjusting for neat sperm concentration (all partial r_s ≥ 0.493, all p < 0.01). Similarly, AB-staining was significantly correlated with the percentage point change in sperm DNA fragmentation (r_s = 0.366, p = 0.014) and necrosis (r_s = 0.403, p = 0.009), both of which remained significant after adjusting for neat sperm concentration (both partial r_s ≥ 0.404, both p < 0.01), and borderline significantly correlated with percentage point change in late apoptosis (r_s = 0.307, p = 0.051). Conclusions: Sperm with poorly condensed chromatin may be more susceptible to cellular damage during the freeze–thaw process, independent of pre-freeze sperm concentration. These findings may help to explain the intrinsic variation in sperm resistance to cryodamage within and between individuals that is poorly understood.