Background The rate at which lethal intracellular ice formation occurs during cryopreservation is highly dependent on several variables. The objective of this study was to determine the optimal rate at which rhesus sperm can be cooled.
Methods Experiments were performed using three rates of cooling. Sperm motility was evaluated by computer-assisted semen analysis, and post-thaw viability was determined using propidium iodide labeling and flow cytometry. Semen was frozen at three cooling rates: (i) fast, (ii) slow, and (iii) standard. Straws were thawed for 30 s at 37°C for analysis of motility and viability.
Results Post-thaw motility and viability were comparable between freezing curves. Sperm cryopreserved using the slow freeze curve exhibited lowest motility and viability.
Conclusions This study indicates that macaque sperm survive cooling optimally when cooling rates range from −17 to −34°C/minute. Conversely, slow cooling was detrimental and resulted in poor quality sperm.