Effect of Cooling, Freezing and Thawing Rates and Storage Conditions on Preservation of Human Spermatozoa

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Abstract
Human spermatozoa was relatively resistant to cooling shock. When diluted semen was cooled faster than 10.degree. C/min from room temperature (RT) to 5.degree. C and rewarmed to RT, percentage motility and percentage alive of spermatozoa decreased when compared to the slower cooling rates (< 5.degree. C/min). The optimum cooling rate from RT to 5.degree. C resulting in maximum survival of human spermatozoa was 0.5 to 1.degree. C/min when cooled from RT to 5.degree. C and subsequently frozen-thawed in liquid N (LN2). The optimal freezing rate of 10.degree. C/min from 5.degree. to -80.degree. C, resulted in higher survival of human spermatozoa than slower (1.1.degree. C/min) or faster (87.1.degree. C/min) freezing rates. Slow thawing in 20.degree. or 35.degree. C air, on a dry bench, resulted in better survival than the other slower or faster thawing methods used. The temperature at which human semen samples were transferred to LN2 significantly influenced spermatozoa survival. Survival was higher when transferred at -30.degree. C or lower when compared with samples transferred at -15.degree. C or higher. Maximal spermatozoa survival was obtained when the samples were transferred at -80.degree. C or lower. Transfer of human semen from LN2 to -25 to -30.degree. C and storage for 24 h significantly reduced spermatozoa viability when compared with storage at 196.degree. C or -80 to -85.degree. C. No significant differences were found between storage temperatures of -80 to -85.degree. C and -196.degree. C in the maintenance of spermatozoa viability for up to 90 days.