Effect of cooling, freezing and thawing rates and storage conditions on preservation of human spermatozoa.

Human spermatozoa was relatively resistant to cooling shock. However, when diluted semen was cooled faster than 10 degrees C per minute from room temperature (RT) to 5 degrees C and rewarmed to RT, percentage motility and percentage alive of spermatozoa decreased when compared to the slower cooling rates (less than 5 degrees C/min). The optimum cooling rate from RT to 5 degrees C resulting in maximum survival of human spermatozoa was found to be 0.5 to 1 degree per minute when cooled from RT to 5 degrees C and subsequently frozen-thawed in liquid nitrogen (LN2). The optimal freezing rate of 10 degrees C per minute, from 5 degrees to -80 degrees C, resulted in higher survival of human spermatozoa than slower (1.1 degrees C/min) or faster (87.1 degrees C/min) freezing rates. Slow thawing in 20 or 35 degrees 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 degrees C or lower when compared with samples transferred at -15 degrees C or higher. However, maximal spermatozoa survival was obtained when the samples were transferred at -80 degrees C or lower. Transfer of human semen from LN2 to -25 to -30 degrees C and storage for 24 hours significantly reduced spermatozoa viability when compared with storage at 196 degrees C or -80 to -85 degrees C. No significant differences were found between storage temperatures of -80 to -85 degrees C and -196 degrees C in the maintenance of spermatozoa viability for up to 90 days.