Cryopreservation of human sperm in the presence of Zn²⁺ increases the motility rate

Introduction
The aim of the study was to study the effect of Zn²⁺ addition to human sperm prior to and after cryopreservation on post-thaw sperm motility.

Material and methods
Human semen was liquefied, and the semen was loaded on a gradient and centrifuged for 10 min at 3000 g at room temperature. The lower layer containing the sperm was collected, resuspended in Ham’s F-10 medium then centrifuged again, and the sperm were allowed to “swim up” after the last wash at 37°C. The motile cells were collected, resuspended in capacitation medium and supplemented with 0.1% human serum albumin. Samples with or without ZnCl2 were kept at room temperature for 10 min, after which they were mixed with equal volumes of cryoprotective medium and aliquots were transferred to screw-top cryovials and stored in liquid nitrogen for several days. The samples were removed from liquid nitrogen, subjected to rapid thawing and washed in HamF-10 medium. Sperm motility was analyzed using computer-assisted sperm analysis (CASA). Results:

Results
Freezing of human sperm in the presence of Zn²⁺ led to a significant increase in the number of motile sperm as well as preserving motility for a longer period of time. The percentage of sperm presenting progressive motility was also increased by Zn²⁺ depending on the fluid volume used for freezing. The presence of Zn²⁺ in the freezing medium also protects sperm motility after second freezing.

Conclusions
In order to achieve good motility after freezing, human sperm should be frozen in 0.1 ml of medium containing 50 µM Zn²⁺.