Impact of warming of vitrified semen at different temperatures on cryosurvival

INTRODUCTION

Vitrification is a semen cryopreservation technique that preserves male fertility. Optimum devitrification temperature of vitrified semen is important as it may result in intra- and extracellular ice crystallization that damages cell structure.^[1] Also, Sanchez et al. have reported that for proper sperm function and for maintaining sperm membrane integrity, optimum devitrification temperature is vital.^[2]

Mansilla et al. attempted to find out the optimum warming temperature and found that progressive motility was higher in those sperm samples, which were warmed at 42°C (65%) than those samples at 38°C (26%) and 40°C (57%) and also plasma membrane function was best at 42°C.^[3] However, Vutyavanich et al. warmed the vitrified semen sample in 25°C to 28°C tap water and observed improved postwarming motility and cryosurvival than conventional freezing.^[4] Another study showed warming of 21 vitrified samples of human sperm (30 µl pellets) in a water bath at 37°C for 5 minutes resulted in motile and fertile spermatozoa.^[5] There is yet no optimum temperature which is known to give better postwarming results.

Therefore, this study was conducted to compare the postwarming semen parameters at different temperatures, that is, at room temperature (RT), 37°C, and 42°C to know about an optimum devitrification temperature. These three are the conventional temperatures used for devitrification; hence, our study aim was to find out the better one out of these. The temperature at which cryosurvival of spermatozoa was best recorded was the optimum devitrification temperature. The cryosurvival was defined as the viability of spermatozoa after 30 minutes postwarming incubation.

MATERIALS AND METHODS

The fresh semen parameters (count, motility, progressive motility, and kinematics) were analyzed using computer-assisted semen analyzer system after liquefaction of the raw sample.

RESULTS

The mean of sperm motility in raw semen sample, at RT, at 37°C, and at 42°C was 80.55 ± 13.631, 38.76 ± 14.279, 44.64 ± 12.465, and 56.69 ± 11.126, respectively. The sperm motility in postwarm samples was significantly lower at all temperatures compared to the raw sample.

Among various warming temperatures, sperm motility was the best at 42°C, and this difference was more significant when compared with RT and 37°C. Among various warming temperatures, cryosurvival factor was the best at 42°C, and this difference was more significant when compared with RT and 37°C.

The kinematic parameters, that is, curvilinear velocity (VCL), straight-line velocity (VSL), average path velocity (VAP) showed a significant decrease in postwarming at different temperatures. The difference was significant at the RT, 37°C, and 42°C (P < 0.049, P < 0.033, and P < 0.0001, respectively). But kinematic parameters were the best at 37°C [Table 1 and Figure 1].

Table 1 Basal semen parameters, cryosurvival factor, and velocity parameter values after warming at room temperature, 37°C, and 42°C

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Figure 1

Comparison of count (million/ml), motility (%), progressive motility (%), curvilinear velocity (VCL) (μm/s), straight-line velocity (VSL) (μm/s), average path velocity (VAP) (μm/s), and cryosurvival factor (%) between different warming temperatures, that is, room temperature, 37°C, and 42°C.

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DISCUSSION

There was significant decrease in all basal parameters in the semen samples warmed at different temperatures such as RT, at 42°C, and at 37°C compared to the raw semen sample. Stanic et al. reported that the various processes such as osmotic shock, cellular dehydration, and intracellular and extracellular ice crystal formation leading to thermal shock occur during freezing–warming procedure, affecting the various semen parameters.^[6]

Sperm motility has been reported as a predictive factor of fertility.^[7,8] It was found that there was a significant difference (P < 0.0001) of sperm motility when all three warming temperatures were compared to each other. The least sperm motility was found upon thawing the semen sample at RT, and the maximum sperm motility was found after thawing at 42°C. Similar to motility, the progressive motility was also significantly decreased P < 0.0001 in the semen samples that were cryopreserved. The maximum progressive motility was found in the semen samples that were thawed at 42°C.

The adenosine triphosphate (ATP) formed due to oxidative phosphorylation is transferred to the microtubules which is responsible for the sperm motility. The reason behind the decreased motility of sperm is due to the impairment of the mitochondrial activity of the spermatozoa. The mitochondrial bilayered membrane gets damaged due to which the oxidative phosphorylation and ATP generation is impaired leading to the decreased sperm motility.^[9] However, oxidative phosphorylation in the mitochondria does not generate all of the ATP required for spermatozoa motility. Glycolysis is, in fact, another source of ATP that powers sperm movement.^[10]

A faster recovery rate of sperm enzymatic antioxidant activity could be one explanation for the observed increase in motility recovery following warming at 42°C. There are two contradictory processes that indicate the rate of cell damage during warming: (1) the recovery rate of enzymatic antioxidant activity and (2) the magnitude of oxygen radical generation. Because the recovery of enzymatic antioxidant activity is faster at higher temperatures, sperm will be able to neutralize the increase in oxygen radical generation recorded after warming more effectively at 42°C than at RT and 37°C. Greater ATP production in the spermatozoa axoneme could be another cause for increased motility after warming at higher temperatures.^[11] There are a few studies which showed that a high warming rate resulted in an increase in motility recovery.^[12,13] Other research, however, reported no difference in motility recovery when different warming temperatures were used.^[14,15]

There was a significant difference (P < 0.0001) in sperm cryosurvival factor, when all three warming temperatures were compared to each other. The maximum cryosurvival factor of sperm (%) was observed at 42°C. As the cryosurvival factor is directly dependent upon the post-thaw motility. As the motility was maximum at 42°C, the cryosurvival factor was found to be maximum at 42°C.

Hirano et al. reported that the velocity parameters can be the indicators of pregnancy outcome.^[16] VCL, VSL, and VAP are the commonly used velocity values that describe the human sperm centroid movement.^[17] Therefore, the motility parameters such as VCL, VSL, and VAP were compared between all warming temperatures. The significant difference in the VCL, VSL, and VAP, that is, P = 0.049, P = 0.033, and P = 0.0001, respectively, was found in the cryopreserved semen samples. Similar type of results have been reported in various studies.^[18,19] In our study, the kinematic parameters resulted best at 37°C rather than at RT or 42°C.

CONCLUSION

The warming of semen samples at 42°C resulted in a significant increase in cryosurvival factor when compared with 37°C and RT. However, the velocity parameters were best at 37°C rather than 42°C and RT. The cryopreservation of the semen affected the basal sperm parameters such as sperm motility, progressive motility, cryosurvival, and kinematics. These were found to be decreased in the cryopreserved semen samples. It can also be concluded that upon thawing the semen sample at 42°C temperature, there was maximum sperm count, motility, and progressive motility in comparison to the semen samples thawed at RT and 37°C. The recovery of functioning human spermatozoa after cryopreservation may therefore be facilitated by warming it at 42°C for 5 minutes. Although results are significant, but to know if clinically this study will make any difference, the fertilization rates should be compared using a warmed sample at 37°C and 42°C.