Comparison of rapid freezing versus vitrification for human sperm cryopreservation using sucrose in closed straw systems

Rapid freezing and vitrification using sucrose are two simple and cost-effective sperm cryopreservation methods. However, it is still unclear which method is better and what the optimal concentration of sucrose is. This study aimed to determine the optimal sucrose concentration for human sperm cryopreservation and compare the cryoprotective effects of rapid freezing versus vitrification using different closed straw systems in terms of sperm motility and DNA integrity. Our data showed that: (1) The optimal sucrose concentration for vitrification was 0.25 mol/l among the tested 0, 0.125, 0.25 and 0.5 mol/l concentrations; (2) Sperm total motility and progressive motility were cryopreserved significantly better by rapid freezing than vitrification in standard 0.5 ml cryostraws ( P < 0.05); and (3) Sperm total motility and progressive motility were cryopreserved significantly better by vitrification in the straw-in-straw system than rapid freezing in the standard 0.5 cryostraw ( P < 0.05), but no difference was found in sperm nuclear DNA fragmentation level between the two cryopreservation methods ( P > 0.05). It was concluded that sucrose at 0.25 mol/l concentration is suitable for human sperm rapid freezing and vitrification, and sperm cryopreservation can be achieved by rapid freezing using closed standard 0.5 ml straws or by vitrification using the novel straw-in-straw system made of standard 0.25 and 0.5 ml straws.