Vitrification of Donkey Sperm: Is It Better Using Permeable Cryoprotectants?

Advances in Donkey and Mule Research

Donkeys (Equus asinus) and mules represent approximately 50% of the entire domestic equine population in the world and play an essential role in the lives of thousands of people, primarily in developing countries [...].

Kinetic vitrification: concepts and perspectives in animal sperm cryopreservation

Sperm cryopreservation is an important tool for genetic diversity management programs and the conservation of endangered breeds and species. The most widely used method of sperm conservation is slow freezing, however, during the process, sperm cells suffer from cryoinjury, which reduces their viability and fertility rates. One of the alternatives to slow freezing is vitrification, that consist on rapid freezing, in which viable cells undergo glass-like solidification. This technology requires large concentrations of permeable cryoprotectants (P- CPA's) which increase the viscosity of the medium to prevent intracellular ice formation during cooling and warming, obtaining successful results in vitrification of oocytes and embryos. Unfortunately, this technology failed when applied to vitrification of sperm due to its higher sensitivity to increasing concentrations of P-CPAs. Alternatively, a technique termed 'kinetic sperm vitrification' has been used and consists in a technique of permeant cryoprotectant-free cryopreservation by direct plunging of a sperm suspension into liquid nitrogen. Some of the advantages of kinetic vitrification are the speed of execution and no rate-controlled equipment required. This technique has been used successfully and with better results for motility in human (50-70% motility recovery), dog (42%), fish (82%) and donkey (21.7%). However, more studies are required to improve sperm viability after devitrification, especially when it comes to motility recovery. The objective of this review is to present the principles of kinetic vitrification, the main findings in the literature, and the perspectives for the utilization of this technique as a cryopreservation method.

Donkey semen cryopreservation: Alternatives with permeable, non-permeable cryoprotectants and seminal plasma

Cryopreservation of semen is an important technique to preserve genetic material. Yet, pregnancy rates in jennies after artificial insemination with frozen-thawed donkey semen are poor. This condition has been attributed to the impact of permeable cryoprotectants, that could cause high post-breeding endometritis. Removal of seminal plasma (SP) prior to semen freezing process is another contributing factor. SP is involved in a multitude of sperm functions and events preceding fertilization and has a mediating effect of sperm capacitation and postcoital uterine inflammatory response. The aim of this study was to evaluate different alternatives in donkey semen cryopreservation with permeable, non-permeable cryoprotectants, BSA and SP. Thirty ejaculates from 10 donkeys were cryopreserved with different combinations of dimethylformamide (DMF, 5%), sucrose (SUC, 200 mM) and homologous SP (10%): DMF (T1), DMF/SP (T2), SUC/BSA (T3), SUC/BSA/SP (T4), DMF/SUC/BSA (T5), DMF/SUC/BSA/SP (T6), DMF/BSA (T7) and DMF/BSA/SP (T8). After thawing, sperm motility and kinetics were assessed by computerized semen analysis. Sperm vitality (SV) was evaluated by fluorescence microscopy, functional membrane integrity (FMI) by the HOST test, abnormal morphology by eosin-nigrosin staining and sperm membrane stability by flow cytometry. For statistical analysis, sperm quality indexes (SQi) were obtained, general linear models were carried out and mean comparisons were made by the Tukey test. T1, T2, T5, T6, and T7 had higher and equivalent results for motility, most kinetic parameters and function membrane integrity. Cryopreservation of donkey semen without permeable cryoprotectant (T3 and T4) showed a reduction in motility, kinetics, SV, FMI and SQi. T5 showed a reduction in progressive motility, sperm velocities, IMF and SQi compared to other DMF treatments. T6 and T8 achieved higher SQi values compared to T1, but they were not different compared to T2 and T7. T1 had a smaller sperm population with low-M540 compared to T3. It is concluded that the use of permeable cryoprotectant is essential to achieve higher post-thaw quality of donkey semen. In addition, the combined use of BSA, SUC and/or PS may provide additional sperm protection compared to the individual use of DMF.

Recent advances in donkey sperm vitrification

Artificial insemination (AI) with cryopreserved semen is an important tool to preserve endangered species, including European donkey breeds. Sperm vitrification is an alternative method to conventional freezing using high cooling rates and non-permeable cryoprotectant agents (CPAs). In donkeys, sperm vitrification was firstly developed in spheres by directly dropping the sperm (30 µl) into the liquid nitrogen. The vitrification media contained a combination of sucrose and bovine serum albumin as non-permeable CPAs and resulted in better sperm parameters after warming than extenders containing glycerol. Thereafter, sperm vitrification was optimized using an aseptic protocol, which consists of volumes up to 160 µl vitrified at 300 million sperm/ml using 0.25-ml straws with outer covers, obtaining similar sperm parameters as conventional freezing for total motility (52.7 ± 15.6% versus. 58.2 ± 16.1%), progressive motility (44.3 ± 15.0% versus. 44.7 ± 18.2%) and plasma membrane integrity (49.2 ± 11.2% versus. 55.4 ± 9.0%), respectively. In order to vitrify larger volumes of sperm, a procedure using 0.5-ml straws was evaluated; however, this methodology failed when compared to conventional freezing or other vitrification protocols, obtaining poor sperm quality after warming. Recently, a new methodology was developed for warming 0.25-ml straws in a water bath and after AI using the vitrified sperm, the uterine inflammatory response solved faster, and pregnancy rates were greater (22%) than frozen semen (10%) but not statistically different. In conclusion, all these findings confirm that sperm vitrification can be performed in donkeys as an alternative to conventional freezing for AI in jennies.