Field fertility of frozen boar semen: A retrospective report comprising over 2600 AI services spanning a four year period

Boar semen cryopreservation: State of the art, and international trade vision

Biosecurity is a major concern in the global pig production. The separation in time of semen collection, processing and insemination in the pig farm is a few days for chilled semen but it can be indefinite when using cryopreserved semen. Field fertility results of boar cryopreserved semen are close to chilled semen, which makes it a valuable resource for the establishment of semen genebanks, long-distance semen trade, and the implementation of other technologies such as the sex-sorted semen. But cryopreserved semen is far from being routine in pig farms. The most recent research efforts to facilitate its implementation include the use of additives before freezing, or in the thawing extender. Long-term preserved semen trade is a biosecurity challenge. To harmonize international trade of germplasm, the World Organization of Animal Health (WOAH) established a regulatory framework for all member countries. The present paper aims to review the latest advances of boar semen cryopreservation with special focus on the benefits of its inclusion as a routine tool in the pig industry. We also review recently reported field fertility results of cryopreserved semen, its international trade compared to chilled semen, and the regulatory framework involved. Boar cryopreserved semen is a valuable tool to control biosecurity risk, implement other technologies, and facilitate international trade. Research already demonstrated good field fertility results, but it still represents less than 0.1 % of the international trade. As boar cryopreserved semen gets closer to implementation, the correspondent authorities are reviewing the trade rules.

Effects of Nobiletin supplementation on the freezing diluent on porcine sperm cryo-survival and subsequent in vitro embryo development

Nobiletin (NOB) is a bioflavonoid compound isolated from citrus fruit peels. The present study aimed to elucidate whether NOB facilitates the porcine sperm cryosurvival and embryo development after in vitro fertilization (IVF). To this end, spermatozoa were diluted and cryopreserved in a freezing extender supplemented with 0 (control), 50, 100, 150, and 200 μM Nobiletin. The kinematic patterns of frozen-thawed (FT) sperm were assessed after 30 and 90 min incubation using a Sperm Class Analyzer (SCA). Viability, acrosome integrity, and mitochondrial membrane potential (MMP) were measured by fluorescence microscopy 30 min after thawing using SYBR-14/PI, PSA/FITC, and R123/PI, respectively. Lipid peroxidation was determined using MDA assay after incubation for 90 min. The addition of 100 μM and 150 μM NOB to the extender significantly improved sperm progressive motility, and acrosome integrity compared to the control group (P < 0.05). The proportion of viable spermatozoa was significantly higher in the 150 μM NOB group. MDA levels were less in 50 μM and 150 μM NOB treated groups compared to the control. In addition, IVF with FT sperm was used to assess the embryo developmental competence. Treatment with 150 μM NOB before cryopreservation increased the cleavage and blastocyst formation rates compared to the control group. Furthermore, the relative expression of POU5F1 and AMPK, genes related to pluripotency and cell differentiation were significantly upregulated in embryos resulting from NOB-treated sperm compared to the control group. These results suggest that Nobiletin is a functionally novel phytochemical to mitigate oxidative stress during the freezing-thawing of porcine spermatozoa as reflected by improved FT sperm quality and IVF outcome.

Vaginal and vestibular electrical resistance as an alternative marker for optimum timing of artificial insemination with liquid-stored and frozen-thawed spermatozoa in sows

Development of a pen-side test to objectively determine the ideal time for artificial insemination (AI) in the sow would save producers time and money. Current processes rely on identification of oestrus via subjective behavioural and physiological markers that are indicative of high blood oestrogen concentrations. This study attempted to use measurements of electrical resistance (ER) in the cervical mucus to pinpoint timing of AI accurately enough to lead to equivalent pregnancy rates as a natural mating. Thirty-six sows were divided into 3 groups and observed for signs of oestrus. Seven sows did not show any oestrus behaviour and were excluded from the study. The remaining 29 sows were inseminated via natural mating and conventional oestrus detection (n = 14), or inseminated artificially with either liquid-stored semen (n = 8) or frozen-thawed semen (n = 7) according to timing indicated from electrical resistance measurements in the vagina and vestibule. Sows that were artificially inseminated on the basis of the electrical resistance readings had a lower pregnancy rate (P = 0.034) and less piglets born alive per litter (P < 0.05) than those that were naturally mated according to a conventional oestrus detection regime. However, the pregnancy rate and total piglets born alive did not differ between the two groups that underwent artificial insemination. Change in electrical resistance in the vagina has the potential to accurately predict ovulation timing, but more work is required to refine the timing of AI in relation to the readings before the technique can be adopted by industry.

Carboxylated ε-Poly-L-Lysine Supplementation of the Freezing Extender Improves the Post-Thawing Boar Sperm Quality

Simple Summary

Frozen boar sperm is used on a very limited scale in pig artificial insemination owing to the low quality of post-thaw sperm. Cryoprotectant is usually used in boar sperm freezing extender, which is important for improving the post-thaw sperm quality. The carboxylated ε-poly-L-lysine, an efficient and non-toxic cryoprotectant, has been used as a food and cell preservative, as well as for tissue engineering and drug delivery in the biomedical applications. However, whether addition of carboxylated ε-poly-L-lysine to the freezing medium improves the post-thaw boar sperm quality or not is unknown. In this study, the addition of 0.25% carboxylated ε-poly-L-lysine to the freezing medium significantly improved the post-thaw boar sperm quality by protecting sperm mitochondrial function and antioxidant defense system.

Abstract

Frozen boar sperm is used on a minimal scale in consequence of the cryo-injuries induced by biochemical and physical modifications during the freezing and thawing processes. The present study investigates whether the addition of carboxylated ε-poly-L-lysine (CPLL) to the freezing medium could improve post-thaw boar sperm quality or not. Boar sperm was diluted with freezing medium contained different doses of carboxylated ε-poly-L-lysine (0, 0.125%, 0.25%, 0.5%, and 1%; v/v). The motility patterns, membrane integrity, acrosome integrity, mitochondrial membrane potential, NADH-CoQ activity, ATP level, malondialdehyde (MDA) level, and antioxidant defense system, as well as apoptosis in post-thaw boar sperm, were measured. It was observed that 0.25% CPLL treatment significantly improved the post-thaw boar sperm total motility, progressive motility, straight-linear velocity (VSL), curvilinear velocity (VCL), average path velocity (VAP), linearity (LIN), straightness (STR), membrane integrity, and acrosome integrity. Interestingly, the addition of CPLL also significantly increased the post-thaw sperm mitochondrial membrane potential, NADH-CoQ activity, and ATP level. Moreover, post-thaw boar sperm catalase (CAT) activity, glutathione peroxidase (GPx) activity, and superoxide dismutase (SOD) activity were increased with the addition of CPLL from 0.125% to 0.5% concentration levels. Furthermore, reduction of post-thaw sperm MDA level and apoptosis in 0.25% CPLL treatment was also observed. Those observations suggested that the addition of 0.25% CPLL to the freezing medium increased post-thaw boar sperm quality by protecting sperm mitochondrial function and antioxidant defense system. These findings provided novel insights that CPLL can be used as an efficient cryoprotectant to improve the post-thaw boar sperm quality during cryopreservation.

Reducing the Glucose Level in Pre-treatment Solution Improves Post-thaw Boar Sperm Quality

Frozen–thawed boar sperm was not widely used in pig artificial insemination as the sperm quality was damaged by biochemical and physical modifications during the cryopreservation process. The aim of this study was to investigate whether reduction of the glucose level in diluted medium could protect the post-thaw boar sperm or not. Boar sperm was diluted with the pre-treatment medium with different doses of glucose (153, 122.4, 91.8, 61.2, 30.6, and 0 mM) during the cooling process. The sperm motility patterns and glycolysis were evaluated during the cooling process. Meanwhile, the post-thaw sperm quality, ATP level, mitochondrial function as well as apoptosis were also measured. It was observed that 153 mM glucose treatment showed the highest glycolysis in boar sperm as the activities of hexokinase, fructose-bisphosphate aldolase A, and lactate dehydrogenase are the highest as well as the lactate level. Reduction of the glucose level from 153 to 30.6 mM suppressed sperm glycolysis. In addition, treatment with 153 mM glucose made the sperm demonstrate a circle-like movement along with a high value of curvilinear velocity and amplitude of the lateral head, while decreasing the glucose level reduced those patterns in the cooling process. Moreover, reduction of the glucose level also significantly increased the post-thaw sperm's total motility, progressive motility, straight-linear velocity, membrane integrity, and acrosome integrity. The treatment with 30.6 mM glucose showed the highest value among the treatments. Furthermore, the post-thaw sperm's succinate dehydrogenase activity, malate dehydrogenase activity, mitochondrial membrane potential as well as ATP level were increased by reducing the glucose level from 153 to 30.6 mM. Interestingly, the treatment with 30.6 mM glucose showed the lowest apoptosis of post-thaw sperm among the treatments. Those observations suggest that reduction of the glucose level in diluted medium increased the post-thaw boar sperm quality via decreasing the glycolytic metabolism. These findings provide novel insights that reduction of boar sperm activity via decreasing sperm glycolysis during the cooling process helps to improve the post-thaw sperm quality during cryopreservation.

Clinical Application of Bioextracts in Supporting the Reproductive System of Animals and Humans: Potential and Limitations

There is an increasing demand of spices and herbs in developing countries due to the beneficial effects of plants and herbal preparations as medicines. The basic technological process of obtaining extracts from natural raw materials is extraction, consisting in etching with solvents. Plant extracts are extremely complex, multicomponent mixtures obtained from flowers, fruits, leaves, stems, twigs, or seeds of various plant materials. They are a rich source of polyphenols, flavonoids, phytosterols, carotenoids, and vitamins. The search for alternative methods of treatment is increasingly replacing the scientists' excessive focus on the healing properties of bioextracts. Recent research offers great hope for the development of alternative methods to improve the reproductive system. The use of animal models in experimental research has increased knowledge regarding the beneficial effects of bioextracts on both male and female reproductive systems and reproductive cells. Demonstrating the positive effect of plant extracts creates new opportunities for the use of biowaste, which is a by-product in various production sectors. The aim of this review is to present the functional properties of extracts of natural origin, a cross section of modern methods of their preparation, and a discussion of the possibilities of their use in the auxiliary reproductive system.

Role of Antioxidants in Cooled Liquid Storage of Mammal Spermatozoa

Cooled preservation of semen is usually associated with artificial insemination and genetic improvement programs in livestock species. Several studies have reported an increase in reactive oxidative species and a decrease in antioxidant substances and sperm quality parameters during long-term semen storage at refrigerated temperatures. The supplementation of antioxidants in extenders before refrigeration could reduce this detrimental effect. Various antioxidants have been tested, both enzymatic, such as superoxide dismutase and catalase, and non-enzymatic, such as reduced glutathione, vitamins E and C and melatonin. However, the problem of oxidative stress in semen storage has not been fully resolved. The effects of antioxidants for semen-cooled storage have not been reviewed in depth. Therefore, the objective of the present study was to review the efficiency of the supplementation of antioxidants in the extender during cooled storage of semen in livestock species.

Oxidative and nitrosative stress in frozen-thawed pig spermatozoa. I: Protective effect of melatonin and butylhydroxytoluene on sperm function

The addition of antioxidants to the cryopreservation medium has been shown to exert a positive effect on the quality of frozen-thawed sperm in different species. The objective of the present study was to evaluate the effects of supplementing the freezing medium with butylhydroxytoluene (BHT) and melatonin (MEL) in frozen-thawed pig spermatozoa. With this purpose, six ejaculates coming from six separate boars were cryopreserved in traditional freezing medium (i.e. lactose/egg-yolk/glycerol; Control) supplemented with 1.0 mM BHT (BHT-1), 2.0 mM BHT (BHT-2), 0.01 μM MEL (MEL-1) and 1.0 μM MEL (MEL-2). We evaluated sperm viability, membrane lipid disorder, acrosome integrity, mitochondrial membrane potential, lipid peroxidation, oxidation of thiol groups, and levels of total reactive oxygen species (ROS), peroxynitrite and superoxide anion (·O₂^-). We also analysed total (TM) and progressive sperm motilities (PM), and kinetic parameters at post-thaw (T0, T30 and T60). The BHT-2 and MEL-2 groups presented higher viability and acrosome integrity, and lower levels of peroxynitrite, ·O₂^- and lipid peroxidation than the control (P < 0.05), whereas MEL-2 diminished the levels of total ROS (P < 0.05). TM and PM were not affected by the treatment, while, LIN and STR shows differences between experimental groups. In conclusion, the addition of BHT and MEL to cryopreservation medium diminishes oxidative and nitrosative stress markers, which has repercussions for the integrity of plasma and acrosomal membranes of frozen-thawed spermatozoa.

Oxidative and nitrosative stress in frozen-thawed pig spermatozoa. II: Effect of the addition of saccharides to freezing medium on sperm function

Saccharides have bioprotective properties, with a high capacity to preserve biological proteins and membranes during sperm cryopreservation. The aim of this study was to evaluate how replacing the lactose of cryopreservation media by sucrose (SUC) or trehalose (TRE) at concentrations of 0.2 M (SUC-1 and TRE-1) and 0.25 M (SUC-2 and TRE-2) affects frozen/thawed pig spermatozoa. The media used were composed of medium A (saccharide/egg yolk) and B (saccharide/egg yolk/glycerol), their osmolality being determined prior to freezing. Cell viability, membrane lipid disorder, acrosome integrity, mitochondrial membrane potential (MMP), lipid peroxidation, thiol group oxidation, total reactive oxygen species (ROS), peroxynitrite and superoxide anion (O₂^●-) were determined through flow cytometry; total motility (TM), progressive motility (PM) and kinetic parameters motility were determined immediately after thawing (T0) and again 30 (T30) and 60 (T60) minutes later. The SUC-2 and TRE-2 groups maintained viability significantly and presented fewer lipid membrane disorders, respectively, both with a significant increase in MMP. The production of O₂^●- and peroxynitrite was lower in the TRE-2 groups compared to the control (P < 0.05). Total motility at T0 was greater in the TRE-2 group (P < 0.05). Sperm kinetics was not affected by the treatment. The use of saccharides SUC and TRE at a concentration of 0.25 M improves sperm quality, so that both non-penetrating cryoprotectants can be utilized in pig sperm freezing media.

Improvement of the motility of boar sperm after cryopreservation

After the sperm cryopreservation process, there frequently is a marked decrease in sperm motility, however, this loss of motility could be avoided or reduced by modifying the thawing extender. The aim of conducting this study was to evaluate treatments resulting in differences in pH of the thawing extender on post-thawing sperm quality, especially motility. The associated effects were evaluated of the thawing extender pH when there were pH values of 7, 8 and 9 on values for sperm viability, acrosome integrity, motility and kinematic variables. To evaluate pH effects of the thawing extender on sperm motility, the procedures for evaluating the sperm variables were conducted with or without inclusion of caffeine in the extender. In addition, effects of thawing extender pH on the sperm subpopulations were analyzed. The results indicated that by increasing the pH of thawing extender to 9, sperm viability and acrosome status were not affected, and motility was markedly greater with there being similar values when there was a pH of greater alkalinity and pH 7 when caffeine was included for conducting the evaluations. Furthermore, for values of kinematic variables and sperm subpopulations, when there was a pH of 8 of thawing extender, sperm motility was maintained to a greater extent as storage duration advanced and the proportions of different sperm populations were also maintained during storage. In conclusion, when there was use of the thawing extender at pH 8, there was enhanced post-thawing sperm motility and preservation of motility as storage time advanced.

Rutin protects boar sperm from cryodamage via enhancing the antioxidative defense

This study was aimed to investigate whether and how Rutin protects boar sperm against cryoinjury during cryopreservation. Five concentrations of Rutin with 0.2, 0.4, 0.6, 1.0, and 2.0 mM were added to the freezing extender of boar sperm, respectively, and the effects on quality and function of boar sperm after freezing-thawing were assessed. The results showed that the sperm motility, mitochondrial activity, plasma membrane integrity, and acrosomal integrity were significantly improved in 0.4 mM and 0.6 mM Rutin groups (p < .05). Compared with ganoderma lucidum polysaccharide (GLP) or Tanshinone IIA, Rutin exhibited higher rates of mitochondrial activity and acrosome integrity (p < .05). Mechanistically, the addition of Rutin at the concentration of 0.6, 0.8, and 1.0 mM significantly attenuated ROS accumulation and MDA production by improving antioxidant enzymatic activity, including SOD, CAT, and GSH-Px (p < .05). Functionally, a higher penetration rate and the increased total efficiency of fertilization were observed in the 0.4, 0.6, and 1.0 mM Rutin groups than in the control group (p < .05). Moreover, the addition of Rutin (0.6 mM) significantly induced an increase in both the cleavage and blastocyst rates (p < .05). In summary, supplementation with Rutin in cryopreservation medium protects boar sperm against ROS attack by enhancing the antioxidative defense.

Effects of different equilibration times at 5 °C on boar sperm cryotolerance

Equilibration time (ET) is the period during which sperm cells are in contact with cooling/freezing media components at a temperature of 5 °C, providing a proper osmotic balance between the intra- and extra-cellular milieu. The present study aimed to determine the ET (0, 2, and 4 h) that results in greater post-thaw sperm quality and functions. Based on the post-thaw sperm membrane integrity and motility ratios, 20 ejaculates collected from five boars were classified as having good (GFE, n = 5) or poor (PFE, n = 15) freezing capacity. Ratios of post-thaw sperm with intact plasma membrane and acrosome were similar between ET (0 h: 37.02 % ± 2.85 %; 2 h: 34.59 % ± 7.12 %; 4 h: 37.87 % ± 4.44 %) in GFE samples. In PFE, ratios of sperm with intact plasma membrane and acrosome at post-thaw were greater (P < 0.05) after an ET of 2 h than after an ET of 0 h (2 h: 26.16 % ± 1.54 % and 0 h: 16.74 % ± 1.59 %). Also, ratios of post-thaw sperm with relatively lesser membrane lipids disorder were greater (P < 0.05) after an ET of 2 h than for other ET in both GFE (2 h: 21.97 % ± 4.24 % and 0 h: 16.63 % ± 2.38 %) and PFE (2 h: 16.65 % ± 1.40 % and 0 h: 13.23 % ± 1.25 %) samples. In conclusion, an ET of 2 h results in greater sperm cryotolerance in both GFE and PFE samples, which suggests that modifying the freezing protocol lead to an increase post-thaw sperm function and survival.

Improvement of in vitro fertilization by a tannin rich vegetal extract addition to frozen thawed boar sperm

Boar spermatozoa are very susceptible to cryopreservation injuries and, for this reason, pig remains one of the few species in which fresh semen is still preferred to thawed one for routine artificial insemination (AI). The present work evaluated the effect of supplementing boar sperm thawing medium with Silvafeed SP (SSP), a mixture of Chestnut and Quebracho wood extracts (60/40 w/w) rich in polyphenols (92.4% tannin content) on in vitro fertilization (IVF) and on the following sperm parameters: sperm motility (assessed by CASA), viability, acrosome integrity, mitochondrial function and lipid peroxidation (assessed by flow cytometry) and capacitation status (immunolocalization of tyrosine phosphorylated proteins). Thawed spermatozoa were incubated 1 h at 37°C in BTS without (CTR) or with (5, 10, 20 µg/mL) SSP. After incubation sperm suspension was divided in three aliquots: one was used for IVF trials, one for sperm analysis, and the last one was capacitated for 1 h at 39°C 5% CO2 in IVF medium. Sperm motility parameters, viability, acrosome integrity, mitochondrial functionality, lipid peroxidation and tyrosine phosphorylated protein immunolocalization, used as capacitation parameter, were not influenced by SSP. However, oocytes inseminated with thawed spermatozoa pretreated with all the different SSP concentrations presented a significant (P < 0.01) increase in penetration rate compared to CTR. In addition, 5 µg/mL SSP exerted a positive effect (P<0.05) on the total efficiency of fertilization. These results encourage the use of SSP in the thawing medium since post-thawing fertility is a limit for the large-scale use of boar frozen semen.

Gene Polymorphisms in Boar Spermatozoa and Their Associations with Post-Thaw Semen Quality

Genetic markers have been used to assess the freezability of semen. With the advancement in molecular genetic techniques, it is possible to assess the relationships between sperm functions and gene polymorphisms. In this study, variant calling analysis of RNA-Seq datasets was used to identify single nucleotide polymorphisms (SNPs) in boar spermatozoa and to explore the associations between SNPs and post-thaw semen quality. Assessment of post-thaw sperm quality characteristics showed that 21 boars were considered as having good semen freezability (GSF), while 19 boars were classified as having poor semen freezability (PSF). Variant calling demonstrated that most of the polymorphisms (67%) detected in boar spermatozoa were at the 3’-untranslated regions (3’-UTRs). Analysis of SNP abundance in various functional gene categories showed that gene ontology (GO) terms were related to response to stress, motility, metabolism, reproduction, and embryo development. Genomic DNA was isolated from sperm samples of 40 boars. Forty SNPs were selected and genotyped, and several SNPs were significantly associated with motility and membrane integrity of frozen-thawed (FT) spermatozoa. Polymorphism in SCLT1 gene was associated with significantly higher motility and plasma membrane integrity of FT spermatozoa from boars of the GSF group compared with those of the PSF group. Likewise, polymorphisms in MAP3K20, MS4A2, and ROBO1 genes were significantly associated with reduced cryo-induced lipid peroxidation and DNA damage of FT spermatozoa from boars of the GSF group. Candidate genes with significant SNP associations, including APPL1, PLBD1, FBXO16, EML5, RAB3C, OXSR1,PRICKLE1, and MAP3K20 genes, represent potential markers for post-thaw semen quality, and they might be relevant for future improvement in the selection procedure of boars for cryopreservation. The findings of this study provide evidence indicating that polymorphisms in genes expressed in spermatozoa could be considered as factors associated with post-thaw semen quality.

Reproductive technologies in swine

This chapter highlights the importance of reproductive technologies that are applied to porcine breeds. Nowadays the porcine industry, part of a high technological and specialized sector, offers high-quality protein food. The development of the swine industry is founded in the development of breeding/genetics, nutrition, animal husbandry, and animal health. The implementation of reproductive technologies in swine has conducted to levels of productivity never reached before. In addition, the pig is becoming an important species for biomedicine. The generation of pig models for human disease, xenotransplantation, or production of therapeutic proteins for human medicine has in fact generated a growing field of interest.

C60 Fullerenes Suppress Reactive Oxygen Species Toxicity Damage in Boar Sperm

We report the carboxylated C₆₀ improved the survival and quality of boar sperm during liquid storage at 4 °C and thus propose the use of carboxylated C₆₀ as a novel antioxidant semen extender supplement. Our results demonstrated that the sperm treated with 2 μg mL^-1 carboxylated C₆₀ had higher motility than the control group (58.6% and 35.4%, respectively; P ˂ 0.05). Moreover, after incubation with carboxylated C₆₀ for 10 days, acrosome integrity and mitochondrial activity of sperm increased by 18.1% and 34%, respectively, compared with that in the control group. Similarly, the antioxidation abilities and adenosine triphosphate levels in boar sperm treated with carboxylated C₆₀ significantly increased (P ˂ 0.05) compared with those in the control group. The presence of carboxylated C₆₀ in semen extender increases sperm motility probably by suppressing reactive oxygen species (ROS) toxicity damage. Interestingly, carboxylated C₆₀ could protect boar sperm from oxidative stress and energy deficiency by inhibiting the ROS-induced protein dephosphorylation via the cAMP-PKA signaling pathway. In addition, the safety of carboxylated C₆₀ as an alternative antioxidant was also comprehensively evaluated by assessing the mean litter size and number of live offspring in the carboxylated C₆₀ treatment group. Our findings confirm carboxylated C₆₀ as a novel antioxidant agent and suggest its use as a semen extender supplement for assisted reproductive technology in domestic animals.

Relative content of Niemann-Pick C2 protein (NPC2) in seminal plasma, but not that of spermadhesin AQN-1, is related to boar sperm cryotolerance

Variation between and within boar ejaculates in terms of their ability to withstand freeze-thawing is a limitation for sperm cryopreservation. Consequently, searching for freezability markers not only in sperm but also in seminal plasma (SP) is imperative. The present study aimed to evaluate the relationship between cholesterol content, relative levels of NPC2 and AQN-1 at two different holding times (0 h: HT0 and 24 h: HT24) at 17 °C, and boar sperm freezability. Forty-five ejaculates were cryopreserved and subsequently classified as of good (GFE) or poor (PFE) freezability according to their post-thaw sperm viability and total motility. Prior to cryopreservation, relative abundances of two SP proteins (NPC2 and AQN-1) and cholesterol content in sperm and SP were determined through immunoblotting and colorimetric methods, respectively. These determinations were made after ejaculation (HT0) and after 24 h of storage at 17 °C (HT24). Two bands for NPC2 protein (16 kDa and 19 kDa) were identified. Relative amounts of the 16 kDa-band were significantly (P < 0.05) higher in poor (PFE) than in good (GFE) freezability ejaculates both at HT0 and HT24, whereas those of the 19 kDa-band were significantly (P < 0.05) higher in PFE than in GFE at HT24 only. In the case of AQN-1, no significant differences between GFE and PFE were observed. In addition, no variations in the cholesterol content of sperm and SP were observed either between HT0 and HT24 or between GFE and PFE. We can conclude that the content of two NPC2 isoforms in SP, but not of that of spermadhesin AQN-1, may be involved in the sperm resilience to withstand freeze-thawing procedures and may predict ejaculate freezability. While a possible mechanism through which NPC2 during HT could affect boar sperm cryotolerance is suggested to be related to its ability to bind the plasma membrane cholesterol, further research is warranted.

Effect of Astragalus polysaccharide addition to thawed boar sperm on in vitro fertilization and embryo development

It is important to utilize an antioxidant to ameliorate oxidative damage during boar sperm cryopreservation and thawing. Some studies have shown that Astragalus polysaccharide (APS) has antioxidant capabilities in sperm storage at low temperatures. However, the effects of APS on thawed sperm are unclear. In this study, the effect of supplementation of thawing boar semen extender with APS (0.5, 1, 5, 10 mg/mL) on sperm quality parameters (viability, motility, acrosome integrity and mitochondrial activity) was evaluated. Next, we investigated the effect of APS (0.5 mg/mL) supplementation on antioxidant parameters. Semen from two straws was thawed and diluted with three volumes of Beltsville Thawing Solution (BTS) and immediately divided into a control group without addition of antioxidants (CTR) and the APS group. Sperm and antioxidant parameters were evaluated in the CTR and APS groups after 1 h of incubation at 37 °C. Finally, we studied the effect of APS (0.5 mg/mL) supplementation on in vitro fertilization (IVF) and embryo development. The addition of different doses of APS to thawed sperm did not induce any significant effects on the sperm viability or motility compared to the sperm without APS treatment. However, the addition of 0.5 mg/mL APS to thawed sperm showed improved mitochondrial activity, higher penetration rate and increased total IVF efficiency compared with those of the control group. Moreover, our results indicate that the supplementation of APS in thawed sperm decreased the concentration of reactive oxygen species (ROS) and improved the activity of superoxide dismutase (SOD) and catalase (CAT). Finally, the addition of APS significantly increased the cleavage rate and blastocyst rate compared to those of the control group. In conclusion, the addition of APS to thawed boar sperm can enhance the antioxidant ability of sperm and improve in vitro fertilization (IVF) parameters and the outcomes of embryonic development. These results imply that APS has practical potential to enhance boar sperm reproductive performance.

A mechanism by which Astragalus polysaccharide protects against ROS toxicity through inhibiting the protein dephosphorylation of boar sperm preserved at 4 °C

Numerous studies have shown that Astragalus polysaccharide (APS) has strong antioxidant effects and high practical value for preserving semen at low temperatures in vitro. However, to date, little attention has been paid to the precise mechanism of APS in sperm preservation at 4 °C. Thus, to gain further insight into the protective effects of APS, the present study was performed to assess the changes in sperm quality parameters, antioxidant capacity, ATP content, and protein phosphorylation levels. Here, we demonstrated that supplementation with APS could effectively preserve boar sperm quality parameters such as sperm motility, acrosome integrity, and mitochondrial membrane potential. Moreover, we found that the positive effects of APS on boar sperm quality were mainly due to the elimination of excessive mitochondrial ROS, the improvement of antioxidant capacities and the enhancement of ATP levels. Interestingly, by conducting a series of studies on protein phosphorylation, we also discovered that APS could protect boar sperm from oxidative stress and energy deficiency through inhibiting the protein dephosphorylation caused by ROS via the cAMP-PKA signaling pathway. To our knowledge, this is the first exploration of the molecular mechanism underlying the protective roles of APS toward ROS toxicity from the perspective of energy metabolism and protein modification. This study comprehensively provides novel insights into the action mechanism of the protective effects of antioxidants on sperm stored at 4 °C and reveals the practical feasibility of using APS as a boar semen extender supplement for assisted reproductive technology.

Artificial insemination with frozen-thawed boar sperm

Artificial insemination with frozen-thawed semen in pigs is not a routine technique; its use is restricted to specific cases, such as preservation of valuable genetic material (germplasm banks), safety strategies in case of natural disasters, long-distance transport of sperm, and in combination with sex-sorting. Cryoinjuries resulting from freeze-thawing protocols are a major concern with regard to the fertilization capacity of the treated sperm, which is lower than that of liquid-stored semen. Here, we provide an overview of artificial insemination using cryopreserved sperm, and summarize the factors that influence cryopreservation success before, during, and after freeze-thaw (i.e., sperm selection before starting the cryopreservation process, holding time, use of cryoprotectants, and rates of freezing and thawing) and that are driving the identification of biomarkers to predict sensitivity to cryodamage. Three different artificial insemination techniques (conventional or intracervical; intrauterine; and deep intrauterine) are also discussed with regards to their relevance when using frozen-thawed semen. Finally, we review the use of additives to freezing and thawing media, given reports that they may maintain and improve the quality and fertilizing capacity of frozen-thawed sperm. In sum, artificial insemination with frozen-thawed boar sperm can provide reasonable fertility outcomes, if freezable ejaculates, specific additives, and appropriate insemination techniques are used.

The Fertility of Frozen Boar Sperm When used for Artificial Insemination

One of the limits to practical use of frozen boar sperm involves the lowered fertility when used for artificial insemination. Years of studies have shown that 5-6 billion sperm (approximately 3 billion viable) used in single or multiple inseminations results in pregnancy rates most often between 60 and 70% and with litter sizes between nine and 10 pigs. Yet today, it is not uncommon for studies to report pregnancy rates from 70 to 85% and litter sizes with 11-12 pigs. While global statements about the incidence and reasons for higher fertility are not conclusive, incremental fertility improvements appear independently associated with use of a minimum number of viable sperm (1-2 billion), insemination timing that increases the probability that sperm will be present close to ovulation for groups of females, selection for boar sperm survival following cryopreservation, and modification of the freeze and thaw conditions using additives to protect sperm from oxidative damage. Studies show that techniques such as intrauterine and deep uterine insemination can provide an opportunity to reduce sperm numbers and that control of time of ovulation in groups of females can reduce the need for multiple inseminations and improve the chance for AI close to ovulation. However, optimal and consistent fertility with cryopreserved boar sperm may require a multifaceted approach that includes boar selection and screening, strategic use of additives during the freezing and thawing process, post-thaw evaluation of sperm and adjustments in sperm numbers for AI, assessment of female fertility and ovulation induction for single insemination. These sequenced procedures should be developed and incorporated into a quality control system for improved fertility when using minimal numbers of cryopreserved boar sperm.

Recent Advances in Boar Sperm Cryopreservation: State of the Art and Current Perspectives

While sperm cryopreservation is the best technology to store boar semen for long-term periods, only 1% of all artificial inseminations (AI) conducted worldwide are made using frozen-thawed boar sperm. With the emergence of long-term extenders for liquid storage, the use of cryopreserved sperm in routine AI is less required. However, banks of boar semen contain cryopreserved sperm and planning inseminations in AI centres may benefit from the use of frozen-thawed semen. Therefore, there is an interest in the use of this technology to preserve boar sperm. In this regard, although the first attempts to cryopreserve boar semen date back to the seventies and this technology is still considered as optimal, some relevant improvements have been made in the last decade. After giving a general picture about boar sperm cryodamage, the present review seeks to shed light on these recent cryopreservation advances. These contributions regard to protein markers for predicting ejaculate freezability, sperm selection prior to start cryopreservation procedures, additives to freezing and thawing extenders, relevance of the AI-technique and insemination-to-ovulation interval. In conclusion, most of these progresses have allowed counteracting better boar sperm cryodamage and are thus considered as forward steps for this storage method. It is also worth noting that, despite being lower than fresh/extended semen, reproductive performance outcomes following AI with frozen-thawed boar sperm are currently acceptable.

Stress preconditioning of semen before cryopreservation improves fertility and increases the number of offspring born: a prospective randomised study using a porcine model

The aim of the present study was to investigate the effect of applying sublethal stress treatment at room temperature, before cryopreservation (hydrostatic pressure (HP): 40 MPa, 80 min) of 34 boar ejaculate samples, on post-thawed motility and sow fertility. Sows (n = 102) were randomly allocated into equal groups inseminated with HP-treated or untreated frozen–thawed semen. Sows were inseminated twice, 10 h apart, with 6 × 109 spermatozoa per dose without oestrus synchronisation. Rates of non-return of oestrus and pregnancy, and total numbers of piglets and live piglets were significantly higher (P < 0.05) in the HP-treated group. There was also a numerical, albeit non-significant (P > 0.05), improvement in the farrowing rate in the HP-treated group. Although the number of live piglets per litter decreased approximately 15% in both groups by 42 days after farrowing, but this remained significantly higher in the HP-treated group. Although total and progressive sperm motility were significantly (P < 0.001) higher in the HP-treated group, there were no significant differences (P > 0.05) in these parameters between pregnant and non-pregnant sows in either group; thus motility can indicate, but not predict, improved fertility. In conclusion, HP treatment, with sperm cryopreservation, increases in vitro sperm motility and improves reproductive performance without adversely affecting the health of the piglets.

Will AI in pigs become more efficient?

AI is commercially applied worldwide to breed pigs, yielding fertility outcomes similar to those of natural mating. However, it is not fully efficient, as only liquid-stored semen is used, with a single boar inseminating about 2000 sows yearly. The use of liquid semen, moreover, constrains international trade and slows genetic improvement. Research efforts, reviewed hereby, are underway to reverse this inefficient scenario. Special attention is paid to studies intended to decrease the number of sperm used per pregnant sow, facilitating the practical use of sexed frozen-thawed semen in swine commercial insemination programs.

Sperm cryopreservation update: Cryodamage, markers, and factors affecting the sperm freezability in pigs

Cryopreservation is the most efficient method for long-term preservation of mammalian sperm. However, freeze-thawing procedures may strongly impair the sperm function and survival and thus decrease the reproductive performance. In addition, the sperm resilience to withstand cryopreservation, also known as freezability, presents a high individual variability. The present work summarizes the principles of cryoinjury and the relevance of permeating and nonpermeating cryoprotective agents. Descriptions about sperm cryodamage are mainly focused on boar sperm, but reference to other mammalian species is also made when relevant. Main cryoinjuries not only regard to sperm motility and membrane integrity, but also to the degradation effect exerted by freeze-thawing on other important components for sperm fertilizing ability, such as mRNAs. After delving into the main differences between good and poor freezability boar ejaculates, those protein markers predicting the sperm ability to sustain cryopreservation are also mentioned. Moreover, factors that may influence sperm freezability, such as season, diet, breed, or ejaculate fractions are discussed, together with the effects of different additives, like seminal plasma and antioxidants. After briefly referring to the effects of long-term sperm preservation in frozen state and the reproductive performance of frozen-thawed boar sperm, this work speculates with new research horizons on the preservation of boar sperm, such as vitrification and freeze-drying.

Storage of sexed boar spermatozoa: Limits and perspectives

Despite the great potential application of sex-sorted spermatozoa in swine, the technology is not practiced in the pig industry because of technical factors and species-specific issues. The susceptibility of boar spermatozoa to stresses induced by the sorting procedure, the relative slowness of the sex-sorting process together with the high sperm numbers required for routine artificial insemination in pig are some of the main factors limiting the commercial application of this technology in pigs. This review briefly describes the damage to spermatozoa during sex sorting, focusing on an additional limiting factor: increased susceptibility of sexed boar spermatozoa to injuries induced by liquid storage and cryopreservation that, in turn, impairs sperm quality leading to unsatisfactory results in vivo. Strategies to extend the lifespan of sex-sorted boar spermatozoa and to improve their fertilizing ability after liquid storage or cryopreservation need to be implemented before this technology can be used in pig farms. In this regard, encapsulation in barium alginate membranes could be a promising technique to optimize the in vivo use of sexed boar spermatozoa, by protecting, targeting, and controlling the release of sperm into the female genital tract.

Enhanced fertility prediction of cryopreserved boar spermatozoa using novel sperm function assessment

Due to reduced fertility, cryopreserved semen is seldom used for commercial porcine artificial insemination (AI). Predicting the fertility of individual frozen ejaculates for selection of higher quality semen prior to AI would increase overall success. Our objective was to test novel and traditional laboratory analyses to identify characteristics of cryopreserved spermatozoa that are related to boar fertility. Traditional post-thaw analyses of motility, viability, and acrosome integrity were performed on each ejaculate. In vitro fertilization, cleavage, and blastocyst development were also determined. Finally, spermatozoa-oviduct binding and competitive zona-binding assays were applied to assess sperm adhesion to these two matrices. Fertility of the same ejaculates subjected to laboratory assays was determined for each boar by multi-sire AI and defined as (i) the mean percentage of the litter sired and (ii) the mean number of piglets sired in each litter. Means of each laboratory evaluation were calculated for each boar and those values were applied to multiple linear regression analyses to determine which sperm traits could collectively estimate fertility in the simplest model. The regression model to predict the percent of litter sired by each boar was highly effective (p < 0.001, r(2) = 0.87) and included five traits; acrosome-compromised spermatozoa, percent live spermatozoa (0 and 60 min post-thaw), percent total motility, and the number of zona-bound spermatozoa. A second model to predict the number of piglets sired by boar was also effective (p < 0.05, r(2) = 0.57). These models indicate that the fertility of cryopreserved boar spermatozoa can be predicted effectively by including traditional and novel laboratory assays that consider functions of spermatozoa.

Cryopreservation of semen from domestic livestock

In modern livestock breeding, cryopreserved semen is routinely used for artificial insemination. Sperm cryopreservation secures future reproduction, and insemination doses can be easily shipped. Processing of semen for cryopreservation can be done with minimal efforts and relatively low costs. In this chapter we describe the entire cryopreservation process for stallion and bull sperm including dilution of sperm in primary and freezing extender, cooling and packaging in straws, freezing in liquid nitrogen vapor, cryogenic storage, and thawing. Special emphasis is given on preparation of commonly used primary and freezing extenders (skim milk extender "INRA-82", TRIS-egg yolk extender "TEY") used in a two-step dilution approach. Furthermore the different cooling rates needed in different temperature regimes during the freezing process are being described. Cryopreservation procedures are described in case of using both specialized automated equipment and simple equipment.

Boar sperm cryosurvival is better after exposure to seminal plasma from selected fractions than to those from entire ejaculate

Boar bulk ejaculates are now being collected instead of usual sperm-rich fractions (SRF) for artificial insemination purpose. The present study evaluated the influence of holding boar sperm samples before freezing surrounded in their own seminal plasma (SP), from either fractions/portions or the entire ejaculate, on post-thawing sperm quality and functionality. Ejaculates collected as bulk (BE) or as separate (first 10 mL of SRF [P1] and rest of SRF [P2]) from 10 boars were held 24h at 15-17°C and then frozen. Some bulk ejaculate samples were frozen immediately after collections as Control. In addition, epididymal sperm samples from the same 10 boars were collected post-mortem and extended in SP from P1 (EP1), P2 (EP2) and post SRF (EP3), and also held 24h before freezing for a better understanding of the influence of SP on boar sperm cryopreservation. The sperm quality (motility, evaluated by CASA, and viability, evaluated by flow cytometry) and functionality (flow cytometry assessment of plasma membrane fluidity, mitochondrial membrane potential and intracellular generation of reactive oxygen species [ROS] in viable sperm) were evaluated at 30, 150 and 300 min post-thaw. Post-thawing sperm quality and functionality of P1 and P2 were similar but higher (p < 0.01) than BE samples. Control samples showed higher (p < 0.01) post-thaw sperm quality and functionality than BE samples. Post-thawing sperm quality and functionality of EP1 and EP2 were similar but higher (p < 0.05) than EP3. These results showed that boar sperm from BE are more cryosensitive than those from the SRF, particularly when held 24h before freezing, which would be attributable to the cryonegative effects exerted by the SP from post SRF.

Conception rate and litter size in multiparous sows after intrauterine insemination using frozen-thawed boar semen in a commercial swine herd in Thailand

The aim of the present study was to determine the conception rate and litter size in sows after fixed time intra-uterine insemination using frozen-thawed boar semen in a commercial swine herd in Thailand. Sixty-nine Landrace multiparous sows were randomly allocated into two groups, including control (n=36) and treatment (n=33). The control sows were inseminated with extended fresh semen (3 × 10⁹ motile sperm/dose, 100 ml) at 24, 36 and 48 hr after the onset of estrus. The treatment sows were inseminated with frozen-thawed semen (2 × 10⁹ motile sperm/dose, 20 ml) at 24 and 36 hr after induction of ovulation by human chorionic gonadotropin. All inseminations were carried out by using an intra-uterine insemination technique. The time of ovulation was determined by using transrectal real-time B-mode ultrasonography. The conception rate, farrowing rate, total number of piglets born/litter (TB) and number of piglets born alive/litter (BA) were evaluated. The sows inseminated with extended fresh semen yield a higher TB (10.8 versus 9.0 piglets/l, P=0.015) and tended to have a higher conception rate (88.9% versus 75.8%, P=0.150) than sows inseminated with frozen-thawed semen. In conclusion, insemination using frozen-thawed boar semen can be practiced with convinced fertility under field conditions by fixed-time intrauterine insemination with 2 × 10⁹ sperm/ dose of 20 ml at 24 and 36 hr after the onset of estrus.