Cryo-scanning electron microscopy (Cryo-SEM) of boar semen frozen in medium-straws and MiniFlatPacks

Enrichment of thawed boar spermatozoa with an intact membrane using Magnetic Activated Cell Sorting

Not all boar sperm samples survive cryopreservation well. A method of eliminating damaged sperm might enable more cryopreserved boar semen to be used for pig breeding. In this study we investigated the use of Magnetic Activated Cell sorting (MACS) to eliminate damaged sperm from thawed boar semen samples. The thawed samples were mixed with Dead cell removal particles and were applied to the column in a SuperMACS II. Different fractions were collected: Original sample (O), Flow-through (FT), and Eluate (E). Sperm membrane integrity, mitochondrial membrane potential and reactive oxygen species were evaluated by flow cytometry after staining with SYBR 14 and propidium iodide, or 5', 6, 6'-tetrachloro-1, 1', 3, 3'-tetraethylbenzimidazolylcarbocyanine iodide, or hydroethidine and dichlorodihydrofluorescein diacetate, respectively. The FT samples had increased membrane integrity, a greater proportion of sperm with high mitochondrial membrane potential and a greater proportion of sperm negative for hydrogen peroxide than O samples (P<0.0001), which in turn had increased membrane integrity than E samples (P <0.0001). However, differences were seen between boars. The FT samples had increased values of live, superoxide positive sperm than O samples (P <0.0001) and O samples had greater values than E samples (P <0.0001), while there was no effect of boar. Sperm quality was best in the FT fraction, comprising approximately 32% of the sperm sample. In conclusion, although there were differences between boars, MACS separation can improve sperm quality in thawed semen samples. It would be interesting to see if this improvement is reflected in fertility outcomes.

Effect of Magnetized Freezing Extender on Membrane Damages, Motility, and Fertility of Boar Sperm Following Cryopreservation

Magnetized water is defined as the amount of water that has passed through a magnet. The magnetic field weakens the hydrogen bonds between the water molecules, leading to the magnetized liquid acquiring special characteristics such as easy supercooling and forming smaller ice crystals. We researched the influences of a magnetized freezing extender on cell membrane damage and in vitro fertilization of boar sperm during cryopreservation. The freezing extenders were passed through 0, 2000, 4000, and 6000 gausses (G) of magnetic devices using a liquid cycling pump system and then used for the sperm freezing process. The damage to plasma, acrosomal, and mitochondrial membranes in frozen-thawed spermatozoa was investigated by flow cytometry, and motility was assessed using the CASA system. The fertility of frozen-thawed sperm was estimated using in vitro fertilization. The damage to the membranes was significantly decreased in the magnetized freezing extender by the 6000 G magnetic field compared to that of the control in frozen-thawed sperm, and motility was increased in the 6000 G group. Although there were no significant differences in the cleavage rates of in vitro fertilized oocytes among the treatment groups, the ratio of blastocyst formation increased in the magnetized freezing extender groups compared with that in the control group. The number of blastocysts was significantly higher in the 4000 G group than in the 0 G group. In conclusion, these results suggest that a magnetized freezing extender could improve the freezability of sperm and the development of oocytes fertilized in vitro with frozen-thawed sperm.

Cryo-scanning electron microscopy demonstrates that ice morphology is not associated with the post-thaw survival of domestic boar (Sus domesticus) spermatozoa: A comparison of directional and conventional freezing methods

Directional freezing (in 2 or 10 ml hollow glass tubes) has been reported to improve post-thaw sperm survival parameters compared to conventional methods (in 0.5 ml straws). However, the biophysical properties that increase post-thaw survival are poorly understood. Therefore, the aim for the current study was to investigate the effect of ice morphology on the post-thaw survival of domestic boar spermatozoa directionally and conventionally cryopreserved in 0.5 ml straws. Ice morphology was quantitatively analyzed using a combination of cryo-scanning electron microscopy and Fiji Shape Descriptors. Multivariate analysis found a significant, non-linear effect (p < 0.05) of interface velocity on ice morphology, with an increase in both ice-lake size, as indicated by area and in aspect ratio, at an interface velocity of 0.2 mm/s. By contrast, post-thaw sperm survival (defined as spermatozoa with both intact plasma membranes and acrosomes) was biphasic, with peaks of survival at interface velocities of 0.2 mm/s (54.2 ± 1.9%), and 1.0 or 1.5 mm/s (56.5 ± 1.5%, 56.7 ± 1.7% respectively), and lowest survival at 0.5 (52.1 ± 1.6%) and 3.0 mm/s (51.4 ± 1.9%). Despite numerical differences in Shape Descriptors, there was no difference (p > 0.05) in the post-thaw survival between conventionally and directionally cryopreserved samples at optimal interface velocities of 1.0 or 1.5 mm/s. These findings suggest that: 1) ice morphology has little impact on post-thaw survival of boar spermatozoa, and 2) directional freezing in 0.5 ml straws (rather than 2 or 10 ml hollow glass tubes) may attenuate benefits of directional freezing.

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.

Ultra-rapid cooling of ibex sperm by spheres method does not induce a vitreous extracellular state and increases the membrane damages

Sperm cryopreservation by ultra-rapid cooling based on dropping small volumes of sperm suspension directly into liquid nitrogen, has been successful in some wild ruminant species, including the Iberian ibex (Capra pyrenaica). In ultra-rapid cooling, the contents of these droplets are expected to enter a stable, glass-like state, but to the best of our knowledge no information exists regarding the presence or absence of ice formation in the extracellular milieu when using this technique. Different modifications to the extracellular milieu likely inflict different types of damage on the plasmalemma, the acrosome and mitochondrial membranes. The aims of the present work were: 1) to examine the physical state of the extracellular milieu after cryopreservation at slow and ultra-rapid cooling rates—and thus determine whether ultra-rapid cooling vitrifies the extracellular milieu; and 2) to compare, using conventional sperm analysis techniques and scanning and transmission electron microscopy, the damage to sperm caused by these two methods. Sperm samples were obtained by the transrectal ultrasound-guided massage method (TUMASG) from anesthetized Iberian ibexes, and cryopreserved using slow and ultra-rapid cooling techniques. Sperm motility (22.95 ± 3.22% vs 4.42 ± 0.86%), viability (25.64 ± 3.71% vs 12.8 ± 2.50%), acrosome integrity (41.45± 3.73% vs 27.00 ± 1.84%) and mitochondrial membrane integrity (16.52 ± 3.75% vs 4.00 ± 0.65%) were better after slow cooling (P<0.001) than after ultra-rapid technique. Cryo-scanning electron microscopy (Cryo-SEM) suggested that the vitrified state was not achieved by ultra-rapid cooling, and that the ice crystals formed were smaller and had more stretchmarks (P<0.001) than after slow cooling. Scanning electron microscopy revealed no differences in the types of damage caused by the examined techniques, although transmission electron microscopy showed the damage to the plasmalemma and mitochondrial membrane to be worse after ultra-rapid cooling. In conclusion ultra-rapid cooling provoked more membrane damage than slow cooling, perhaps due to the extracellular ice crystals formed.

Post-thaw boar sperm motility is affected by prolonged storage of sperm in liquid nitrogen. A retrospective study

Owing to the quick genetic turnover of the pig industry, most AI-boar sires live 2-3 yr, a period during which for 1-2 yr their semen is extended and used in liquid form for AI. Despite showing low cryosurvival, affecting fertility after AI, boar semen is frozen for easiness of transport overseas and reposition of valuable genetics. For the latter, semen is stored in liquid nitrogen (LN₂, cryostorage) for many years, a controversial practice. Here we studied how length of cryostorage could affect sperm quality. Straws (0.5 mL) frozen using the same cryopreservation protocol at one specific location from AI- sires of proven fertility were stored in LN₂ for up to 8 yr. Post-thaw sperm quality was evaluated after 2, 4 or 8 yr of cryostorage, always compared to early thawing (15 d after freezing). Sperm motility and kinematics were evaluated post-thaw using CASA and sperm viability was cytometrically evaluated using specific fluorophores. Sperm viability was not affected by length of cryostorage, but total and progressive sperm motility were lower (p < 0.01) in sperm samples cryostored for 4 or 8 yr compared to those thawed 15 d after freezing. Cryostorage time affected sperm kinetics, but with greater intensity in the samples cryostored for 4 yr (p < 0.001) than in those for 2 yr (p < 0.01). The fact that the major phenotypic characteristic of boar spermatozoa, motility, is constrained by time of cryostorage should be considered when building cryobanks of pig semen. Attention should be placed on the finding that >2 yr of cryostorage time can be particularly detrimental for the post-thaw motility of some sires, which might require increasing sperm numbers for AI.

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.

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.

Emerging applications of sperm, embryo and somatic cell cryopreservation in maintenance, relocation and rederivation of swine genetics

Advances in porcine assisted reproductive technology (ART) make it possible to use cryopreserved sperm, embryos and somatic cells in the maintenance, relocation and regeneration of swine genetics. In this review, development of key application-limiting technology is discussed in each cell type, focusing on the efficiencies, ease of storage and transportation, and minimization of pathogen transmission. Methods to regenerate swine genetics and/or models using frozen sperm, embryos and somatic cells in combination with other porcine ARTs, such as in vitro fertilization (IVF), intracytoplasmic sperm injection (ICSI), and somatic cell nuclear transplantation (SCNT), are also discussed. The applications of these ARTs utilizing cryopreserved cells will greatly increase the efficiency as well as biosecurity for maintenance, relocation and rederivation of swine genetics/models.

Freezing injury: the special case of the sperm cell

The cellular damage that spermatozoa encounter at rapid rates of cooling has often been attributed to the formation of intracellular ice although no convincing evidence of intracellular ice formation has ever been obtained. We demonstrate that the high intracellular protein content together with the osmotic shrinkage associated with extracellular ice formation leads to intracellular vitrification of spermatozoa during cooling. At rapid rates of cooling the cell damage to spermatozoa is a result of an osmotic imbalance encountered during thawing, not intracellular ice formation. The osmotic imbalance occurs at rapid cooling rates due to a diffusion limited ice crystallisation in the extracellular fluid, i.e. the amount of ice forming during the cooling is less than expected from the equilibrium phase diagram. This explanation allows insights into other aspects of the cryobiology of spermatozoa and it is anticipated that this understanding will lead to specific improved methods of conventional cryopreservation for mammalian spermatozoa. It is also likely that this model will be relevant to the development of novel technologies for sperm preservation including vitrification and freeze drying.

Advances in boar semen cryopreservation

The present paper highlights aspects of the cryopreservation of boar semen, a species with particular large, fractionated ejaculates, and a cumbersome cryotechnology that had prevented its commercial application. With the dramatic increase of use of liquid pig semen for artificial breeding over the past decade, developments on cryopreservation alongside the routine use of stud boar semen for AI had been promoted. Recent advances in our laboratory, accommodating the best use of portions of the sperm-rich fraction of the ejaculate for cryopreservation of the sperm-peak portion (P1) and parallel use of the rest of the collected ejaculated spermatozoa, appears as a suitable commercial alternative.