Cryo-scanning Electron Microscopy Discloses Differences in Dehydration of Frozen Boar Semen Stored in Large Containers

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.

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.

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.

Do physical forces contribute to cryodamage?

To achieve the ultimate goal of both cryosurgery and cryopreservation, a thorough understanding of the processes responsible for cell and tissue damage is desired. The general belief is that cells are damaged primarily due to osmotic effects at slow cooling rates and intracellular ice formation at high cooling rates, together termed the "two factor theory." The present study deals with a third, largely ignored component--mechanical damage. Using pooled bull sperm cells as a model and directional freezing in large volumes, samples were frozen in the presence or absence of glass balls of three different diameters: 70-110, 250-500, and 1,000-1,250 microm, as a means of altering the surface area with which the cells come in contact. Post-thaw evaluation included motility at 0 h and after 3 h at 37 degrees C, viability, acrosome integrity, and hypoosmotic swelling test. Interactions among glass balls, sperm cells, and ice crystals were observed by directional freezing cryomicroscopy. Intra-container pressure in relation to volume was also evaluated. The series of studies presented here indicate that the higher the surface area with which the cells come in contact, the greater the damage, possibly because the cells are squeezed between the ice crystals and the surface. We further demonstrate that with a decrease in volume, and thus increase in surface area-to-volume ratio, the intra-container pressure during freezing increases. It is suggested that large volume freezing, given that heat dissipation is solved, will inflict less cryodamage to the cells than the current practice of small volume freezing.