Membrane changes during different stages of a freeze-thaw protocol for equine semen cryopreservation.

The research article investigates the impact of freeze-thaw protocols used in cryopreservation of equine sperm on sperm membrane. The study specifically documents changes in membrane function and sperm viability at different stages of the process and explores whether functionality and viability prediction is possible using specific evaluation methods.

Objective and Methods

• The primary objective of the study was identifying which stages of the freeze-thaw process result in membrane alterations. Additionally, the research aimed to determine if chlortetracycline (CTC) stain could be used to predict how resistant horse sperm is to freezing.
• Two techniques were used to simultaneously evaluate sperm viability and capacitation state: CTC and Hoechst 33258 (H258).
• To evaluate membrane function, the researchers conducted the hypo-osmotic swelling test (HOS). Additionally, progressive motility (PM) was examined under light microscopy at every phase of the freeze-thaw protocol.
• The researchers assessed raw semen and samples at various stages, including after dilution and centrifugation, redilution and equilibration, cooling to 5 degrees Celsius, super cooling to -15 degrees Celsius, and finally, post-thawing.

Findings

• The researchers discovered that the most significant damage to sperm membranes and the greatest decline in progressive motility occurred post-thaw.
• CTC/H258 staining showed that cryopreservation had a significant impact on sperm membrane integrity at several stages: post-centrifugation, redilution and equilibration, and cooling to 5 degrees Celsius.
• HOS and H258 test results were found to be similar and inversely correlated with the number of viable, non-capacitated sperm cells.
• Interestingly, it was noted that the subpopulation of capacitated live cells was unaffected at all stages of the freeze-thaw process, and the number of live cells having completed acrosome reaction increased four-fold.

Implications and Limitations

• The increase in live acrosome-reacted cells suggests that the group of functional sperm cells capable of binding to the zona pellucida may increase post-thawing, despite the overall detrimental effect of the process on the total live sperm population.
• However, it wasn’t conclusively determined whether the changes in CTC patterns reflect a true capacitation phenomenon or an intermediate, destabilized state of the sperm cell membrane.
• The findings could potentially help refine cryopreservation processes or develop new prediction models for sperm resistivity to freezing, although more research would be required.