Effect of seminal plasma on the cryopreservation of equine spermatozoa

Seminal plasma is generally removed from equine spermatozoa prior to cryopreservation. Two experiments were designed to determine if adding seminal plasma back to spermatozoa, prior to cryopreservation, would benefit the spermatozoa. Experiment 1 determined if different concentrations of seminal plasma affected post-thaw sperm motility, viability and acrosomal integrity of frozen/thawed stallion spermatozoa. Semen was washed through 15% Percoll® to remove seminal plasma and spermatozoa resuspended to 350 × 106 sperm/mL in a clear Hepes buffered diluent containing either 0, 5, 10, 20, 40 or 80% seminal plasma for 15 min, prior to being diluted to a final concentration of 50 × 106 sperm/mL in a Lactose-EDTA freezing diluent and cryopreserved. Sperm motility was analyzed at 10 and 90 min after thawing, while sperm viability and acrosomal integrity were analyzed 20 min after thawing. Seminal plasma did not affect sperm motility, viability or acrosomal integrity (P > 0.05). Experiment 2 tested the main affects of seminal plasma level (5 or 20%), incubation temperature (5 or 20 °C) and incubation time (2, 4 or 6 h) prior to cryopreservation. In this experiment, spermatozoa were incubated with 5 or 20% seminal plasma for up to 6 h at either 5 or 20 °C prior to cryopreservation in a skim milk, egg yolk freezing extender. Samples cooled immediately to 5 °C, prior to freezing had higher percentages of progressively motile spermatozoa than treatments incubated at 20 °C (31 versus 25%, respectively; P < 0.05), when analyzed 10 min after thawing. At 90 min post-thaw, total motility was higher for samples incubated at 5 °C (42%) compared to 20 °C (35%; P < 0.05). In addition, samples containing 5% seminal plasma had higher percentages of total and progressively motile spermatozoa (45 and 15%) than samples exposed to 20% seminal plasma (33 and 9%; P < 0.05). In conclusion, although the short-term exposure of sperm to seminal plasma had no significant effect on the motility of cryopreserved equine spermatozoa, prolonged exposure to seminal plasma, prior to cryopreservation, was deleterious.