Post-thaw functional status of boar spermatozoa cryopreserved using three controlled rate freezers: a comparison

Abstract

This study compared variation in the quality of cryopreserved boar spermatozoa and the control and accuracy of cooling rates between three semen freezers (CryoLogic Freeze Control CL3000, Planer Products Kryo Save Compact KS1.7/Kryo 10 Control module and a controlled rate ‘Watson’ freezing machine developed within our laboratory). Five ejaculates were collected from each of 15 boars (five boars from each of three breeds). Semen was diluted into a commercial freezing buffer (700 mOsm/kg, 3% v/v glycerol) and placed into 0.5 ml straws. Three straws per treatment, from each ejaculate were cooled to −5 °C at 6 °C/min, held at −5 °C for 30 s while ice crystal formation was induced, then further cooled from −5 to 80 °C at either 40 °C/min (Kryo Save Compact KS1.7 and Watson) or 6 °C/min (Freeze Control CL3000). Precise measurements of temperature fluctuations during the programmed cooling curves were made by inserting thermocouples into the semen filled straws. Semen was assessed for %motile cells, motility characteristics using computer-assisted semen analysis (CASA), plasma membrane integrity (%SYBR-14 positive stained spermatozoa) and acrosome integrity (%FITC-PNA positive stained spermatozoa). Spermatozoa cryopreserved using the Freeze Control CL3000 system (maximum rate of 6 °C/min) exhibited reduced post-thaw viability (14.2±2.8% mean plasma membrane intact spermatozoa) when compared to both the KS1.7 and Watson freezers (optimal rate of 40 °C/min) (18.4±3.2 and 25.7±3.7% mean plasma membrane intact spermatozoa, respectively). Differences in motility characteristics were observed between spermatozoa cryopreserved at 40 °C/min with the Watson apparatus preserving a larger proportion of sperm with progressive motility. Cooling curves in the CL3000 and KS1.7 were interrupted by a pronounced increase in temperature at −5 °C that corresponded with the latent heat of fusion released with ice crystal formation. This temperature change was significantly reduced in the cooling curves produced by the Watson freezer. These findings suggest that preserving spermatozoa using the Watson freezer improved post-thaw semen quality, with regard to sperm motility characteristics. Furthermore, that post-thaw semen viability was enhanced by minimising temperature fluctuations resulting from the release of the latent heat of fusion at ice crystal formation.