A comparison of the freezing of human semen in the uncirculated vapour above liquid nitrogen and in a commercial semi-programmable freezer

The cooling rates inside 0.25-ml semen straws filled with glycerol, egg yolk, citrate buffer were compared between a standard vapour freezing procedure and freezing in a Nicool LM-10 semi-programmable freezer. During vapour freezing, the cooling rate at the bottom of the straw was much faster than at the top and there was considerable variation between replicates. By contrast, position within the straw did not affect the cooling rate in the Nicool LM-10 procedure and replicates were more consistent. More motile spermatozoa survived the Nicool freezing procedure and their lateral head displacement was greater than vapour frozen spermatozoa. However the percentage of intact spermatozoa and their velocity was very similar after the two procedures. We conclude that the freezing procedure using the Nicool LM-10 provides a comparatively economical way to achieve consistent semen freezing for research studies.