Sperm Flagellum Volume Determines Freezability in Red Deer Spermatozoa

Abstract

The factors affecting the inter-individual differences in sperm freezability is a major line of research in spermatology. Poor sperm freezability is mainly characterised by a low sperm velocity, which in turn is associated with low fertility rates in most animal species. Studies concerning the implications of sperm morphometry on freezability are quite limited, and most of them are based on sperm head size regardless of the structural parts of the flagellum, which provides sperm motility. Here, for the first time, we determined the volumes of the flagellum structures in fresh epididymal red deer spermatozoa using a stereological method under phase contrast microscopy. Sperm samples from thirty-three stags were frozen and classified as good freezers (GF) or bad freezers (BF) at two hours post-thawing using three sperm kinetic parameters which are strongly correlated with fertility in this species. Fourteen stags were clearly identified as GF, whereas nineteen were BF. No significant difference in sperm head size between the two groups was found. On the contrary, the GF exhibited a lower principal piece volume than the BF (6.13 µm3 vs 6.61 µm3, respectively, p = 0.006). The volume of the flagellum structures showed a strong negative relationship with post-thawing sperm velocity. For instance, the volume of the sperm principal piece was negatively correlated with sperm velocity at two hours post-thawing (r = −0.60; p<0.001). Our results clearly show that a higher volume of the sperm principal piece results in poor freezability, and highlights the key role of flagellum size in sperm cryopreservation success.

Effects of vitrification on ram spermatozoa using free-egg yolk extenders

“…The differences between studies could be due to the species. Shape and size of the sperm head and the flagellum are factors that define the cryosensivity of the cell [4,33,35]. A small size of head has been related with a higher cryoresistance in some species [4].…”

Current status and potential of morphometric sperm analysis

“…Some authors have explored the morphological response to diluting or washing the sperm sample,105106 to capacitation107 or to cryopreservation 26108109110…”

“…Despite most of the studies on the effect of cryopreservation on sperm morphology have been focused on the evaluation of sperm head features, Ros-Santaella et al 109. recently reported that stags not differing in sperm head dimensions showed significant differences in sperm cryoresistance that were strongly related to the volume of the sperm principal piece.…”

“…Several studies have addressed how sperm morphology and sperm velocity may be related,23445199109132 and their impact on reproductive performance 2398133. However, results are contradictory on how sperm diversity translates into variation in sperm velocity 4399132.…”

“…Since the importance of sperm flagellar dimensions on sperm function has been demonstrated,98109 CASA-Morph systems should be modified to incorporate new tools that allow an automatic and accurate evaluation of these sperm structures. These advances will allow the undertaking of basic studies of sperm morphometry since most of the research done so far has focused only on the sperm head.…”